# The National Grid

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## The National Grid

The National Grid supplies electricity to homes and businesses all over Great Britain. It is made up of a system of cables and transformers that link power stations to consumers

## Efficiency of the National Grid

The National Grid transfers electricity using a high potential difference and a low current

This is because the National Grid needs to transfer a very large amount of energy. A high current cannot be used because the wires would get very hot and a lot of energy would be lost to the surroundings as heat energy

It is cheaper and more efficient to use a very high potential difference (approximately $400 \, 000\text{ V}$) and to keep the current very low.

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## Transformers

Transformers are used throughout the National Grid to increase and decrease the potential difference when it is needed.

Transformers are made up of two coils: the primary and secondary coil. These coils are wrapped around an iron core

If the transformer increases the potential difference, there are more turns on the secondary coil. This is called a step-up transformer and it is used to increase the potential difference of electricity for transportation via cables. If the transformer decreases the potential difference, there will be fewer turns on the secondary coil. This is called a step-down transformer and is used to reduce the potential difference electricity reaches homes.

Transformers are nearly $100\%$ efficient.

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## Transformer Calculations

Because transformers are nearly $100\%$ efficient, almost no power $(\text{P}=\text{VI})$ is lost when energy moves across a transformer and so we can write:

$\text{V}_2\times \text{I}_2=\text{V}_1 \times \text{I}_1$

• $V_2 =$ the potential difference in the secondary coil in volts $\text{(V)}$
• $I_2 =$ the current in the secondary coil in amps $\text{(A)}$
• $V_1 =$ the potential difference in the primary coil in volts $\text{(V)}$
• $I_1 =$ the current in the primary coil in amps $\text{(A)}$
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@mmerevise

## Example: Transformer Calculation

The diagram on the right shows the current and potential difference going into and coming out of a transformer. Calculate the missing value of current.

[2 marks]

$\bold{\text{V}_2\times \text{I}_2=\text{V}_1 \times \text{I}_1} \\ \\ \text{I}_2=\dfrac{\text{V}_1 \times \text{I}_1}{\text{V}_2}=\dfrac{\textcolor{00d865}{250} \times \textcolor{f21cc2}{10}}{\textcolor{10a6f3}{1000}}=\bold{2.5 \text{ A}}$
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## The National Grid Example Questions

The National Grid is a system of cables and transformers that link power stations to consumers and hence supply Great Britain with electricity

Gold Standard Education

The National Grid is required to transfer a large amount of energy

The current needs to be kept low to reduce heat loss and therefore make the National Grid more efficient

Gold Standard Education

To increase or decrease the potential difference of the electricity supply.

Gold Standard Education

$\bold{\text{V}_2 \times \text{I}_2=\text{V}_1 \times \text{I}_1}$

Rearrange for $\text{V}_2$:

$\text{V}_2 = \dfrac{\text{V}_1\times \text{I}_1}{\text{I}_2}=\dfrac{5 000\text{ V} \times 12\text{ A}}{6\text{ A}} = \bold{10\,000\text{ V}}$

Gold Standard Education

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