A Compass Near a Wire Swings the Moment the Current Starts
Follow the field lines around a bar magnet and a current-carrying wire, use the right-hand thumb rule to get the direction without guessing, and compare the field of a straight wire, a circular loop and a solenoid.
How can an electric current behave like a magnet?
Place a compass needle below a straight wire, connect the wire to a cell, and the needle swings aside the instant the current begins. Break the circuit and it returns. Reverse the cell and it swings the other way.
That one observation joins two subjects that look unrelated. A current-carrying conductor produces a magnetic field around it, and the field is real enough to deflect a compass — which is, after all, just a small magnet free to turn.
Everything in this chapter follows from that. Once a current can produce a field:
- A wire wound into a coil can produce a field as strong and as neatly shaped as a bar magnet's, and can be switched off
- A current-carrying wire placed in a field experiences a force, which is how a motor turns
- Moving a magnet near a coil produces a current, which is how electricity is generated
This first part builds the tools. You need to be able to draw field lines correctly, and you need a rule that gives the direction of the field without guessing — the right-hand thumb rule. Direction questions carry as many marks as anything else in the chapter, and they cannot be answered by intuition.
A magnetic field is a vector quantity: it has both a magnitude and a direction at every point, which is exactly why the field-line picture is used. Its SI unit is the tesla.
This page covers the first part of the CBSE Class 10 Science chapter on magnetic effects of electric current: magnetic field lines, the field around a straight conductor, the right-hand thumb rule, and the fields of a loop and a solenoid.
That one observation joins two subjects that look unrelated. A current-carrying conductor produces a magnetic field around it, and the field is real enough to deflect a compass — which is, after all, just a small magnet free to turn.
Everything in this chapter follows from that. Once a current can produce a field:
- A wire wound into a coil can produce a field as strong and as neatly shaped as a bar magnet's, and can be switched off
- A current-carrying wire placed in a field experiences a force, which is how a motor turns
- Moving a magnet near a coil produces a current, which is how electricity is generated
This first part builds the tools. You need to be able to draw field lines correctly, and you need a rule that gives the direction of the field without guessing — the right-hand thumb rule. Direction questions carry as many marks as anything else in the chapter, and they cannot be answered by intuition.
A magnetic field is a vector quantity: it has both a magnitude and a direction at every point, which is exactly why the field-line picture is used. Its SI unit is the tesla.
This page covers the first part of the CBSE Class 10 Science chapter on magnetic effects of electric current: magnetic field lines, the field around a straight conductor, the right-hand thumb rule, and the fields of a loop and a solenoid.
What are magnetic field lines and what rules do they obey?
A magnetic field line is the path a free north pole would follow in the field, and the pattern of lines is a picture of both the direction and the strength of the field.
Around a bar magnet the lines emerge from the north pole, curve around through the space outside, and enter the south pole — and inside the magnet they run from south to north, so every line forms a closed loop.
The four properties, each of which is an examined statement.
- The direction of the field at a point is the direction of the tangent to the field line there, which is also the direction a compass needle would point
- Field lines are closed and continuous curves — they never begin or end anywhere
- The lines are crowded where the field is strong and spread out where it is weak, which is why they bunch at the poles
- No two field lines ever intersect
Why lines can never cross, and this is the reason to give. If two lines crossed, then at the crossing point the compass needle would have to point in two different directions at the same time, which is impossible. A field has one definite direction at each point.
How to draw the lines around a bar magnet. Place the magnet on paper, put a small compass near it, mark the two ends of the needle, move the compass so its tail sits where the head was, and repeat. Joining the marks traces one field line. Iron filings sprinkled around the magnet show the same pattern at once, because each filing becomes a tiny magnet and lines up along the field.
Where the field is strongest and weakest.
- Strongest at the poles, where the lines are closest together
- Weakest near the middle of the side face, where they are furthest apart
- At a neutral point the fields of the magnet and of some other source cancel, and the resultant field is zero
One point worth being careful about. The lines are a model, not physical threads. They are drawn to record two things at once — direction through their tangent and strength through their spacing — and the fact that they never cross is a consequence of the field having a single direction everywhere.
