A Plain Copper Wire Can Move a Compass Needle
Learn the experiment that shows a current has a magnetic effect, predict how the needle moves when the current changes, build an electromagnet and control its strength, and see why devices prefer it to a permanent magnet.
Can an ordinary wire behave like a magnet?
Only while a current flows through it. Place a compass under a wire and nothing happens — close the circuit and the needle swings aside at once.
That single observation links electricity to magnetism, and everything else in this chapter follows from it. This page covers everything in the CBSE Class 8 Science chapter's first part: the compass needle experiment, predicting the deflection, making an electromagnet, and its uses.
That single observation links electricity to magnetism, and everything else in this chapter follows from it. This page covers everything in the CBSE Class 8 Science chapter's first part: the compass needle experiment, predicting the deflection, making an electromagnet, and its uses.
What experiment shows that a current has a magnetic effect?
A straight wire is placed over a compass, lying parallel to the needle, and connected through a switch to a cell.
The steps:
1. Let the compass settle. Its needle points north-south, as it always does.
2. Arrange the wire above the needle, along the same north-south direction.
3. Close the switch so current flows.
4. Watch the needle.
The result: the needle deflects to one side. Open the switch and it swings back to north-south.
The conclusion is that a current-carrying wire produces a magnetic field around itself — the wire behaves like a magnet, which is why a magnetic needle responds to it.
The apparatus needed is only a cell, a switch, connecting wires and a compass, which is why this is a standard classroom activity.
The wire must lie parallel to the needle at the start, and that detail is what makes the experiment work. If the wire is placed at right angles to the needle, the needle is already pointing the way the field would push it, so there is little visible movement — and the experiment appears to fail when the physics is perfectly fine.
The steps:
1. Let the compass settle. Its needle points north-south, as it always does.
2. Arrange the wire above the needle, along the same north-south direction.
3. Close the switch so current flows.
4. Watch the needle.
The result: the needle deflects to one side. Open the switch and it swings back to north-south.
The conclusion is that a current-carrying wire produces a magnetic field around itself — the wire behaves like a magnet, which is why a magnetic needle responds to it.
The apparatus needed is only a cell, a switch, connecting wires and a compass, which is why this is a standard classroom activity.
The wire must lie parallel to the needle at the start, and that detail is what makes the experiment work. If the wire is placed at right angles to the needle, the needle is already pointing the way the field would push it, so there is little visible movement — and the experiment appears to fail when the physics is perfectly fine.
How does the deflection change when you alter the current?
Three changes give three predictable results, and together they prove the deflection is caused by the current itself.
- Increase the current — by using two cells instead of one — and the deflection becomes larger. A bigger current makes a stronger magnetic field.
- Reverse the current — by swapping the cell's connections — and the needle deflects to the opposite side. The field's direction depends on the current's direction.
- Switch the current off — and the needle returns to north-south. No current, no magnetic field.
A fourth observation completes the picture: moving the compass to the other side of the wire also reverses the deflection, because the magnetic field forms circles around the wire rather than pointing one fixed way.
So the field is not a property the wire keeps. It exists only while charge is moving, and it vanishes the instant the circuit is broken.
The reversal test is the most convincing of the three, and it is the one examiners ask for. A needle that merely moved could be reacting to something else in the room — but a needle that flips to the opposite side when the cell is turned around can only be responding to the direction of the current.
- Increase the current — by using two cells instead of one — and the deflection becomes larger. A bigger current makes a stronger magnetic field.
- Reverse the current — by swapping the cell's connections — and the needle deflects to the opposite side. The field's direction depends on the current's direction.
- Switch the current off — and the needle returns to north-south. No current, no magnetic field.
A fourth observation completes the picture: moving the compass to the other side of the wire also reverses the deflection, because the magnetic field forms circles around the wire rather than pointing one fixed way.
So the field is not a property the wire keeps. It exists only while charge is moving, and it vanishes the instant the circuit is broken.
