Two Coils That Never Touch Can Still Hand Energy to Each Other
Follow the compass-needle experiment that links current to magnetism, use the right-hand thumb rule on a wire and a loop, compare an electromagnet with a permanent magnet, apply both of Fleming's rules, and work out a transformer's output from its turns ratio.
How can electricity and magnetism turn out to be the same subject?
Lay a compass needle under a straight wire and connect the wire to a cell. The needle swings aside the moment the current begins, returns when the circuit is broken, and swings the other way when the cell is reversed. That observation, known as Oersted's experiment, is where this part of the chapter starts, and everything else follows from it.
A current-carrying conductor produces a magnetic field around it. Electricity and magnetism are not two separate topics that happen to sit in the same chapter — they are two faces of one thing.
And the connection runs both ways, which is the more remarkable half.
- A current produces a magnetic field. Wind the wire into a coil around an iron core and you have an electromagnet you can switch on and off
- A current in a magnetic field feels a force. That force is what makes an electric motor turn, and its direction comes from Fleming's left-hand rule
- A changing magnetic field produces a current. That is electromagnetic induction, and its direction comes from Fleming's right-hand rule
The last of those is the one with the widest consequences. Because a changing field induces a current, two coils wound on the same iron core can pass energy to each other without any electrical connection between them at all — which is exactly what a transformer does, and why the entire electricity supply of the previous part is possible.
So this part closes the circle. The high-voltage transmission you met earlier depends on being able to step a voltage up and down, and stepping a voltage needs a transformer, and a transformer needs a changing magnetic field. None of it works with a steady direct current, and that is why the mains supply alternates.
This page covers the fourth part of the ICSE Class 10 Physics chapter on electricity and magnetism: the magnetic field due to a current, electromagnets, Fleming's two rules, electromagnetic induction and the transformer.
A current-carrying conductor produces a magnetic field around it. Electricity and magnetism are not two separate topics that happen to sit in the same chapter — they are two faces of one thing.
And the connection runs both ways, which is the more remarkable half.
- A current produces a magnetic field. Wind the wire into a coil around an iron core and you have an electromagnet you can switch on and off
- A current in a magnetic field feels a force. That force is what makes an electric motor turn, and its direction comes from Fleming's left-hand rule
- A changing magnetic field produces a current. That is electromagnetic induction, and its direction comes from Fleming's right-hand rule
The last of those is the one with the widest consequences. Because a changing field induces a current, two coils wound on the same iron core can pass energy to each other without any electrical connection between them at all — which is exactly what a transformer does, and why the entire electricity supply of the previous part is possible.
So this part closes the circle. The high-voltage transmission you met earlier depends on being able to step a voltage up and down, and stepping a voltage needs a transformer, and a transformer needs a changing magnetic field. None of it works with a steady direct current, and that is why the mains supply alternates.
This page covers the fourth part of the ICSE Class 10 Physics chapter on electricity and magnetism: the magnetic field due to a current, electromagnets, Fleming's two rules, electromagnetic induction and the transformer.
What does the compass-needle experiment show, and how do you find the field direction?
It shows that a current produces a magnetic field around the conductor, and the right-hand thumb rule gives that field's direction.
The experiment, set out so that it can be described. A straight wire is held horizontally above a compass needle, in the direction the needle points, and connected through a key to a cell and a rheostat.
- With the key open there is no current, and the needle points along the wire as usual
- Close the key and the needle deflects, showing that a magnetic field has appeared
- Reverse the cell and the needle deflects the other way, showing that reversing the current reverses the field
- Increase the current with the rheostat and the deflection increases
- Move the needle further from the wire and the deflection decreases
Four conclusions, one from each observation: a current produces a magnetic field; the field's direction depends on the current's direction; the field is stronger for a larger current; and the field is weaker further from the wire.
The right-hand thumb rule. Hold the conductor in your right hand with the thumb pointing along the direction of the current. Your curled fingers then give the direction in which the field lines circle the wire.
The shape of the field around a straight wire. The field lines are concentric circles in a plane perpendicular to the wire, with the wire at the centre.
- They are not straight lines and they do not point away from the wire — that is the commonest drawing error in the chapter
- They are crowded close to the wire and further apart away from it, because the field weakens with distance
- The field is proportional to the current and inversely proportional to the distance from the wire
Worked reasoning — doubling both. If the current in a wire is doubled and the point of observation is moved to twice its distance, what happens to the field?
Doubling the current doubles the field; doubling the distance halves it. The two changes cancel exactly, so the field is unchanged. That is a favourite one-line question and it is answered purely from the two proportionalities.
The dot and cross convention, needed for every diagram. A dot means the current is coming out of the page; a cross means it is going into the page.
- Current out of the page gives field circles that are anticlockwise when seen from in front
- Current into the page gives circles that are clockwise
The field due to a circular loop. Bend the wire into a loop and apply the rule to each small part of it.
- Near the wire the lines are still circles around it
- At the centre of the loop the contributions of every part of the wire point the same way, so they add, and the field there is nearly straight and perpendicular to the plane of the loop
- **A coil of turns gives times the field at the centre**, because the current passes the same point times and each turn contributes in the same direction
- A smaller loop gives a larger field at its centre, for the same current
Which is the whole reason coils are wound with many turns. A single loop produces a weak field; a hundred turns produce a hundred times as much for the same current, and that is the step from a bent wire to a useful electromagnet.