Around a bar magnet the lines emerge from the north pole, curve around through the space outside, and enter the south pole — and inside the magnet they run from south to north, so every line forms a closed loop.
The four properties, each of which is an examined statement.
- The direction of the field at a point is the direction of the tangent to the field line there, which is also the direction a compass needle would point
- Field lines are closed and continuous curves — they never begin or end anywhere
- The lines are crowded where the field is strong and spread out where it is weak, which is why they bunch at the poles
- No two field lines ever intersect
Why lines can never cross, and this is the reason to give. If two lines crossed, then at the crossing point the compass needle would have to point in two different directions at the same time, which is impossible. A field has one definite direction at each point.
How to draw the lines around a bar magnet. Place the magnet on paper, put a small compass near it, mark the two ends of the needle, move the compass so its tail sits where the head was, and repeat. Joining the marks traces one field line. Iron filings sprinkled around the magnet show the same pattern at once, because each filing becomes a tiny magnet and lines up along the field.
Where the field is strongest and weakest.
- Strongest at the poles, where the lines are closest together
- Weakest near the middle of the side face, where they are furthest apart
- At a neutral point the fields of the magnet and of some other source cancel, and the resultant field is zero
One point worth being careful about. The lines are a model, not physical threads. They are drawn to record two things at once — direction through their tangent and strength through their spacing — and the fact that they never cross is a consequence of the field having a single direction everywhere.
How do you find the direction of the field around a straight wire?
Use the right-hand thumb rule. Hold the wire in your right hand with the thumb pointing along the current; your curled fingers give the direction in which the field lines circle the wire.
The shape of the field first. Around a long straight current-carrying conductor the field lines are concentric circles lying 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.
How strong the field is. Two proportionalities cover every qualitative question:
- The field is directly proportional to the current, so doubling the current doubles the field
- The field is inversely proportional to the distance from the wire, so the circles further out represent a weaker field
That is why the concentric circles are drawn close together near the wire and further apart as you move away — the spacing is carrying information.
Worked reasoning — doubling both. Suppose the current in a wire is doubled and at the same time the point of observation is moved to twice its distance from the wire. What happens to the field?
Doubling the current doubles the field; doubling the distance halves it. The two changes cancel exactly, and the field is unchanged. This is a favourite one-line question, and it is answered purely from the two proportionalities.
Using the rule in the two standard cases.
- Current flowing upward in a vertical wire: point the right thumb up, and the fingers curl anticlockwise when seen from above
- Current flowing downward: the circles are clockwise seen from above
- Reverse the current and every field line reverses direction, which is why the compass needle swings the other way when the cell is reversed
The notation for current going into or out of the page. A dot at the centre means the current is coming out of the page, and a cross means it is going into the page. With the current out of the page the field circles are anticlockwise; with it into the page they are clockwise. Getting this convention right is often the whole of a direction question.
One consequence worth stating. Because the field lines are circles, the field at a point directly above the wire and the field directly below it point in opposite directions. That is why a compass placed below the wire deflects one way and a compass above it deflects the other — a standard experimental observation to explain.
The shape of the field first. Around a long straight current-carrying conductor the field lines are concentric circles lying 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.
How strong the field is. Two proportionalities cover every qualitative question:
- The field is directly proportional to the current, so doubling the current doubles the field
- The field is inversely proportional to the distance from the wire, so the circles further out represent a weaker field
That is why the concentric circles are drawn close together near the wire and further apart as you move away — the spacing is carrying information.
Worked reasoning — doubling both. Suppose the current in a wire is doubled and at the same time the point of observation is moved to twice its distance from the wire. What happens to the field?
Doubling the current doubles the field; doubling the distance halves it. The two changes cancel exactly, and the field is unchanged. This is a favourite one-line question, and it is answered purely from the two proportionalities.
Using the rule in the two standard cases.
- Current flowing upward in a vertical wire: point the right thumb up, and the fingers curl anticlockwise when seen from above
- Current flowing downward: the circles are clockwise seen from above
- Reverse the current and every field line reverses direction, which is why the compass needle swings the other way when the cell is reversed
The notation for current going into or out of the page. A dot at the centre means the current is coming out of the page, and a cross means it is going into the page. With the current out of the page the field circles are anticlockwise; with it into the page they are clockwise. Getting this convention right is often the whole of a direction question.