The reversal test is the most convincing of the three, and it is the one examiners ask for. A needle that merely moved could be reacting to something else in the room — but a needle that flips to the opposite side when the cell is turned around can only be responding to the direction of the current.
How do you make an electromagnet and control its strength?
Wind insulated copper wire in many close turns around an iron nail or rod, and connect the ends through a switch to a battery. While current flows the nail picks up pins; break the circuit and they drop off.
The wire must be insulated so that the current travels the whole length of the winding instead of short-circuiting between touching turns.
Three factors decide the strength:
- Number of turns — more turns give a stronger electromagnet.
- Current — a larger current, from more cells, gives a stronger electromagnet.
- Nature of the core — a soft iron core gives a much stronger magnet than a core of wood, plastic or air.
Testing it is straightforward: count how many pins the nail lifts with 50 turns, then with 100 turns, then with two cells instead of one. The pin count rises each time.
An electromagnet has a north and a south pole like any magnet, and reversing the current swaps them.
The reason the core is soft iron rather than steel matters, and it decides the whole design. Soft iron magnetises strongly but loses almost all its magnetism the moment the current stops — which is exactly what a switchable magnet needs. Steel keeps its magnetism, making it right for permanent magnets and wrong here.
The wire must be insulated so that the current travels the whole length of the winding instead of short-circuiting between touching turns.
Three factors decide the strength:
- Number of turns — more turns give a stronger electromagnet.
- Current — a larger current, from more cells, gives a stronger electromagnet.
- Nature of the core — a soft iron core gives a much stronger magnet than a core of wood, plastic or air.
Testing it is straightforward: count how many pins the nail lifts with 50 turns, then with 100 turns, then with two cells instead of one. The pin count rises each time.
An electromagnet has a north and a south pole like any magnet, and reversing the current swaps them.
The reason the core is soft iron rather than steel matters, and it decides the whole design. Soft iron magnetises strongly but loses almost all its magnetism the moment the current stops — which is exactly what a switchable magnet needs. Steel keeps its magnetism, making it right for permanent magnets and wrong here.
Why do devices use an electromagnet rather than a permanent magnet?
Because an electromagnet can be switched on and off, and a permanent magnet cannot.
Its uses follow from that one advantage:
- Electric bell — the electromagnet attracts an iron armature so a hammer strikes the gong. The armature's movement breaks the circuit, the magnetism dies, a spring pulls it back and remakes the contact. That self-interrupting cycle repeats many times a second, which is why the bell rings continuously instead of giving one clang.
- Cranes in scrap yards — a huge electromagnet lifts a load of iron, swings it across and drops the whole load simply by cutting the current. No hand ever touches the scrap.
- Electric motors — found in fans, mixers, pumps and washing machines, where electromagnets whose poles keep reversing produce continuous rotation.
- Loudspeakers, telephone earpieces and magnetic separators for removing iron from a mixture.
Comparing the two:
- Electromagnet — can be switched on and off, its strength can be changed by altering turns or current, it can be made very strong, and its poles reverse with the current.
- Permanent magnet — needs no current, but has a fixed and usually weaker strength and fixed poles.
The bell shows why the choice is not merely convenient but necessary. Built with a permanent magnet, the armature would be pulled in once and simply stay there — no interruption, no repeat, no ringing. The device depends on the magnetism being able to disappear.
Its uses follow from that one advantage:
- Electric bell — the electromagnet attracts an iron armature so a hammer strikes the gong. The armature's movement breaks the circuit, the magnetism dies, a spring pulls it back and remakes the contact. That self-interrupting cycle repeats many times a second, which is why the bell rings continuously instead of giving one clang.
- Cranes in scrap yards — a huge electromagnet lifts a load of iron, swings it across and drops the whole load simply by cutting the current. No hand ever touches the scrap.
- Electric motors — found in fans, mixers, pumps and washing machines, where electromagnets whose poles keep reversing produce continuous rotation.
- Loudspeakers, telephone earpieces and magnetic separators for removing iron from a mixture.