One boundary case worth stating. A straight current-carrying wire has no north or south pole. Its field lines are closed circles that never begin or end anywhere, so there is no region they leave from and none they enter. Poles appear only when the wire is wound into a coil, which is the subject of the next section.
The experiment, set out so that it can be described. A straight wire is held horizontally above a compass needle, in the direction the needle points, and connected through a key to a cell and a rheostat.
- With the key open there is no current, and the needle points along the wire as usual
- Close the key and the needle deflects, showing that a magnetic field has appeared
- Reverse the cell and the needle deflects the other way, showing that reversing the current reverses the field
- Increase the current with the rheostat and the deflection increases
- Move the needle further from the wire and the deflection decreases
Four conclusions, one from each observation: a current produces a magnetic field; the field's direction depends on the current's direction; the field is stronger for a larger current; and the field is weaker further from the wire.
The right-hand thumb rule. Hold the conductor in your right hand with the thumb pointing along the direction of the current. Your curled fingers then give the direction in which the field lines circle the wire.
The shape of the field around a straight wire. The field lines are concentric circles in a plane perpendicular to the wire, with the wire at the centre.
- They are not straight lines and they do not point away from the wire — that is the commonest drawing error in the chapter
- They are crowded close to the wire and further apart away from it, because the field weakens with distance
- The field is proportional to the current and inversely proportional to the distance from the wire
Worked reasoning — doubling both. If the current in a wire is doubled and the point of observation is moved to twice its distance, what happens to the field?
Doubling the current doubles the field; doubling the distance halves it. The two changes cancel exactly, so the field is unchanged. That is a favourite one-line question and it is answered purely from the two proportionalities.
The dot and cross convention, needed for every diagram. A dot means the current is coming out of the page; a cross means it is going into the page.
- Current out of the page gives field circles that are anticlockwise when seen from in front
- Current into the page gives circles that are clockwise
The field due to a circular loop. Bend the wire into a loop and apply the rule to each small part of it.
- Near the wire the lines are still circles around it
- At the centre of the loop the contributions of every part of the wire point the same way, so they add, and the field there is nearly straight and perpendicular to the plane of the loop
- **A coil of turns gives times the field at the centre**, because the current passes the same point times and each turn contributes in the same direction
- A smaller loop gives a larger field at its centre, for the same current
Which is the whole reason coils are wound with many turns. A single loop produces a weak field; a hundred turns produce a hundred times as much for the same current, and that is the step from a bent wire to a useful electromagnet.
One boundary case worth stating. A straight current-carrying wire has no north or south pole. Its field lines are closed circles that never begin or end anywhere, so there is no region they leave from and none they enter. Poles appear only when the wire is wound into a coil, which is the subject of the next section.
What is an electromagnet, and how does it compare with a permanent magnet?
An electromagnet is a soft iron core placed inside a solenoid, which becomes a strong magnet while a current flows and loses its magnetism when the current stops.
How it is made. A long insulated copper wire is wound closely in the form of a cylinder — a solenoid — and a rod of soft iron is placed inside it. When a current is passed:
- The solenoid alone produces a magnetic field whose lines inside it are parallel and straight, so the field there is uniform, and whose outside pattern is that of a bar magnet
- One end behaves as a north pole and the other as a south pole, and the poles swap if the current is reversed
- The soft iron core becomes magnetised and greatly strengthens the field
How to identify which end is the north pole. Look at one end of the solenoid and see which way the current circulates in the nearest turn. If it is anticlockwise as you look at that face, that face is the north pole; if clockwise, it is the south pole.
Three ways to make an electromagnet stronger:
- Increase the current through the coil
- Increase the number of turns per unit length
- Use a soft iron core rather than air, and make it of a suitable shape
Why the core must be soft iron and not steel. Soft iron magnetises strongly and loses its magnetism as soon as the current stops, which is exactly what a switchable magnet needs. Steel retains its magnetism, which makes it the right material for a permanent magnet and the wrong material for an electromagnet core.
Uses of electromagnets, which is a standard list question:
- An electric bell, where the electromagnet pulls an armature to strike a gong and breaks its own circuit as it does so
- A relay, where a small current operates an electromagnet that closes a switch in a separate high-current circuit
- A loudspeaker, where a varying current in a coil in a magnetic field moves a cone to produce sound
- Electric motors and generators
- A lifting magnet on a crane, for moving scrap iron and steel in a yard
- Separating iron and steel from other scrap or from waste
- Removing iron or steel splinters from a patient's eye
- Magnetic recording and medical imaging machines, which use very strong electromagnets
The comparison with a permanent magnet, point by point, which is the most examined part of this section:
- Strength. An electromagnet can be made very strong; a permanent magnet is comparatively weak
- Can it be varied? An electromagnet's strength can be changed by changing the current or the turns; a permanent magnet's cannot
- Can it be switched off? An electromagnet can be switched off by breaking the circuit; a permanent magnet cannot
- Can the poles be reversed? An electromagnet's polarity reverses with the current; a permanent magnet's polarity is fixed
- Does it need a supply? An electromagnet needs a continuous current, so it costs energy to keep magnetised; a permanent magnet needs none
- Does it keep its magnetism? An electromagnet with a soft iron core loses its magnetism almost completely; a permanent magnet of steel retains it
Which of the two to use, in one sentence each. Use an electromagnet wherever the magnetism must be switched, varied or made very strong — a crane, a bell, a relay. Use a permanent magnet wherever a steady, supply-free field is wanted — a compass, a fridge door catch, the field magnet of a small motor.
