A magnetic field is created when an electric current flows through a wire. Electromagnets have strong magnetic fields due to the coiling of wire around a soft iron core. Electromagnets are used in many appliances including electric bells and relay switches. When a magnetic field from a wire is placed into another magnetic field, it causes the wire to move. This principle is utilised in electric motors and loudspeakers.

Electromagnetism

When an electric current flows through a wire, it produces a magnetic field around the wire. This magnetic field is only present while the current is flowing.

This effect is used in electromagnets. Wire is wrapped around a soft iron core, and an electric current passed through it. The electromagnet behaves as if it were a bar magnet, except that it can be switched on and off.

Applications of electromagnets

The ability of electromagnets to attract magnetic materials (iron, steel, nickel and cobalt) makes them useful in many ways. For example, electromagnets are used on cranes to lift and drop iron and steel in scrapyards, recycling centres and steel works.You should be able to explain how electromagnetic appliances work by interpreting diagrams. Three examples of appliances that use electromagnets are given below

The electric bell

Electric bells work due to the action of electromagnets.

1. When the current flows through the circuit, the electromagnet makes a magnetic field.

2. The electromagnet attracts the springy metal arm.

3. The arm hits the gong, which makes a sound.

4. The circuit is broken now the arm is out of position.

5. The electromagnet is turned off and the springy metal arm moves back.

6. The circuit is complete again.

7. The cycle repeats as long as the switch is closed.

The circuit breaker

The circuit breaker does the same job as a fuse, but it works in a different way.

1. A spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt.

2. An electromagnet is arranged so that it can pull the bolt away from the switch.

3. If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position.

The loudspeaker

Loudspeakers transform electrical signals into sound. Inside a loudspeaker there is a permanent magnet. An electromagnet attached to the speaker cone is inside the magnet field of the permanent magnet.

1. The electrical current from the amplifier is continually changing direction which, in turn, causes the magnetic field around the electromagnet to continually change.

2. The changing attraction and repulsion between the permanent magnet’s magnetic field and the electromagnet’s magnetic field make the electromagnet move back and forth.

3. In turn, the speaker cone vibrates back and forth, which generates sound waves. The frequency at which the current changes direction is the frequency of the sound that the speaker produces.

The motor effect

A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves. This is called the motor effect.This size of the force is greatest when the wire is perpendicular to the magnetic field of the permanent magnet. In other words, it cuts through the magnetic field at 90°. If the wire is parallel to the magnetic field, it will not experience any force.

Working out the direction of the force

The direction of the force - and therefore the movement of the wire - can be determined using Fleming’s left hand rule.To do this, spread out your left thumb, forefinger (index finger) and second finger so they are all at 90° to one another:

• point your forefinger (index finger) in the direction of the magnetic field (north to south)

• point your second finger in the direction of the electric current (positive to negative)

Your thumb will point in the direction of movement.

Remember:

• thuMB – Movement

• Forefinger – magnetic Field

• seCond finger – Current

Note that the direction of the force is reversed if either the direction of the current is reversed, or if the direction of the magnetic field is reversed.

Electric motors

Electric motors use the motor effect. A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles.When an electric current flows through the coil, the coil experiences a force and moves. One side moves up and the other side moves down (based on Fleming’s left hand rule).The direction of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or ‘commutator’.

Increasing the size of the force

The size of the force on a wire carrying a current in a magnetic field can be increased by:

• increasing the size of the current

• increasing the strength of the magnetic field

The speed of a motor can be increased by either increasing the size of the current or by increasing the strength of the magnetic field.

A wire moving in a magnetic field can induce an electric current. This principle is used in electricity generation, but it is also used in transformers to change the potential difference of the electricity. Modern electronic devices tend not to use 230 V mains electricity, and therefore switch mode transformers allow the potential difference to be reduced.

