The Magnet You Can Switch Off
Learn the properties of magnets and their poles, see how a current deflects a compass needle, build an electromagnet and control its strength, and follow the working of an electric bell.
Can a magnet be switched on and off?
Yes. Wind an insulated wire around an iron nail and connect it to a cell, and the nail picks up pins. Break the circuit and the pins drop off at once.
That is an electromagnet, and it is a magnet only while current flows. This page covers everything in the ICSE Class 7 Physics chapter's second half: types and properties of magnets, the magnetic effect of an electric current, making an electromagnet, and the electric bell.
That is an electromagnet, and it is a magnet only while current flows. This page covers everything in the ICSE Class 7 Physics chapter's second half: types and properties of magnets, the magnetic effect of an electric current, making an electromagnet, and the electric bell.
What are the types and properties of magnets?
A magnet is a body that attracts iron, cobalt and nickel, and points north-south when suspended freely.
Magnets are natural — pieces of the ore magnetite, irregular in shape and weak — or artificial, made to a chosen shape and strength. The artificial ones in the syllabus are the bar magnet, the horseshoe magnet, the cylindrical magnet and the magnetic needle used in a compass.
The properties to state are these:
- A magnet has two poles, north and south, where its attraction is strongest.
- Like poles repel; unlike poles attract.
- Poles always exist in pairs — break a bar magnet in half and each piece has its own north and south pole.
- A freely suspended magnet always comes to rest along the north-south direction, which is how a compass works.
A compass needle in a geography lab shows the last property directly: however you turn the case, the needle swings back to the same line.
The first property is the reliable test for a magnet, and it rests on repulsion, not attraction. An unmagnetised iron bar is attracted by a magnet too, so attraction proves nothing. Only repulsion proves that both objects are magnets.
Magnets are natural — pieces of the ore magnetite, irregular in shape and weak — or artificial, made to a chosen shape and strength. The artificial ones in the syllabus are the bar magnet, the horseshoe magnet, the cylindrical magnet and the magnetic needle used in a compass.
The properties to state are these:
- A magnet has two poles, north and south, where its attraction is strongest.
- Like poles repel; unlike poles attract.
- Poles always exist in pairs — break a bar magnet in half and each piece has its own north and south pole.
- A freely suspended magnet always comes to rest along the north-south direction, which is how a compass works.
A compass needle in a geography lab shows the last property directly: however you turn the case, the needle swings back to the same line.
The first property is the reliable test for a magnet, and it rests on repulsion, not attraction. An unmagnetised iron bar is attracted by a magnet too, so attraction proves nothing. Only repulsion proves that both objects are magnets.
How does an electric current produce a magnetic effect?
A current-carrying wire behaves like a magnet — it creates a magnetic field around itself.
The demonstration, known as Oersted's experiment, is simple. Place a straight wire above a compass needle, lying parallel to the needle. With the circuit open, the needle points north-south as usual. Close the circuit and the needle deflects to one side.
Three observations complete the experiment, and each carries a mark:
- Reverse the current and the needle deflects to the opposite side.
- Increase the current and the deflection becomes larger.
- Switch the current off and the needle returns to north-south.
The magnetic field itself forms circles around the wire, which is why a compass placed on the other side of the wire deflects the other way.
This is the link the rest of the chapter is built on: electricity and magnetism are not separate subjects. A current makes a magnetic field, and that is what makes an electromagnet possible at all.
The demonstration, known as Oersted's experiment, is simple. Place a straight wire above a compass needle, lying parallel to the needle. With the circuit open, the needle points north-south as usual. Close the circuit and the needle deflects to one side.
Three observations complete the experiment, and each carries a mark:
- Reverse the current and the needle deflects to the opposite side.
- Increase the current and the deflection becomes larger.
- Switch the current off and the needle returns to north-south.
The magnetic field itself forms circles around the wire, which is why a compass placed on the other side of the wire deflects the other way.
This is the link the rest of the chapter is built on: electricity and magnetism are not separate subjects. A current makes a magnetic field, and that is what makes an electromagnet possible at all.
How do you make an electromagnet and control its strength?
Wind an insulated copper wire in many close turns around a soft-iron core, and pass a current through the coil. The core becomes a strong magnet while the current flows and loses almost all its magnetism when the current stops.
The coil wire must be insulated so the current travels the whole length of the winding instead of short-circuiting between touching turns.
Three factors decide the strength:
- The number of turns — more turns give a stronger magnet.
- The current — a larger current gives a stronger magnet.
- The nature of the core — soft iron gives a much stronger electromagnet than a core of steel, wood or plastic.
So a coil of 100 turns carrying is stronger than the same coil carrying , and stronger still than a 50-turn coil at the same current.
A scrap-yard crane is the everyday example: it lifts a load of iron, moves it, and drops the whole load simply by cutting the current.
The reason the core is soft iron rather than steel matters. Soft iron magnetises strongly but loses its magnetism as soon as the current stops, which is exactly what a switchable magnet needs. Steel keeps its magnetism, which makes it right for permanent magnets and wrong here.
The coil wire must be insulated so the current travels the whole length of the winding instead of short-circuiting between touching turns.
Three factors decide the strength:
- The number of turns — more turns give a stronger magnet.
- The current — a larger current gives a stronger magnet.
- The nature of the core — soft iron gives a much stronger electromagnet than a core of steel, wood or plastic.
So a coil of 100 turns carrying is stronger than the same coil carrying , and stronger still than a 50-turn coil at the same current.
A scrap-yard crane is the everyday example: it lifts a load of iron, moves it, and drops the whole load simply by cutting the current.
