Electromagnetic induction

Electromagnetic induction is all about creating electricity (inducing a voltage) by placing a wire in a moving magnetic field. Transformers are devices which are able to change the voltage of electricity to reduce energy losses when electricity is delivered to our homes.

 
 

Electromagnetic Induction

Connecting a wire to a power supply isn’t the only way to produce a current in a wire. Electricity can also be induced in a wire by placing it in a magnetic field. This is known as electromagnetic induction - creating electricity using a magnet. It works because the magnetic field around the coil is changing when the coil ‘cuts through’ the magnetic field line. It can be done by moving a bar magnet inside a coil of wire or by moving a coil of wire in a magnetic field. Because the magnet is being moved back and forth, this changes the direction of the current, creating alternating (a.c.) current.

 
 

You can think of it as the opposite of the motor effect. The motor effect is when the an electric current produces a movement of a wire (due to the interaction of magnetic fields). Electromagnetic induction is when movement of a wire (or a magnet) creates an electric current.

The size of the induced voltage will increase if:

  1. the strength of the magnet is increased

  2. the number of turns on the coil is increased

  3. the magnet is moved into the coil of wire faster

Electromagnetic field can have astonishing effects. Watch the video below made by Veritasium in conjuction with Palais de la Decouverte. In the video they use a huge alternating current to produce an alternating magnetic field. The effect of this is to create an alternating current in an aluminium plate which results in an opposing magnetic field, causing the plate to levitate. Who needs magic carpets when you’ve got physics?

Transformers

Transformers can change the voltage of an alternating current. Transformers are useful because the voltage produced by power stations is too low to be transmitted efficiently. Transformers are used to increase the voltage (which lowers the current) of electricity moving through power lines. The lower the current, the less energy wasted as it travels through the cables. Another transformer is used to reduce the voltage before being delivered to homes to make it safe to use. Transformers therefore make transmitting electricity much more efficient and minimise the amount of electricity that is wasted.

Transformers consist of a rectangular iron core with two coils of wire wrapped around it - the primary coil and the secondary coil. The voltage running through the wires can be changed by altering the number of turns between the primary and secondary coils.

Step-up transformers increase the voltage. They work by having more turns on the secondary coil than the primary coil. Step-down transformers decrease the voltage. They work by having more turns on the primary coil than the secondary coil.

The following equation can be used to calculate the output voltage from a particular transformer:

 
 

Transformers are essentially 100% efficient so the amount of power input is equal to the amount of power output. Since power is equal to the current multiplied by the voltage (power = current x voltage), then we know that the current x voltage of the primary coil will be equal to the current x voltage of the secondary coil. This can be summarised in the equation: