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  1. Describe the construction of a basic iron-cored transformer as used for voltage transformations.

A transformer is an electrical device that changes the voltage of an A.C. current supply.

A transformer has two coils of wires wound around an iron core, a primary coil and a secondary coil, which are connected to the primary circuit and secondary circuit respectively. When an alternating current is applied to the primary coil through the primary circuit, an alternating voltage is induced in the secondary coil and thus an alternating current is induced in the secondary circuit. This happens due to electromagnetic induction.


A transformer can increase or decrease the voltage this way. When there are lesser turns on the secondary coil than the primary coil, the voltage produced in the secondary coil decreases. These are called step-down transformers. When the secondary coil has more turns than the primary coil, the voltage is ‘stepped-up’ so it is called a step-up transformer. The above is a step- down transformer as the no. of turns on the primary coil exceed the no. of turns on the secondary coil.

  1. Recall and use the equation (Vp / Vs) = (Np / Ns)

The alternating voltage induced in the secondary coil Vs depends upon:

the alternating voltage applied to the primary coil Vpthe number of turns of wire in the primary coil Npthe number of turns of wire in the secodnary coil NsThus these factors are all inter-connected and can be used in a formula:

Vp ÷ Vs   =    Np ÷ Ns

You can use this formula and rearrange it to calculate any of those values.

  1. Describe the use of the transformer in high-voltage transmission of electricity.

When an electricity grid needs to supply electricity to many different places, a high current is needed; however this results in a lot of heat energy being lost. So the grid transmits electricity at a low current by increasing the voltage. Since a high voltage is dangerous to use in homes, transformers are used to step down this power supply, making it safe to use. So electricity is transmitted at high voltage and supplied to homes and buildings at low voltage.

  1. Recall and use the equation Vp Ip = Vs Is (for 100% efficiency).

We know that power = current * voltage

So, the power supplied to a transformer (primary power) will be Ip * VpThe power delivered to homes by the transformer (secondary power) will be Is * VsIn order for transformers to achieve 100% efficiency, the primary and secondary powers need to be equal, that is:

Ip * Vp =  Is * VsExplain why energy losses in cables are lower when the voltage is high.

Energy is lost as heat when a current is run through a wire. From the equation P = I * V we know that voltage and current are inversely proportional.. If the voltage is high, the current is low, resulting in lesser energy loss.

Notes submitted by Lintha

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4 thoughts on “P14.3 – Transformer

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