Introduction

The voltage of the domestic electric supply is 110 volts in North America and 230 volts in Europe. However, many electrical devices such as radios, computers, and clocks require only a few volts to operate. Inside all these devices is a transformer that changes (or steps down) the voltage to a lower value.

The voltage of the domestic electric supply is 110 volts in North America and 230 volts in Europe. However, many electrical devices such as radios, computers, and clocks require only a few volts to operate. Inside all these devices is a transformer that changes (or steps down) the voltage to a lower value.

Double helix

A **transformer**is made of two separate coils of wire, both wrapped around the same piece of metal, usually iron.

Transformers come in many different shapes and sizes.

The two coils in a

**transformer**Click on the figure below to interact with the model.

In Fig.4 a 9 V

**direct current**

**d.c.**The primary coil behaves like a solenoid. When a current flows through it, a

**magnetic field**The secondary coil behaves like an induction coil. A current is only induced in the secondary coil when the magnetic field inside this coil changes.

The bulb only lights up when the current through the primary coil changes.

Click on the figure below to interact with the model.

The circuit in Fig.5 above uses an

**alternating current**

**a.c.**Push and hold down the switch in the circuit and see what happens.

Because the current supply is alternating, the current flowing through the primary coil is always changing. This means that the magnetic field generated by this coil also changes. As both coils share the same metal core, the changing field always induces a voltage across the secondary coil. This explains why the lamp is always on whenever the switch is pressed.

Transforming voltage

Transformers can be used to change (or transform) the voltage of an alternating electrical signal.
The source in Fig.6 below generates a 12 V a.c. supply. The voltmeter on the left-hand side measures the voltage across the
primary coil of the transformer. It displays a reading of 12.0 V.Click on the figure below to interact with the model.

Click on the push button to send a current through the transformer.

The voltmeter on the right-hand side shows the voltage across the secondary coil.

This type of transformer is called a step-down transformer because the voltage across the secondary coil is less than the voltage across the primary coil. The voltage has therefore been stepped down. A step-up transformer increases the voltage from the primary to the secondary coil.

The two numbers near to the transformer 3:1 mean that there are three times as many turns on the primary coil than on the secondary coil. The voltage on the primary coil is three times the voltage on the secondary coil.

Click on the number '3' and change it to '4'.

Calculating the voltage

The voltages across the primary and secondary coils are related to the number of turns on the coils. If you know the number
of turns and the primary voltage, you can calculate the secondary voltage. The equation to use is shown below:
We can apply these equations to the previous experiments in Figs.5 and 6 above.

In the first case, the voltage across the primary coil (V_{1}) was 12 volts. |

We do not know the exact number of turns on each coil, but we do not need to. We do know the ratio of turns. |

Now click on V_{2} in the equation above to rearrange the equation in terms of V_{2}. |

We can now use the rearranged equation to calculate V_{2}. |

V_{2} = 4 V |

In a similar way, we can calculate the secondary voltage (V

_{2}) when the ratio of turns is .

Set the turns ratio of the transformer in Fig.6 above to 6:1. Check whether your calculation in the question above was correct.

Summary

Transformers can change the voltage of a supply. Step-down transformers reduce the voltage and step-up transformers increase the voltage.

Transformers only work with alternating current. This is because a changing magnetic field is needed to induce a current.

The ratio of turns on the primary and secondary coils is equal to the ratio of the voltage across the primary and secondary coils.

Transformers can change the voltage of a supply. Step-down transformers reduce the voltage and step-up transformers increase the voltage.

Transformers only work with alternating current. This is because a changing magnetic field is needed to induce a current.

The ratio of turns on the primary and secondary coils is equal to the ratio of the voltage across the primary and secondary coils.

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