Comparing the two:
- Electromagnet — can be switched on and off, its strength can be changed by altering turns or current, it can be made very strong, and its poles reverse with the current.
- Permanent magnet — needs no current, but has a fixed and usually weaker strength and fixed poles.
The bell shows why the choice is not merely convenient but necessary. Built with a permanent magnet, the armature would be pulled in once and simply stay there — no interruption, no repeat, no ringing. The device depends on the magnetism being able to disappear.
Exam tip
Exam tip: describing the reversal, not just the deflection
Two answers come up again and again in this chapter, and both are marked point by point.
For the compass experiment, state the three observations: the needle deflects, it deflects the other way when the current is reversed, and it returns to north-south when the current is switched off. Reporting only the first misses most of the marks.
For electromagnet strength, name all three factors — number of turns, current, and a soft iron core — and say why soft iron is used.
For the electric bell, write the cycle as numbered steps and make sure the step where the circuit breaks appears. That break is the whole mechanism.
When comparing the two kinds of magnet, lead with switchable on and off, since that is the property every application depends on.
And say the coil wire is insulated — it is a one-word detail that carries a mark.
For the compass experiment, state the three observations: the needle deflects, it deflects the other way when the current is reversed, and it returns to north-south when the current is switched off. Reporting only the first misses most of the marks.
For electromagnet strength, name all three factors — number of turns, current, and a soft iron core — and say why soft iron is used.
For the electric bell, write the cycle as numbered steps and make sure the step where the circuit breaks appears. That break is the whole mechanism.
When comparing the two kinds of magnet, lead with switchable on and off, since that is the property every application depends on.
And say the coil wire is insulated — it is a one-word detail that carries a mark.
Did you know
Why do the pins fall the instant you open the switch?
Because the nail was never a magnet in its own right — it merely borrowed its magnetism from the current.
While current flows through the coil, its magnetic field lines up the iron inside the nail so the whole nail behaves as a magnet. Cut the current and there is nothing left to maintain that arrangement, so the iron relaxes almost immediately.
Soft iron is chosen precisely because it lets go so readily. A steel rod in the same coil would stay magnetised after the switch opened, and the pins would cling on — useful for a fridge magnet, useless for a scrapyard crane that has to drop its load.
While current flows through the coil, its magnetic field lines up the iron inside the nail so the whole nail behaves as a magnet. Cut the current and there is nothing left to maintain that arrangement, so the iron relaxes almost immediately.
Soft iron is chosen precisely because it lets go so readily. A steel rod in the same coil would stay magnetised after the switch opened, and the pins would cling on — useful for a fridge magnet, useless for a scrapyard crane that has to drop its load.
Key takeaways
Magnetic effect of current: quick revision
- A compass needle placed parallel to a current-carrying wire deflects, showing that a current produces a magnetic field around the wire.
- Increasing the current increases the deflection, reversing it flips the deflection, and switching off returns the needle to north-south.
- The field forms circles around the wire, so the compass deflects the other way on the opposite side.
- An electromagnet is an insulated coil wound on a soft iron core, strengthened by more turns, a larger current and that iron core.
- Soft iron is used because it loses its magnetism as soon as the current stops.
- Electromagnets are used in electric bells, scrapyard cranes, motors and loudspeakers, because unlike a permanent magnet they can be switched on and off and have their strength and poles changed.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Increasing the current increases the deflection, reversing it flips the deflection, and switching off returns the needle to north-south.
- The field forms circles around the wire, so the compass deflects the other way on the opposite side.
- An electromagnet is an insulated coil wound on a soft iron core, strengthened by more turns, a larger current and that iron core.
- Soft iron is used because it loses its magnetism as soon as the current stops.
- Electromagnets are used in electric bells, scrapyard cranes, motors and loudspeakers, because unlike a permanent magnet they can be switched on and off and have their strength and poles changed.
You will remember all of this far better after answering five questions on it than after reading it twice.