The reason the core is soft iron rather than steel matters. Soft iron magnetises strongly but loses its magnetism as soon as the current stops, which is exactly what a switchable magnet needs. Steel keeps its magnetism, which makes it right for permanent magnets and wrong here.
How does an electric bell work?
It uses an electromagnet that repeatedly breaks its own circuit, so the hammer strikes the gong again and again.
The parts are an electromagnet, a soft-iron armature carrying a hammer, a contact screw touching a springy strip, and a gong.
The cycle runs like this:
1. Pressing the switch completes the circuit, so current flows and the electromagnet is energised.
2. The electromagnet attracts the armature, and the hammer strikes the gong — the sound.
3. Moving towards the magnet pulls the strip away from the contact screw, which breaks the circuit.
4. With no current, the electromagnet loses its magnetism, and the spring pulls the armature back, remaking the contact.
5. The circuit is complete again, and the cycle repeats many times a second.
That self-interrupting loop is why the bell rings continuously while the button is held, rather than giving one single clang.
Comparing the two kinds of magnet:
- An electromagnet can be switched on and off, its strength can be changed, it can be made very strong, and its poles can be reversed by reversing the current.
- A permanent magnet needs no current and keeps a fixed, usually weaker strength with fixed poles.
The bell depends on the first property of the list. Built with a permanent magnet, the armature would be pulled in once and simply stay there.
The parts are an electromagnet, a soft-iron armature carrying a hammer, a contact screw touching a springy strip, and a gong.
The cycle runs like this:
1. Pressing the switch completes the circuit, so current flows and the electromagnet is energised.
2. The electromagnet attracts the armature, and the hammer strikes the gong — the sound.
3. Moving towards the magnet pulls the strip away from the contact screw, which breaks the circuit.
4. With no current, the electromagnet loses its magnetism, and the spring pulls the armature back, remaking the contact.
5. The circuit is complete again, and the cycle repeats many times a second.
That self-interrupting loop is why the bell rings continuously while the button is held, rather than giving one single clang.
Comparing the two kinds of magnet:
- An electromagnet can be switched on and off, its strength can be changed, it can be made very strong, and its poles can be reversed by reversing the current.
- A permanent magnet needs no current and keeps a fixed, usually weaker strength with fixed poles.
The bell depends on the first property of the list. Built with a permanent magnet, the armature would be pulled in once and simply stay there.
Exam tip
Exam tip: listing the three factors and the bell's full cycle
Two questions come up almost every year in this chapter, and both are marked point by point.
For electromagnet strength, name all three factors — number of turns, current, and nature of the core — and say soft iron for the core. Two out of three earns two-thirds of the mark.
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, and an answer that ends with "the hammer strikes the gong" misses why it rings repeatedly.
For Oersted's experiment, state the deflection and what happens when the current is reversed. The reversal is what proves the effect comes from the current's direction.
And when asked to test whether a bar is a magnet, answer with repulsion. Attraction happens with ordinary iron too, so it settles nothing.
For electromagnet strength, name all three factors — number of turns, current, and nature of the core — and say soft iron for the core. Two out of three earns two-thirds of the mark.
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, and an answer that ends with "the hammer strikes the gong" misses why it rings repeatedly.
For Oersted's experiment, state the deflection and what happens when the current is reversed. The reversal is what proves the effect comes from the current's direction.
And when asked to test whether a bar is a magnet, answer with repulsion. Attraction happens with ordinary iron too, so it settles nothing.
Did you know
Why does breaking a magnet in half give two complete magnets?
Because magnetism belongs to the tiny parts inside, not to the two ends.
A magnet behaves as though it were made of countless minute magnets all lined up the same way. Cut it anywhere and each half still holds a lined-up row, so each half immediately has a north pole at one end and a south pole at the other.
Keep cutting and the pattern survives every time. That is why a single, isolated pole is never found — poles come in pairs, all the way down.
A magnet behaves as though it were made of countless minute magnets all lined up the same way. Cut it anywhere and each half still holds a lined-up row, so each half immediately has a north pole at one end and a south pole at the other.
Keep cutting and the pattern survives every time. That is why a single, isolated pole is never found — poles come in pairs, all the way down.
Key takeaways
Magnets and electromagnets: quick revision
- Magnets are natural (magnetite) or artificial (bar, horseshoe, cylindrical, magnetic needle) and attract iron, cobalt and nickel.
- Like poles repel and unlike poles attract; repulsion is the only sure test for a magnet, and poles always occur in pairs.
- A current-carrying wire has a magnetic field: a compass needle deflects, deflects the other way when the current is reversed, and returns to north-south when it is switched off.
- An electromagnet is an insulated coil wound on a soft-iron core, strengthened by more turns, a larger current, and a soft-iron rather than steel core.
- Soft iron is used because it loses its magnetism the moment the current stops.
- An electric bell rings continuously because the armature's movement breaks the circuit, de-energising the electromagnet so the spring can remake the contact.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Like poles repel and unlike poles attract; repulsion is the only sure test for a magnet, and poles always occur in pairs.
- A current-carrying wire has a magnetic field: a compass needle deflects, deflects the other way when the current is reversed, and returns to north-south when it is switched off.
- An electromagnet is an insulated coil wound on a soft-iron core, strengthened by more turns, a larger current, and a soft-iron rather than steel core.
- Soft iron is used because it loses its magnetism the moment the current stops.
- An electric bell rings continuously because the armature's movement breaks the circuit, de-energising the electromagnet so the spring can remake the contact.
You will remember all of this far better after answering five questions on it than after reading it twice.