# Transformers

## Transformers Revision

**Transformers**

**Transformers** change the **voltage** of **alternating currents**. They do this using **electromagnetic induction**, and are used in the **national grid** to change the voltage we use in our homes.

**Transformers**

**Transformers** can increase or decrease the **voltage** of **alternating currents**. They do not work with direct currents.

A **transformer** consists of two **coils** wound around an **iron core**. There is a **primary coil** and a **secondary coil**. The **primary coil** handles the input potential difference, and the **secondary coi****l** handles the output potential difference.

When an **alternating potential difference** is applied across the primary coil, the iron core will magnetise and then quickly demagnetise. An alternating potential difference is induced across the secondary coil due to this changing magnetic field. This is an example of **electromagnetic induction**.

**Step-up transformers**have more turns on the secondary coil than the primary coil and therefore they**i****ncrease the potential difference**. A higher p.d means a lower current. This reduces energy loss via heat when the electricity is travelling through wires around the national grid.**Step-down transformers**have more turns on the primary coil than the secondary coil and they**decrease the potential difference**. They are used to reduce the voltage to a level that can be used in homes.

**The Transformer Equation**

The ratio of the **potential differences** across the **primary** and **secondary coils** is dependent on the ratio of the number of turns on each coil. The** transformer equation **allows us to calculate the output potential difference of a transformer as long as we know the number of turns on each coil and the input potential difference:

\textcolor{f21cc2}{\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}}

- \textcolor{f21cc2}{V_p} is the
**potential difference across the primary coil**in**volts**\left(V\right) - \textcolor{f21cc2}{V_s} is the
**potential difference across the secondary coil**in**volts**\left(V\right) - \textcolor{f21cc2}{n_p} is the
**number of turns on the primary coil** - \textcolor{f21cc2}{n_s} is the
**number of turns on the secondary coil**

For **step-up transformers**, \textcolor{f21cc2}{V_s} > \textcolor{f21cc2}{V_p}.

For **step-down transformers**, \textcolor{f21cc2}{V_s} < \textcolor{f21cc2}{V_p}.

**Transformers and Power**

**Transformers** are not quite 100% efficient. However, they are very close. Therefore if we assume they are 100% efficient, then the **electrical power output is equal to the electrical power input**. This assumption allows us to use P = VI:

\textcolor{00bfa8}{\text{electric power output} = \text{electric power input}}

\textcolor{00bfa8}{V_s I_s = V_p I_p}

- \textcolor{00bfa8}{V_p} is the
**input potential difference**across the primary coil in**volts**\left(V\right) - \textcolor{00bfa8}{I_p} is the
**current through the primary coil**in**amps**\left(A\right) - \textcolor{00bfa8}{V_s} is the
**output potential difference**across the secondary coil in**volts**\left(V\right) - \textcolor{00bfa8}{I_s} is the
**current through the secondary coil**in**amps**\left(A\right)

**Example: Transformer Calculations**

A transformer has a primary coil with \textcolor{bd0000}{50} turns and a secondary coil with \textcolor{2730e9}{100} turns. The input potential difference is \textcolor{00bfa8}{10 \: \text{V}}.

Calculate the output potential difference in volts.

**[3 marks]**

We know the transformer equation is:

\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}

We want to find the output potential difference V_s. We can write the equation as:

V_s = \dfrac{n_s \times V_p}{n_p}

We know that n_s = \textcolor{2730e9}{100}, n_p = \textcolor{bd0000}{50} and V_p = \textcolor{00bfa8}{10 \: \text{V}}, so we can substitute in the values:

V_s = \dfrac{\textcolor{2730e9}{100} \times \textcolor{00bfa8}{10 \: \text{V}}}{\textcolor{bd0000}{50}}

V_s = 20 \: \text{V}

## Transformers Example Questions

**Question 1:** Explain the differences between step-up and step-down transformers.

**[2 marks]**

**Step-up transformers are used to increase potential difference and step-down transformers are used to decrease the potential difference.**

**Step-up transformers have more turns on the secondary coil, whereas step-down transformers have more turns on the primary coil.**

**Question 2: **The output potential difference of a transformer at a substation is 110 \: \text{V}. The primary coil of the transformer has 6000 turns, and the secondary coil has 2 turns. Calculate the input potential difference of the transformer.

**[3 marks]**

**Question 3: **The primary coil of a transformer has a potential difference of 10\: \text{V}and a current of 2\: \text{A}. The secondary coil of the transformer has a current of 4\: \text{A}. Calculate the output potential difference of the transformer.

**[3 marks]**