One connection worth noticing. The crane's lifting magnet is useful precisely because it can be switched off. A permanent magnet strong enough to lift a car would be unable to put it down again — so the ability to lose magnetism, which looks like a weakness, is the feature the application depends on.
How it is made. A long insulated copper wire is wound closely in the form of a cylinder — a solenoid — and a rod of soft iron is placed inside it. When a current is passed:
- The solenoid alone produces a magnetic field whose lines inside it are parallel and straight, so the field there is uniform, and whose outside pattern is that of a bar magnet
- One end behaves as a north pole and the other as a south pole, and the poles swap if the current is reversed
- The soft iron core becomes magnetised and greatly strengthens the field
How to identify which end is the north pole. Look at one end of the solenoid and see which way the current circulates in the nearest turn. If it is anticlockwise as you look at that face, that face is the north pole; if clockwise, it is the south pole.
Three ways to make an electromagnet stronger:
- Increase the current through the coil
- Increase the number of turns per unit length
- Use a soft iron core rather than air, and make it of a suitable shape
Why the core must be soft iron and not steel. Soft iron magnetises strongly and loses its magnetism as soon as the current stops, which is exactly what a switchable magnet needs. Steel retains its magnetism, which makes it the right material for a permanent magnet and the wrong material for an electromagnet core.
Uses of electromagnets, which is a standard list question:
- An electric bell, where the electromagnet pulls an armature to strike a gong and breaks its own circuit as it does so
- A relay, where a small current operates an electromagnet that closes a switch in a separate high-current circuit
- A loudspeaker, where a varying current in a coil in a magnetic field moves a cone to produce sound
- Electric motors and generators
- A lifting magnet on a crane, for moving scrap iron and steel in a yard
- Separating iron and steel from other scrap or from waste
- Removing iron or steel splinters from a patient's eye
- Magnetic recording and medical imaging machines, which use very strong electromagnets
The comparison with a permanent magnet, point by point, which is the most examined part of this section:
- Strength. An electromagnet can be made very strong; a permanent magnet is comparatively weak
- Can it be varied? An electromagnet's strength can be changed by changing the current or the turns; a permanent magnet's cannot
- Can it be switched off? An electromagnet can be switched off by breaking the circuit; a permanent magnet cannot
- Can the poles be reversed? An electromagnet's polarity reverses with the current; a permanent magnet's polarity is fixed
- Does it need a supply? An electromagnet needs a continuous current, so it costs energy to keep magnetised; a permanent magnet needs none
- Does it keep its magnetism? An electromagnet with a soft iron core loses its magnetism almost completely; a permanent magnet of steel retains it
Which of the two to use, in one sentence each. Use an electromagnet wherever the magnetism must be switched, varied or made very strong — a crane, a bell, a relay. Use a permanent magnet wherever a steady, supply-free field is wanted — a compass, a fridge door catch, the field magnet of a small motor.
One connection worth noticing. The crane's lifting magnet is useful precisely because it can be switched off. A permanent magnet strong enough to lift a car would be unable to put it down again — so the ability to lose magnetism, which looks like a weakness, is the feature the application depends on.
How do Fleming's two rules give the direction of a force and of an induced current?
Use the left hand when a current is placed in a field and you want the force; use the right hand when a conductor is moved in a field and you want the induced current.
Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of the left hand so that all three are mutually perpendicular.
- Forefinger — Field
- Middle finger — Current
- Thumb — Thrust, the force on the conductor
When it applies. A current-carrying conductor placed in a magnetic field experiences a force, and this rule gives its direction. The force is perpendicular to both the current and the field, which is why a rule with three perpendicular fingers is needed at all.
What the force depends on:
- It is proportional to the current
- It is proportional to the strength of the field
- It is largest when the conductor is perpendicular to the field, and it is zero when the conductor lies along the field
That zero case is worth holding on to. A wire carrying current parallel to the field feels no force at all, however large the current — because the force must be perpendicular to both, and when the two are parallel there is no such unique direction left.
The demonstration. Hang a short aluminium rod horizontally between the poles of a strong horseshoe magnet and pass a current through it. The rod is thrown sideways. Reverse the current and it goes the other way; reverse the magnet instead and again it reverses. The left-hand rule predicts each case.
Fleming's right-hand rule. Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular.
- Forefinger — Field
- Thumb — Motion of the conductor
- Middle finger — the induced Current
When it applies. A conductor moved across a magnetic field has a current induced in it, and this rule gives that current's direction.
Notice how neatly the two rules mirror each other. Left hand: field and current given, force found — the motor case. Right hand: field and motion given, current found — the generator case. Same three quantities, same three fingers, opposite hands because the cause and the effect are swapped.
Which is why the choice of hand is a real decision and not a formality. Ask what the question gives you before raising a hand:
- Given a current and asked for motion — use the left hand
- Given a motion and asked for a current — use the right hand
Getting this backwards reverses the answer, and since both answers look equally plausible on paper, it is the single most expensive error in the chapter.
Worked reasoning — the electric motor. A rectangular coil carrying current sits between the poles of a magnet.