Electromagnetic induction

If an electrical conductor such as a wire cuts through a magnetic field, a potential difference is induced (made to happen) across the ends of the conductor. If the conductor is part of a complete circuit, an electric current will flow in the circuit.For induction to happen, the conductor must cut through the magnetic field. This can be achieved in two ways:

• a conductor can be moved in a magnetic field

• a magnet can be moved in a coil of wire

Induction does not happen if the conductor moves in the same direction as the magnetic field.The induced potential difference can be increased by:

• moving the magnet or wire faster

• using a stronger magnet

• increasing the number of turns, or loops, on the coil

• increasing the area of the coil

Transformers

A transformer changes the potential difference of electricity. It only works with a.c. (alternating current) electricity:

• a step-down transformer reduces the potential difference

• a step-up transformer increases the potential difference

The structure of a transformer

A transformer consists of a soft iron core with two coils of insulated wire wrapped separately around it. Each coil has a different numbers of turns, or loops.

The primary coil is connected to an a.c. supply. It acts like an electromagnet. The secondary coil is where an alternating potential difference is induced.

How transformers work

This is the basis of how a transformer works:

• An alternating current passes through the primary coil.

• The alternating current produces a magnetic field that continuously changes direction. The soft iron core increases the strength of the magnetic field.

• The secondary coil cuts through the changing magnetic field, inducing an alternating potential difference across the ends of the coil.

• An alternating current flows if a circuit is connected to the secondary coil

It is important to note that there is no electrical connection between the primary and the secondary coils.

Calculating the potential difference across the coils

The potential difference across the primary and secondary coils of a transformer can be shown in the following equation:

where:

Vp is the potential difference across the primary coil in volts, V., Vs is the potential difference across the secondary coil in volts, V.,  np is the number of turns in the primary coil., ns is the number of turns in the secondary coil

This means that:

• step-up transformers have more turns on their secondary coil

• step-down transformers have more turns on their primary coil

Conservation of energy in transformers

In Physics Unit 2 you learnt that electrical power can be calculated using this equation:

P = V × I

where:

P is the power in watts, W

V is the potential difference in volts, V

I is the current in amperes (amps), A

This equation can be used to work out the power for the primary coil and the secondary coil of a transformer. Assuming that the transformer is 100% efficient (no energy is lost between its primary coil and secondary coil), the power output from the secondary coil will be the same as the power input to the primary coil. This can be shown by the equation:

Vp × Ip = Vs × Is

Where:

Vp is the potential difference across the primary coil in volts, V

Ip is the current in the primary coil in amperes (amps), A

Vs is the potential difference across the secondary coil in volts, V

Is is the current in the secondary coil amperes (amps), A

Note that, in reality, the assumption that transformers are 100% efficient is not a valid one. Some energy will be lost to the surroundings as heat from the iron core and the coils.

Switch mode transformers

Switch mode transformers are often found in the power supplies of electronic devices such as laptop and mobile phone chargers.

Devices like these need a smaller potential difference than the 230 V from the mains electricity. Therefore, they need a step-down transformer to reduce the potential difference, built into the plug or power supply.Switch mode transformers achieve this by using complex electronic circuits. These rapidly switch the current on and off, allowing the alternating current to be changed to a higher frequency. This is often between 50 Hz and 200 Hz.At these frequencies, a much smaller and lighter transformer than normal is able to reduce the potential difference. As a result, these transformers are suited for use in power supplies such as mobile phone chargers.When the device is plugged in and the batteries are recharging, a load is being applied (the transformer is drawing power).Switch mode transformers use very little power when the plug is left switched on but no load is applied (such as when the device’s batteries are not charging). This is another advantage for using switch mode transformers in applications such as mobile phone chargers.

Comparing switch mode transformers with iron core transformers

 Switch mode transformersIron core transformers Frequency Operate at a high frequency, often between 50 Hz and 200 Hz Operate at 50 Hz (UK mains frequency) Size

Relatively small and light

Relatively large and heavy due to the iron core) Power usage when no load is applied Very little Same as if a load was being applied because a current continues to flow through the primary coil