One consequence worth stating. Because the field lines are circles, the field at a point directly above the wire and the field directly below it point in opposite directions. That is why a compass placed below the wire deflects one way and a compass above it deflects the other — a standard experimental observation to explain.
How does bending the wire into a loop or a coil change the field?
It concentrates the field. A loop adds the contributions of every part of the wire at its centre, and a solenoid produces a field as uniform and as well shaped as a bar magnet's.
A circular loop. Every small section of the loop produces circular field lines around itself, and near the centre of the loop all those contributions point the same way, adding up. So:
- The field lines are circles near the wire and become straighter as you move toward the centre
- At the centre of the loop the field lines are very nearly straight and perpendicular to the plane of the loop
- The field at the centre is proportional to the current and, for the same current, larger for a smaller loop
Turns multiply the field. If the loop has turns instead of one, the field at the centre becomes times as large, because the current passes the same point times and each turn contributes in the same direction. That single idea is why coils are wound with many turns.
A solenoid. A long coil of many circular turns wound closely in the shape of a cylinder is a solenoid, and its field pattern is the important result of this chapter:
- Inside the solenoid the field lines are parallel straight lines, which means the field there is uniform — the same magnitude and direction at every interior point
- Outside, the pattern is exactly that of a bar magnet, with the lines emerging from one end and entering the other
- One end behaves as a north pole and the other as a south pole, and the poles swap if the current is reversed
How to identify which end is the north pole. Look at the end of the solenoid and see the direction in which 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. The right-hand thumb rule applied to one turn gives the same answer.
The electromagnet. Place a soft iron rod inside a solenoid and switch on the current, and the rod becomes a strong magnet — an electromagnet. Three properties make it useful where a permanent magnet is not:
- It can be switched on and off simply by making or breaking the circuit
- Its strength can be changed by changing the current or the number of turns
- Its polarity can be reversed by reversing the current
Soft iron is chosen for the core, 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 keeps its magnetism, which makes it the right material for a permanent magnet and the wrong material for an electromagnet core.
Three ways to make a solenoid's field stronger, and all three should be named when asked:
- Increase the current
- Increase the number of turns per unit length
- Insert a soft iron core
A circular loop. Every small section of the loop produces circular field lines around itself, and near the centre of the loop all those contributions point the same way, adding up. So:
- The field lines are circles near the wire and become straighter as you move toward the centre
- At the centre of the loop the field lines are very nearly straight and perpendicular to the plane of the loop
- The field at the centre is proportional to the current and, for the same current, larger for a smaller loop
Turns multiply the field. If the loop has turns instead of one, the field at the centre becomes times as large, because the current passes the same point times and each turn contributes in the same direction. That single idea is why coils are wound with many turns.
A solenoid. A long coil of many circular turns wound closely in the shape of a cylinder is a solenoid, and its field pattern is the important result of this chapter:
- Inside the solenoid the field lines are parallel straight lines, which means the field there is uniform — the same magnitude and direction at every interior point
- Outside, the pattern is exactly that of a bar magnet, with the lines emerging from one end and entering the other
- One end behaves as a north pole and the other as a south pole, and the poles swap if the current is reversed
How to identify which end is the north pole. Look at the end of the solenoid and see the direction in which 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. The right-hand thumb rule applied to one turn gives the same answer.
The electromagnet. Place a soft iron rod inside a solenoid and switch on the current, and the rod becomes a strong magnet — an electromagnet. Three properties make it useful where a permanent magnet is not:
- It can be switched on and off simply by making or breaking the circuit
- Its strength can be changed by changing the current or the number of turns
- Its polarity can be reversed by reversing the current
Soft iron is chosen for the core, 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 keeps its magnetism, which makes it the right material for a permanent magnet and the wrong material for an electromagnet core.
Three ways to make a solenoid's field stronger, and all three should be named when asked:
- Increase the current
- Increase the number of turns per unit length
- Insert a soft iron core
Exam tip
What does an examiner want in a magnetic field diagram?