- The two opposite arms of the coil carry current in opposite directions, since the current goes up one side and comes back down the other
- So by the left-hand rule the forces on them are in opposite directions — one arm is pushed up while the other is pushed down
- Two opposite forces on opposite sides produce a turning effect, and the coil rotates
- After half a turn the arms have swapped places, so the current through the coil must be reversed to keep it turning the same way. A split ring or commutator does exactly that
And the essential distinction to state. A motor converts electrical energy into mechanical energy; a generator converts mechanical energy into electrical energy. Both use a coil, a magnet and a rotation — the direction of the energy conversion is what tells them apart, and it is also what tells you which hand to use.
Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of the left hand so that all three are mutually perpendicular.
- Forefinger — Field
- Middle finger — Current
- Thumb — Thrust, the force on the conductor
When it applies. A current-carrying conductor placed in a magnetic field experiences a force, and this rule gives its direction. The force is perpendicular to both the current and the field, which is why a rule with three perpendicular fingers is needed at all.
What the force depends on:
- It is proportional to the current
- It is proportional to the strength of the field
- It is largest when the conductor is perpendicular to the field, and it is zero when the conductor lies along the field
That zero case is worth holding on to. A wire carrying current parallel to the field feels no force at all, however large the current — because the force must be perpendicular to both, and when the two are parallel there is no such unique direction left.
The demonstration. Hang a short aluminium rod horizontally between the poles of a strong horseshoe magnet and pass a current through it. The rod is thrown sideways. Reverse the current and it goes the other way; reverse the magnet instead and again it reverses. The left-hand rule predicts each case.
Fleming's right-hand rule. Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular.
- Forefinger — Field
- Thumb — Motion of the conductor
- Middle finger — the induced Current
When it applies. A conductor moved across a magnetic field has a current induced in it, and this rule gives that current's direction.
Notice how neatly the two rules mirror each other. Left hand: field and current given, force found — the motor case. Right hand: field and motion given, current found — the generator case. Same three quantities, same three fingers, opposite hands because the cause and the effect are swapped.
Which is why the choice of hand is a real decision and not a formality. Ask what the question gives you before raising a hand:
- Given a current and asked for motion — use the left hand
- Given a motion and asked for a current — use the right hand
Getting this backwards reverses the answer, and since both answers look equally plausible on paper, it is the single most expensive error in the chapter.
Worked reasoning — the electric motor. A rectangular coil carrying current sits between the poles of a magnet.
- The two opposite arms of the coil carry current in opposite directions, since the current goes up one side and comes back down the other
- So by the left-hand rule the forces on them are in opposite directions — one arm is pushed up while the other is pushed down
- Two opposite forces on opposite sides produce a turning effect, and the coil rotates
- After half a turn the arms have swapped places, so the current through the coil must be reversed to keep it turning the same way. A split ring or commutator does exactly that
And the essential distinction to state. A motor converts electrical energy into mechanical energy; a generator converts mechanical energy into electrical energy. Both use a coil, a magnet and a rotation — the direction of the energy conversion is what tells them apart, and it is also what tells you which hand to use.
Formula
What is the transformer relation, and why does it need alternating current?
The voltages are in the same ratio as the numbers of turns, and for an ideal transformer the power out equals the power in.
where the subscript refers to the primary coil and to the secondary.
What electromagnetic induction says, in elementary terms. Whenever the magnetic field linked with a coil changes, an emf is induced in the coil, and if the circuit is closed a current flows. The induction requires a change — a steady field, however strong, induces nothing.
Three ways to produce that change:
- Move a magnet toward or away from a coil, or move the coil relative to the magnet
- Rotate a coil in a magnetic field, which is what a generator does
- Change the current in a neighbouring coil, so that its field changes
The third is how a transformer works, and no moving parts are needed at all.
The construction. Two coils of insulated copper wire — the primary and the secondary — are wound on a laminated soft iron core. The two coils are electrically separate; nothing but the core connects them.
The working, in steps:
- An alternating voltage is applied to the primary, so an alternating current flows in it
- That current produces a continuously changing magnetic field, which the soft iron core carries almost entirely through the secondary
- The changing field linked with the secondary induces an alternating emf in it
- The ratio of the voltages equals the ratio of the turns, because each turn of either coil has the same changing field through it
Why it will not work on direct current. A steady direct current produces a steady field, and a steady field linked with the secondary induces no emf at all. So the secondary voltage would be zero except for the brief instants of switching on and off. A transformer works only with alternating current, and that is the single most important condition attached to it.
Two kinds of transformer:
- A step-up transformer has more turns in the secondary, so . The voltage rises and the current falls
- A step-down transformer has fewer turns in the secondary, so . The voltage falls and the current rises
And the current must move the opposite way to the voltage, because the power cannot increase. A transformer changes voltage; it does not create energy.
Worked example 1 — a step-down transformer. A transformer has turns in its primary and in its secondary. The primary is connected to a V alternating supply. Find the secondary voltage.
A step-down transformer, since the secondary has fewer turns.
Worked example 2 — the currents in the same transformer. If the primary draws A, find the secondary current, assuming the transformer is ideal.
Check the powers: W in, and W out. Equal, as an ideal transformer requires — and note that the voltage fell by ten while the current rose by ten.
Worked example 3 — a step-up transformer. A transformer has turns in its primary and in its secondary, with V applied to the primary. Find the secondary voltage, and the primary current if the secondary delivers A.
Check the powers: W and W. Equal. The voltage was multiplied by ten and the current divided by ten.
