We can make a magnet that we can switch on and off, using a coil of wire and a battery. It is called an electromagnet. Remember that, a bar magnet is not an electromagnet, rather it’s a permanent magnet. You cannot switch it off without demagnetizing it completely. In this post, we will discuss different aspects of electromagnets.
- The electromagnetic field around a current carrying wire
- Making an electromagnet
- The strength of an electromagnet
- Differences between Permanent magnets and electromagnets
- Uses of Electromagnets
- X-ray machine and electromagnet
- what is Relay & how to Make a Relay
- Relay to turn on the starter motor in a car
- Using electromagnets in medicine
The electromagnetic field around a current carrying wire
Electromagnets work because when an electric current flows through a wire, a magnetic field is produced. You can use plotting compasses to draw the field around a single piece of wire.
The magnetic field lines around the wire are circles. The magnetic field gets weaker as you get further away from the wire.
Making an electromagnet
A typical electromagnet is made from a coil of copper wire. Copper wire is often used, because of its low resistance, though other metals will do. A coil of wire is called a solenoid. The coil doesn’t have to be made from a magnetic material.
Making an electromagnet requires a magnetic field. This can be done in the following ways:
- You can make a circular loop of wire and pass a current through it. You can see (in figure 2) how magnetic field lines are created.
Note that, the magnetic field lines at the center are straight.
- You can coil a wire into lots of loops and make a coiled cylinder. When you pass an electric current through the coil, a magnetic field is produced. The magnetic field around a solenoid is similar to that around a bar magnet. One end of the coil is a north pole, and the other end is a south pole. Coiling the wire concentrates the magnetic field inside the loops, so the field is much stronger than for a single loop on its own.
To make an electromagnet, we use a coil of insulated wire and some magnetic material called a core in the center of the coil. (figure 4)
The core becomes magnetized by the magnetic field of the coil. This makes the electromagnet much stronger. Most cores are made of iron.
The switch turns the electromagnet on and off because the magnetic field is only produced when a current is flowing in the wire. The iron core does not remain magnetized when you turn the switch off.
The strength of an electromagnet
The strength of an electromagnet depends on:
- the number of turns, or loops, on the coil – more turns will make a stronger electromagnet
- the current flowing in the wire – more current will make a stronger electromagnet
- the type of core – using a magnetic material in the core will make a stronger electromagnet. (adding a soft iron core increases the strength of an electromagnet.)
- The current flowing in the wire depends on the power supply and the type of wire that is used in the electromagnet.
Differences between Permanent magnets and electromagnets
There are two main differences between a permanent magnet and an electromagnet.
- You can turn an electromagnet on and off.
- You can make an electromagnet that is much stronger than a permanent magnet.
Uses of Electromagnets
Electromagnets can be used in electromagnetic cranes to move large pieces of iron or steel in a factory or to move cars in a scrapyard. When the current is turned off, the electromagnet drops the car. Doctors can use a small electromagnet to remove iron or steel splinters from people’s eyes. Electromagnets are used to make relays. Relays are used to switch on and off high voltage circuits like X-ray machines, and starter motors of cars.
X-ray machine and electromagnet
An X-ray machine can be very dangerous. The X-rays can damage your cells if you are exposed to them for too long. The radiographer, who operates the machine, should not be near the machine every time it is switched on and off.
The machine uses a high-voltage circuit. The radiographer turns it on and off using a relay instead of a normal on/off switch.
what is Relay & how to Make a Relay
A relay uses a small current in one circuit to operate a switch in a separate high-voltage or high-current circuit.
You can make a simple relay with a reed switch. This is made of two pieces of magnetic material (called reeds) with a gap between them. If you wind a coil of wire around the outside and pass a current through it, the two reeds are magnetized. They attract each other and touch.
If the reeds are part of another circuit, then the current will flow in the second circuit when they touch.
Relay to turn on the starter motor in a car
A relay is often used to turn on the starter motor in a car.
- The starter motor works from the car battery, which produces a dangerously high current.
- The relay circuit that the driver switches on has only a small current, but it turns on the high-current starter motor circuit.
- When the driver turns the key, a current flows in the coil of wire which produces a magnetic field around it. This magnetic field magnetizes the iron armature.
- The armature is attracted towards the contacts.
- It connects the contacts and completes the starter motor circuit so that the battery powers the starter motor.
- It only takes a small current to operate the electromagnet.
- The current in the starter motor circuit is much bigger and could be dangerous if the driver had to switch it on directly.
Using electromagnets in medicine
Magnets can be used to produce an image of the inside of your body. The device is called a magnetic resonance imaging (MRI) scanner.
An MRI scanner contains very strong electromagnets. These produce a very intense magnetic field that interacts with the hydrogen in the water molecules in your body. The MRI scanner enables doctors to take pictures of people’s organs.