Arrows on the lines, the current direction marked, and lines that never cross. A field diagram without arrows is an unmarked diagram, however neat the curves are.
- Draw the field of a straight wire as concentric circles, closer together near the wire, never as straight radial lines
- Put an arrowhead on every line and mark the current direction on the wire
- State the rule by name — "by the right-hand thumb rule" — before giving a direction. The naming carries a mark
- Use the dot and cross convention correctly: dot means out of the page, cross means into the page
- Inside a solenoid the lines are parallel and straight; drawing them curved loses the uniform-field point
- Label the poles of a solenoid and be ready to justify the labelling from the current direction
- Never let two lines intersect, and know the reason: a compass cannot point two ways at once
- Say soft iron, not iron or steel, for an electromagnet core
The misconception to name. Field lines are not stretched threads pulling on the magnet, and they are not something flowing outward from the wire. They are a way of recording a direction and a strength at every point, and their two conventions — tangent for direction, spacing for strength — are the entire content of the picture.
A second trap. A straight wire does not have a north and a south pole. Poles belong to the solenoid and the bar magnet, whose field lines leave one region and enter another; the circular lines around a straight wire close on themselves without any pole at all.
- Draw the field of a straight wire as concentric circles, closer together near the wire, never as straight radial lines
- Put an arrowhead on every line and mark the current direction on the wire
- State the rule by name — "by the right-hand thumb rule" — before giving a direction. The naming carries a mark
- Use the dot and cross convention correctly: dot means out of the page, cross means into the page
- Inside a solenoid the lines are parallel and straight; drawing them curved loses the uniform-field point
- Label the poles of a solenoid and be ready to justify the labelling from the current direction
- Never let two lines intersect, and know the reason: a compass cannot point two ways at once
- Say soft iron, not iron or steel, for an electromagnet core
The misconception to name. Field lines are not stretched threads pulling on the magnet, and they are not something flowing outward from the wire. They are a way of recording a direction and a strength at every point, and their two conventions — tangent for direction, spacing for strength — are the entire content of the picture.
A second trap. A straight wire does not have a north and a south pole. Poles belong to the solenoid and the bar magnet, whose field lines leave one region and enter another; the circular lines around a straight wire close on themselves without any pole at all.
Did you know
Why does the compass needle itself point north?
Because the Earth behaves as though it has an enormous magnet inside it, and the compass needle is simply a small magnet lining up with that field.
That immediately raises a question worth thinking through. If unlike poles attract, and the north pole of the needle swings toward the Earth's geographic north, then the magnetic pole lying near the geographic north must behave as a south pole. The naming is a historical convention about the needle, not a statement about the Earth's own polarity.
A second observation follows. A compass needle is not perfectly horizontal everywhere. Near the equator it lies almost flat, and as you travel toward the poles the field acquires a steeper downward component and the needle dips. The field is three-dimensional, and a flat compass only shows you its horizontal part.
And the Earth's field is why the wire experiment has to be set up carefully. The compass under the wire is already responding to the Earth's field, pointing north. Switch on the current and the needle settles along the resultant of the Earth's field and the wire's field, not along the wire's field alone. That is why a large current and a short distance are used — to make the wire's contribution clearly dominate.
It also explains the neutral-point idea. Place a bar magnet on a table with its north pole pointing geographic north, and at certain points on either side the magnet's field and the Earth's horizontal field are equal and opposite. A compass at those points has no preferred direction at all — the resultant field there is zero. Iron filings show them as gaps in the pattern.
One last connection to the solenoid. The reason the solenoid's outside pattern matches a bar magnet's so exactly is that both fields come from the same underlying cause: circulating charge. In the solenoid the circulation is the current you supplied; in a permanent magnet it is the motion of electrons within the atoms of the material. A magnet is, at bottom, current that you cannot switch off — which is the single idea that unifies the whole chapter.
That immediately raises a question worth thinking through. If unlike poles attract, and the north pole of the needle swings toward the Earth's geographic north, then the magnetic pole lying near the geographic north must behave as a south pole. The naming is a historical convention about the needle, not a statement about the Earth's own polarity.