The energy losses in a real transformer, and how each is reduced:
- Copper loss — heating of the windings, since they have resistance. Reduced by using thick copper wire of low resistance
- Eddy-current or iron loss — currents induced in the core itself, which heat it. Reduced by using a core made of thin laminated sheets insulated from one another, which breaks up the paths available to such currents
- Hysteresis loss — energy spent in repeatedly magnetising and demagnetising the core. Reduced by using soft iron, which needs little energy for the cycle
- Flux leakage — some of the field from the primary failing to pass through the secondary. Reduced by winding the two coils one over the other on the same part of the core
Which explains the shape of a real transformer. The laminations, the soft iron, the thick windings and the coils wound over each other are each there to remove one of those four losses — and together they make a device that can be well over ninety per cent efficient with no moving parts at all.
where the subscript refers to the primary coil and to the secondary.
What electromagnetic induction says, in elementary terms. Whenever the magnetic field linked with a coil changes, an emf is induced in the coil, and if the circuit is closed a current flows. The induction requires a change — a steady field, however strong, induces nothing.
Three ways to produce that change:
- Move a magnet toward or away from a coil, or move the coil relative to the magnet
- Rotate a coil in a magnetic field, which is what a generator does
- Change the current in a neighbouring coil, so that its field changes
The third is how a transformer works, and no moving parts are needed at all.
The construction. Two coils of insulated copper wire — the primary and the secondary — are wound on a laminated soft iron core. The two coils are electrically separate; nothing but the core connects them.
The working, in steps:
- An alternating voltage is applied to the primary, so an alternating current flows in it
- That current produces a continuously changing magnetic field, which the soft iron core carries almost entirely through the secondary
- The changing field linked with the secondary induces an alternating emf in it
- The ratio of the voltages equals the ratio of the turns, because each turn of either coil has the same changing field through it
Why it will not work on direct current. A steady direct current produces a steady field, and a steady field linked with the secondary induces no emf at all. So the secondary voltage would be zero except for the brief instants of switching on and off. A transformer works only with alternating current, and that is the single most important condition attached to it.
Two kinds of transformer:
- A step-up transformer has more turns in the secondary, so . The voltage rises and the current falls
- A step-down transformer has fewer turns in the secondary, so . The voltage falls and the current rises
And the current must move the opposite way to the voltage, because the power cannot increase. A transformer changes voltage; it does not create energy.
Worked example 1 — a step-down transformer. A transformer has turns in its primary and in its secondary. The primary is connected to a V alternating supply. Find the secondary voltage.
A step-down transformer, since the secondary has fewer turns.
Worked example 2 — the currents in the same transformer. If the primary draws A, find the secondary current, assuming the transformer is ideal.
Check the powers: W in, and W out. Equal, as an ideal transformer requires — and note that the voltage fell by ten while the current rose by ten.
Worked example 3 — a step-up transformer. A transformer has turns in its primary and in its secondary, with V applied to the primary. Find the secondary voltage, and the primary current if the secondary delivers A.
Check the powers: W and W. Equal. The voltage was multiplied by ten and the current divided by ten.
The energy losses in a real transformer, and how each is reduced:
- Copper loss — heating of the windings, since they have resistance. Reduced by using thick copper wire of low resistance
- Eddy-current or iron loss — currents induced in the core itself, which heat it. Reduced by using a core made of thin laminated sheets insulated from one another, which breaks up the paths available to such currents
- Hysteresis loss — energy spent in repeatedly magnetising and demagnetising the core. Reduced by using soft iron, which needs little energy for the cycle
- Flux leakage — some of the field from the primary failing to pass through the secondary. Reduced by winding the two coils one over the other on the same part of the core
Which explains the shape of a real transformer. The laminations, the soft iron, the thick windings and the coils wound over each other are each there to remove one of those four losses — and together they make a device that can be well over ninety per cent efficient with no moving parts at all.
Exam tip
Which habits protect the marks in a magnetism answer?
Draw the field lines with arrowheads, mark the current direction, and name the rule before you use it. A field diagram without arrows earns nothing, however neat the curves.
- Draw the field of a straight wire as concentric circles, closer together near the wire, never as straight radial lines
- Use the dot and cross convention correctly — dot for out of the page, cross for into it
- Name the rule: "by the right-hand thumb rule" or "by Fleming's left-hand rule". The naming carries a mark
- Ask what the question gives you before choosing a hand — a current means the left hand, a motion means the right
- Say soft iron, not iron or steel, for an electromagnet core, and give the reason
- Give the comparison with a permanent magnet as a list of paired points, not as a paragraph
- For induction, always use the word "changing" — a steady field induces nothing
- State that a transformer works only on alternating current, and say why
- **Use ** and check with
- Check that the current moved the opposite way to the voltage in every transformer answer
The misconception to name. A transformer does not increase power. A step-up transformer raises the voltage and lowers the current by the same factor, so the power stays the same — and in a real transformer it falls slightly because of the four losses. Writing that a step-up transformer "gives more electricity" is a marked error, and the guard is the power check: the two products must agree.
A second trap. Saying that a magnetic field induces a current. **A changing magnetic field induces a current**, which is why a magnet held still inside a coil induces nothing at all and why a transformer fails on direct current. Every induction answer needs that word, and questions are set to reward it.
- Draw the field of a straight wire as concentric circles, closer together near the wire, never as straight radial lines
- Use the dot and cross convention correctly — dot for out of the page, cross for into it
- Name the rule: "by the right-hand thumb rule" or "by Fleming's left-hand rule". The naming carries a mark
- Ask what the question gives you before choosing a hand — a current means the left hand, a motion means the right
- Say soft iron, not iron or steel, for an electromagnet core, and give the reason
- Give the comparison with a permanent magnet as a list of paired points, not as a paragraph
- For induction, always use the word "changing" — a steady field induces nothing
- State that a transformer works only on alternating current, and say why
- **Use ** and check with
- Check that the current moved the opposite way to the voltage in every transformer answer
The misconception to name. A transformer does not increase power. A step-up transformer raises the voltage and lowers the current by the same factor, so the power stays the same — and in a real transformer it falls slightly because of the four losses. Writing that a step-up transformer "gives more electricity" is a marked error, and the guard is the power check: the two products must agree.
A second trap. Saying that a magnetic field induces a current. **A changing magnetic field induces a current**, which is why a magnet held still inside a coil induces nothing at all and why a transformer fails on direct current. Every induction answer needs that word, and questions are set to reward it.
Did you know
Why can the same machine be both a motor and a generator?
Set the two devices side by side and the resemblance is uncomfortable. A motor is a coil, a magnet, a shaft and a set of contacts. A generator is a coil, a magnet, a shaft and a set of contacts. The parts list is identical; only what you supply is different.
Supply electricity and you get rotation; supply rotation and you get electricity. A machine built as one can usually be used as the other — and the sign of that symmetry is the pair of Fleming rules, one hand for each direction of the conversion, with the same three quantities on the same three fingers.
You can watch the second effect happening inside a running motor. As its coil spins, it is a conductor moving in a magnetic field, so a current is induced in it as well — and that induced current opposes the one you supplied.
Which explains something you have certainly noticed. A motor draws a large current at the instant of starting, when it is not yet spinning and nothing is opposing the supply, and a much smaller current once it is up to speed. The surge when a fridge compressor or a water pump starts is that effect, and it is why such appliances sometimes dim the lights for a moment.
And it explains why the induced current always opposes. Push the north pole of a magnet toward a coil and the near face of the coil becomes a north pole, resisting the approach. Pull the magnet away and the near face becomes a south pole, resisting the departure. Either way you must do work against the opposition — and that work is exactly where the electrical energy comes from.
So a generator is not free energy, and the reason is clean. The harder the electrical load you connect, the larger the induced current, the stronger the opposition, and the harder the turbine or engine has to push to keep the coil spinning. Switch on more appliances and the power station feels it. Energy is conserved, and the opposing direction of the induced current is how conservation shows up in electromagnetism.
The same pair of rules explains a speaker and a microphone. A loudspeaker uses a current in a coil sitting in a magnetic field to push a cone back and forth — the left-hand rule making sound. A microphone lets sound push a coil back and forth in a magnetic field to produce a current — the right-hand rule listening. The two devices are nearly the same object, and an old loudspeaker will in fact work as a crude microphone.
One last observation about the transformer, which has no moving parts at all. It is the odd member of this family: energy passes from one coil to the other with nothing mechanical happening anywhere. The changing current in the primary does the job that motion does in a generator, which is why it needs alternating current and why it can be made so much more efficient than any machine with a shaft. No friction, no bearings, no wear — and that is why the electricity supply of a whole country can be stepped up and down dozens of times with very little loss.
Supply electricity and you get rotation; supply rotation and you get electricity. A machine built as one can usually be used as the other — and the sign of that symmetry is the pair of Fleming rules, one hand for each direction of the conversion, with the same three quantities on the same three fingers.
You can watch the second effect happening inside a running motor. As its coil spins, it is a conductor moving in a magnetic field, so a current is induced in it as well — and that induced current opposes the one you supplied.
Which explains something you have certainly noticed. A motor draws a large current at the instant of starting, when it is not yet spinning and nothing is opposing the supply, and a much smaller current once it is up to speed. The surge when a fridge compressor or a water pump starts is that effect, and it is why such appliances sometimes dim the lights for a moment.
And it explains why the induced current always opposes. Push the north pole of a magnet toward a coil and the near face of the coil becomes a north pole, resisting the approach. Pull the magnet away and the near face becomes a south pole, resisting the departure. Either way you must do work against the opposition — and that work is exactly where the electrical energy comes from.
So a generator is not free energy, and the reason is clean. The harder the electrical load you connect, the larger the induced current, the stronger the opposition, and the harder the turbine or engine has to push to keep the coil spinning. Switch on more appliances and the power station feels it. Energy is conserved, and the opposing direction of the induced current is how conservation shows up in electromagnetism.
The same pair of rules explains a speaker and a microphone. A loudspeaker uses a current in a coil sitting in a magnetic field to push a cone back and forth — the left-hand rule making sound. A microphone lets sound push a coil back and forth in a magnetic field to produce a current — the right-hand rule listening. The two devices are nearly the same object, and an old loudspeaker will in fact work as a crude microphone.
One last observation about the transformer, which has no moving parts at all. It is the odd member of this family: energy passes from one coil to the other with nothing mechanical happening anywhere. The changing current in the primary does the job that motion does in a generator, which is why it needs alternating current and why it can be made so much more efficient than any machine with a shaft. No friction, no bearings, no wear — and that is why the electricity supply of a whole country can be stepped up and down dozens of times with very little loss.
Exam relevance
How do magnetic effects and the transformer prepare you for JEE and NEET?