A second observation follows. A compass needle is not perfectly horizontal everywhere. Near the equator it lies almost flat, and as you travel toward the poles the field acquires a steeper downward component and the needle dips. The field is three-dimensional, and a flat compass only shows you its horizontal part.
And the Earth's field is why the wire experiment has to be set up carefully. The compass under the wire is already responding to the Earth's field, pointing north. Switch on the current and the needle settles along the resultant of the Earth's field and the wire's field, not along the wire's field alone. That is why a large current and a short distance are used — to make the wire's contribution clearly dominate.
It also explains the neutral-point idea. Place a bar magnet on a table with its north pole pointing geographic north, and at certain points on either side the magnet's field and the Earth's horizontal field are equal and opposite. A compass at those points has no preferred direction at all — the resultant field there is zero. Iron filings show them as gaps in the pattern.
One last connection to the solenoid. The reason the solenoid's outside pattern matches a bar magnet's so exactly is that both fields come from the same underlying cause: circulating charge. In the solenoid the circulation is the current you supplied; in a permanent magnet it is the motion of electrons within the atoms of the material. A magnet is, at bottom, current that you cannot switch off — which is the single idea that unifies the whole chapter.
Exam relevance
How does magnetism at this level prepare you for JEE and NEET?
This is foundation work for Class 12 Moving Charges and Magnetism and Magnetism and Matter, examined heavily in JEE Main, JEE Advanced and NEET.
Where the straight-wire result leads. Class 12 replaces your two proportionalities with a formula: the field at a distance from a long straight wire carrying current is proportional to , with the constant supplied by the Biot–Savart law and by Ampere's circuital law. The proportionalities you use here are exactly that formula without its constant, which is why the doubling-both question at Class 10 and the same question at JEE have the same answer.
Where the loop result leads. Class 12 derives the field at the centre of a circular coil of turns and on its axis, again from the Biot–Savart law. **Your result that turns give times the field is the same statement, and the qualitative fact that a smaller loop gives a larger central field is what the formula makes quantitative.
Where the solenoid leads. Class 12 derives the uniform interior field of a long solenoid from Ampere's law, and extends the same reasoning to the toroid. The Class 10 observation that the interior lines are parallel is precisely the uniformity that makes that derivation possible.
Where the right-hand thumb rule leads. It becomes the general right-hand rule for vector cross products, used for the force on a moving charge and the torque on a current loop. The hand geometry you learn here does not change — only the notation does.
Where the Earth's field leads. Class 12 treats declination, dip and the horizontal component, and the neutral-point idea becomes a calculation. NEET asks the Earth's-field questions more often than JEE does.
Question types to expect. At this level: field patterns, direction by the right-hand thumb rule, why lines never cross, solenoid polarity, and electromagnet properties. In competitive papers: superposition of fields from several wires, field at the centre of arcs and loops, and direction questions in three dimensions.
The single trap that costs marks. Drawing the field of a straight wire as radial straight lines instead of concentric circles. The circles are the whole result, and the error survives all the way into Class 12 superposition problems, where the direction of each contribution must be tangential.
A second trap. Assuming the field lines point away from a current-carrying wire the way an electric field points away from a charge. Magnetic field lines close on themselves — there is no magnetic charge for them to start from, which is why every line is a closed loop.
Board versus competitive emphasis. The CBSE paper marks the diagram, the named rule and the stated properties; a competitive paper marks the resultant direction when several sources act together. The transferable habit is finding the direction of each contribution separately with the right hand before combining anything** — the same discipline, applied more times.
Where the straight-wire result leads. Class 12 replaces your two proportionalities with a formula: the field at a distance from a long straight wire carrying current is proportional to , with the constant supplied by the Biot–Savart law and by Ampere's circuital law. The proportionalities you use here are exactly that formula without its constant, which is why the doubling-both question at Class 10 and the same question at JEE have the same answer.
Where the loop result leads. Class 12 derives the field at the centre of a circular coil of turns and on its axis, again from the Biot–Savart law. **Your result that turns give times the field is the same statement, and the qualitative fact that a smaller loop gives a larger central field is what the formula makes quantitative.