This is foundation work for Class 12 Moving Charges and Magnetism, Electromagnetic Induction and Alternating Current, all examined heavily in JEE Main, JEE Advanced and NEET Physics.
Where the right-hand thumb rule leads. Class 12 replaces your two proportionalities with the Biot-Savart law and Ampere's circuital law, giving the field of a long straight wire as proportional to with the constant supplied. Your doubling-both question has the same answer at JEE level as it does here, because the formula is exactly those two proportionalities with a constant attached. The rule itself becomes the hand geometry of a vector cross product.
Where the loop and solenoid results lead. Class 12 derives the field at the centre of a circular coil of turns and the uniform interior field of a long solenoid. **Your result that turns give times the field is the same statement, and the qualitative fact that the solenoid's interior lines are parallel is precisely the uniformity that makes the Ampere's-law derivation possible.
Where Fleming's left-hand rule leads.** It becomes the force , and the zero-force case for a conductor parallel to the field is the statement that the cross product of parallel vectors vanishes. Class 12 then derives the torque on a current loop and applies it to the moving-coil galvanometer, which is a recurring JEE Main topic.
Where induction leads. Class 12 states Faraday's laws and Lenz's law, which is the precise form of the opposition described in this chapter, and introduces magnetic flux, motional emf, self and mutual inductance. Your insistence on the word "changing" is Faraday's law before it has a formula, and Lenz's law is the induced-current-opposes statement given a sign.
Where the transformer leads. Class 12 treats it quantitatively in Alternating Current, deriving the turns ratio from Faraday's law applied to both coils and discussing the four losses by name. The numericals are identical in form to the ones here, with the addition of efficiency. **JEE Main sets transmission-loss questions that combine the transformer ratio with — which is the link back to the previous part of this chapter.
Where the motor-generator symmetry leads. The induced current opposing the supplied one becomes the back emf of a motor, and the starting-surge observation becomes a standard numerical: given a motor's resistance, supply voltage and back emf, find the current.
Question types to expect. At this level: field diagrams, the right-hand thumb rule, electromagnet comparison, the two Fleming rules, and transformer turns-ratio numericals. In competitive papers: field superposition from several conductors, torque on a loop, Lenz's-law direction problems, flux-change numericals, back emf, and transformer efficiency.
The single trap that costs marks. Using the wrong hand. Left for the force produced by a current, right for the current produced by a motion — and since the two answers are opposite, the error turns a correct method into a wrong direction with no other symptom.
A second trap. Claiming that a transformer increases power, or that it works on direct current. The power check settles the first, and the need for a changing flux settles the second. In Class 12 the same two errors appear as forgetting that the average power in an alternating circuit involves the power factor, and as applying Faraday's law where the flux is constant.
Board versus competitive emphasis. The ICSE paper marks the labelled field diagram, the named rule, the paired comparison and the substituted turns ratio; a competitive paper marks a field magnitude, a torque, an induced emf or an efficiency. The transferable habit is asking whether the flux through the circuit is changing** — because that single question decides whether there is any induced emf at all, and it is the first step of every problem in the Class 12 induction chapter.
Where the right-hand thumb rule leads. Class 12 replaces your two proportionalities with the Biot-Savart law and Ampere's circuital law, giving the field of a long straight wire as proportional to with the constant supplied. Your doubling-both question has the same answer at JEE level as it does here, because the formula is exactly those two proportionalities with a constant attached. The rule itself becomes the hand geometry of a vector cross product.
Where the loop and solenoid results lead. Class 12 derives the field at the centre of a circular coil of turns and the uniform interior field of a long solenoid. **Your result that turns give times the field is the same statement, and the qualitative fact that the solenoid's interior lines are parallel is precisely the uniformity that makes the Ampere's-law derivation possible.
Where Fleming's left-hand rule leads.** It becomes the force , and the zero-force case for a conductor parallel to the field is the statement that the cross product of parallel vectors vanishes. Class 12 then derives the torque on a current loop and applies it to the moving-coil galvanometer, which is a recurring JEE Main topic.
Where induction leads. Class 12 states Faraday's laws and Lenz's law, which is the precise form of the opposition described in this chapter, and introduces magnetic flux, motional emf, self and mutual inductance. Your insistence on the word "changing" is Faraday's law before it has a formula, and Lenz's law is the induced-current-opposes statement given a sign.
Where the transformer leads. Class 12 treats it quantitatively in Alternating Current, deriving the turns ratio from Faraday's law applied to both coils and discussing the four losses by name. The numericals are identical in form to the ones here, with the addition of efficiency. **JEE Main sets transmission-loss questions that combine the transformer ratio with — which is the link back to the previous part of this chapter.
Where the motor-generator symmetry leads. The induced current opposing the supplied one becomes the back emf of a motor, and the starting-surge observation becomes a standard numerical: given a motor's resistance, supply voltage and back emf, find the current.
Question types to expect. At this level: field diagrams, the right-hand thumb rule, electromagnet comparison, the two Fleming rules, and transformer turns-ratio numericals. In competitive papers: field superposition from several conductors, torque on a loop, Lenz's-law direction problems, flux-change numericals, back emf, and transformer efficiency.
The single trap that costs marks. Using the wrong hand. Left for the force produced by a current, right for the current produced by a motion — and since the two answers are opposite, the error turns a correct method into a wrong direction with no other symptom.
A second trap. Claiming that a transformer increases power, or that it works on direct current. The power check settles the first, and the need for a changing flux settles the second. In Class 12 the same two errors appear as forgetting that the average power in an alternating circuit involves the power factor, and as applying Faraday's law where the flux is constant.
Board versus competitive emphasis. The ICSE paper marks the labelled field diagram, the named rule, the paired comparison and the substituted turns ratio; a competitive paper marks a field magnitude, a torque, an induced emf or an efficiency. The transferable habit is asking whether the flux through the circuit is changing** — because that single question decides whether there is any induced emf at all, and it is the first step of every problem in the Class 12 induction chapter.
Key takeaways
What must you be able to do from this part?
One rule for fields, two rules for directions, and one transformer relation.
- A current produces a magnetic field: a compass needle under a wire deflects when the current starts, reverses when the current reverses, deflects more for a larger current and less further away
- Right-hand thumb rule: thumb along the current, curled fingers give the field direction
- Around a straight wire the field lines are concentric circles, closer together near the wire; the field is proportional to the current and inversely proportional to the distance
- Doubling both the current and the distance leaves the field unchanged
- Dot means out of the page and gives anticlockwise circles; cross means into the page and gives clockwise ones
- At the centre of a loop the field is nearly straight and perpendicular to its plane, and turns give times the field
- A straight wire has no poles — its field lines are closed circles
- An electromagnet is a soft iron core in a solenoid, strengthened by more current, more turns per unit length or a core
- Inside a solenoid the field is uniform, and outside it matches a bar magnet's, with poles that reverse with the current
- Uses: electric bell, relay, loudspeaker, motors and generators, lifting magnet on a crane, separating iron from scrap, removing splinters from an eye, and imaging machines
- Against a permanent magnet, an electromagnet can be made stronger, varied, switched off and reversed, but needs a continuous current and loses its magnetism
- Fleming's left-hand rule — forefinger Field, middle finger Current, thumb Thrust — gives the force on a current in a field, the motor case
- The force is zero when the conductor lies along the field
- Fleming's right-hand rule — forefinger Field, thumb Motion, middle finger Current — gives the induced current, the generator case
- In a motor, opposite arms carry current in opposite directions, so the forces are opposite and the coil turns; a split ring reverses the current every half turn
- A motor converts electrical energy to mechanical; a generator converts mechanical to electrical
- **Induction needs a changing field, produced by moving a magnet or coil, rotating a coil, or changing the current in a neighbouring coil
- A transformer has primary and secondary coils on a laminated soft iron core, and with for an ideal one
- to turns on V gives V**, and a primary current of A gives a secondary current of A
- ** to turns on V gives V**, and a secondary current of A needs a primary current of A
- A transformer works only on alternating current, because a steady field induces nothing
- The four losses — copper, eddy-current, hysteresis and flux leakage — are reduced by thick wire, a laminated core, soft iron, and coils wound one over the other
The quickest self-test needs a hand and a diagram. Draw a wire between two magnet poles, pick a current direction, and predict which way it jumps; then draw a magnet entering a coil and predict which way the galvanometer swings — and check whether you reached for the correct hand both times.
- A current produces a magnetic field: a compass needle under a wire deflects when the current starts, reverses when the current reverses, deflects more for a larger current and less further away
- Right-hand thumb rule: thumb along the current, curled fingers give the field direction
- Around a straight wire the field lines are concentric circles, closer together near the wire; the field is proportional to the current and inversely proportional to the distance
- Doubling both the current and the distance leaves the field unchanged
- Dot means out of the page and gives anticlockwise circles; cross means into the page and gives clockwise ones
- At the centre of a loop the field is nearly straight and perpendicular to its plane, and turns give times the field
- A straight wire has no poles — its field lines are closed circles
- An electromagnet is a soft iron core in a solenoid, strengthened by more current, more turns per unit length or a core
- Inside a solenoid the field is uniform, and outside it matches a bar magnet's, with poles that reverse with the current
- Uses: electric bell, relay, loudspeaker, motors and generators, lifting magnet on a crane, separating iron from scrap, removing splinters from an eye, and imaging machines
- Against a permanent magnet, an electromagnet can be made stronger, varied, switched off and reversed, but needs a continuous current and loses its magnetism
- Fleming's left-hand rule — forefinger Field, middle finger Current, thumb Thrust — gives the force on a current in a field, the motor case
- The force is zero when the conductor lies along the field
- Fleming's right-hand rule — forefinger Field, thumb Motion, middle finger Current — gives the induced current, the generator case
- In a motor, opposite arms carry current in opposite directions, so the forces are opposite and the coil turns; a split ring reverses the current every half turn
- A motor converts electrical energy to mechanical; a generator converts mechanical to electrical
- **Induction needs a changing field, produced by moving a magnet or coil, rotating a coil, or changing the current in a neighbouring coil
- A transformer has primary and secondary coils on a laminated soft iron core, and with for an ideal one
- to turns on V gives V**, and a primary current of A gives a secondary current of A
- ** to turns on V gives V**, and a secondary current of A needs a primary current of A
- A transformer works only on alternating current, because a steady field induces nothing
- The four losses — copper, eddy-current, hysteresis and flux leakage — are reduced by thick wire, a laminated core, soft iron, and coils wound one over the other
The quickest self-test needs a hand and a diagram. Draw a wire between two magnet poles, pick a current direction, and predict which way it jumps; then draw a magnet entering a coil and predict which way the galvanometer swings — and check whether you reached for the correct hand both times.