Where the solenoid leads. Class 12 derives the uniform interior field of a long solenoid from Ampere's law, and extends the same reasoning to the toroid. The Class 10 observation that the interior lines are parallel is precisely the uniformity that makes that derivation possible.
Where the right-hand thumb rule leads. It becomes the general right-hand rule for vector cross products, used for the force on a moving charge and the torque on a current loop. The hand geometry you learn here does not change — only the notation does.
Where the Earth's field leads. Class 12 treats declination, dip and the horizontal component, and the neutral-point idea becomes a calculation. NEET asks the Earth's-field questions more often than JEE does.
Question types to expect. At this level: field patterns, direction by the right-hand thumb rule, why lines never cross, solenoid polarity, and electromagnet properties. In competitive papers: superposition of fields from several wires, field at the centre of arcs and loops, and direction questions in three dimensions.
The single trap that costs marks. Drawing the field of a straight wire as radial straight lines instead of concentric circles. The circles are the whole result, and the error survives all the way into Class 12 superposition problems, where the direction of each contribution must be tangential.
A second trap. Assuming the field lines point away from a current-carrying wire the way an electric field points away from a charge. Magnetic field lines close on themselves — there is no magnetic charge for them to start from, which is why every line is a closed loop.
Board versus competitive emphasis. The CBSE paper marks the diagram, the named rule and the stated properties; a competitive paper marks the resultant direction when several sources act together. The transferable habit is finding the direction of each contribution separately with the right hand before combining anything** — the same discipline, applied more times.
Key takeaways
What should you be able to draw and state from this part?
One rule, three field patterns and four properties of field lines.
- A current-carrying conductor produces a magnetic field around it, which is why a compass near a wire deflects when the current starts
- Magnetic field lines are closed continuous curves; the tangent gives the direction, the spacing gives the strength, and no two ever cross because a compass cannot point two ways at once
- Around a bar magnet the lines run from north to south outside and south to north inside, and are most crowded at the poles
- Around a straight wire the lines are concentric circles in a plane perpendicular to the wire
- Right-hand thumb rule: thumb along the current, curled fingers give the direction of the field lines
- **Field current and field , so doubling both leaves the field unchanged
- Dot means current out of the page and gives anticlockwise circles; cross means into the page and gives clockwise circles
- At the centre of a circular loop** the field is nearly straight and perpendicular to the plane of the loop, and turns give times the field
- Inside a solenoid the field is uniform, and outside the pattern is that of a bar magnet, with one end a north pole
- An electromagnet uses a soft iron core so the magnetism disappears when the current stops; strengthen it with more current, more turns per unit length, or a soft iron core
- Magnetic field is a vector, measured in tesla
The quickest way to know this part is secure is to draw it from memory. Sketch a vertical wire with the current going up, put in the field circles with arrows, then sketch a solenoid, mark the current direction in the near turn, and label which end is north — and check both answers with your right hand, not your memory.
- A current-carrying conductor produces a magnetic field around it, which is why a compass near a wire deflects when the current starts
- Magnetic field lines are closed continuous curves; the tangent gives the direction, the spacing gives the strength, and no two ever cross because a compass cannot point two ways at once
- Around a bar magnet the lines run from north to south outside and south to north inside, and are most crowded at the poles
- Around a straight wire the lines are concentric circles in a plane perpendicular to the wire
- Right-hand thumb rule: thumb along the current, curled fingers give the direction of the field lines
- **Field current and field , so doubling both leaves the field unchanged
- Dot means current out of the page and gives anticlockwise circles; cross means into the page and gives clockwise circles
- At the centre of a circular loop** the field is nearly straight and perpendicular to the plane of the loop, and turns give times the field
- Inside a solenoid the field is uniform, and outside the pattern is that of a bar magnet, with one end a north pole
- An electromagnet uses a soft iron core so the magnetism disappears when the current stops; strengthen it with more current, more turns per unit length, or a soft iron core
- Magnetic field is a vector, measured in tesla
The quickest way to know this part is secure is to draw it from memory. Sketch a vertical wire with the current going up, put in the field circles with arrows, then sketch a solenoid, mark the current direction in the near turn, and label which end is north — and check both answers with your right hand, not your memory.