Transformers and the national grid

What you'll learn

How an alternating current in one coil can induce a current in another coil.
How transformers step voltage up or down using different numbers of turns.
How to use the transformer turns ratio and ideal power equations.
Why the National Grid uses very high voltages for long-distance transmission.

In Edexcel 1PH0, much of the detailed transformer explanation is Higher Tier. The spec points ending in P, such as 13.7P and 13.11P, are Separate Physics content, so they are part of GCSE Physics 1PH0.

The prerequisite ideas

Current, potential difference and power

An electric current is the flow of electric charge. Current is measured in amperes (A).

Potential difference, often called voltage, is the energy transferred per unit charge between two points in a circuit. It is measured in volts (V).

Power is the rate of energy transfer. It is measured in watts (W), where 1 watt means 1 joule transferred per second.

You already know one important electricity equation:

P = V I

This means that for the same power transfer, a higher potential difference can be paired with a lower current.

Alternating current

An alternating current, or AC, is a current that repeatedly changes direction. UK mains electricity is AC, with a typical domestic potential difference of 230 V and a frequency of 50 Hz.

A direct current, or DC, flows in one direction only.

Key Idea

Why AC matters

Transformers need a changing current, because a changing current produces a changing magnetic field. That is why transformers work with AC, not steady DC.

Electromagnetic induction

A current in a wire produces a magnetic field around the wire. If the wire is wound into a coil, the magnetic field becomes stronger and more concentrated.

Definition

Electromagnetic induction

Electromagnetic induction is the creation of a potential difference in a conductor when the magnetic field around it changes. If the conductor is part of a complete circuit, the induced potential difference makes a current flow.

Common Mistake

Induced p.d. versus induced current

A changing magnetic field can induce a potential difference even if the circuit is open. An induced current only flows if there is a complete circuit.

What a transformer is

Definition

Transformer

A transformer is an electrical device that uses electromagnetic induction to change the size of an alternating potential difference. It has a primary coil, a secondary coil and an iron core.

The primary coil is connected to the input supply. The secondary coil is connected to the output circuit. The two coils are not joined by a wire; the iron core links them magnetically.

Labelled transformer showing primary coil, secondary coil, iron core, AC input and induced AC output

How a transformer induces a current in another circuit

Here is the chain of events:

An alternating current flows in the primary coil.
This produces a changing magnetic field in the iron core.
The iron core carries this changing magnetic field through the secondary coil.
The changing magnetic field induces an alternating potential difference across the secondary coil.
If the secondary circuit is complete, an alternating current flows in the secondary circuit.

The iron core is usually made from iron because iron is easily magnetised, so it helps transfer the changing magnetic field from one coil to the other.

Example

Deciding whether a transformer works with steady DC

A transformer is connected to a steady DC supply instead of an AC supply. Will there be a continuous output from the secondary coil?

When the DC supply is first switched on, the current changes from zero to a steady value, so the magnetic field changes briefly.
Once the DC current is steady, the magnetic field is also steady, so there is no continuing change in magnetic field through the secondary coil.
Without a changing magnetic field, no continuous potential difference is induced in the secondary coil. The transformer needs AC for continuous operation.

Common Mistake

Thinking the coils have to touch

The primary and secondary coils are electrically separate. Energy is transferred from one circuit to the other by the changing magnetic field in the iron core.

Step-up and step-down transformers

A turn means one complete loop of wire around the core.

A transformer can change the size of an alternating potential difference by using different numbers of turns on the primary and secondary coils.

A step-up transformer increases the potential difference: the secondary coil has more turns than the primary coil.
A step-down transformer decreases the potential difference: the secondary coil has fewer turns than the primary coil.

Key Idea

Turns decide voltage

More turns on the secondary coil means a larger secondary potential difference. Fewer turns on the secondary coil means a smaller secondary potential difference.

The turns ratio equation

For Edexcel 13.7P, you use the transformer turns ratio equation:

VpVs=NpNs\frac{V_p}{V_s} = \frac{N_p}{N_s}

where:

$V_p$ is the potential difference across the primary coil in volts (V)
$V_s$ is the potential difference across the secondary coil in volts (V)
$N_p$ is the number of turns on the primary coil
$N_s$ is the number of turns on the secondary coil

Edexcel lists this as a use equation, rather than a recall statement. In exams where the standard equation sheet is provided, you may be given it, but you still need to choose it and rearrange it correctly.

Example

Calculating secondary voltage

A transformer has 100 turns on the primary coil and 2000 turns on the secondary coil. The primary potential difference is 230 V. Calculate the secondary potential difference.

Compare the turns first: the secondary has more turns than the primary, so you should expect a step-up transformer and a larger secondary potential difference.
Rearrange the turns ratio equation to make $V_s$ the subject:
$Vs=Vp×NsNpV_s = V_p \times \frac{N_s}{N_p}$
Substitute the values:
$VV_s = 230 \text{ V} \times \frac{2000}{100} = 4600 \text{ V}$
The secondary potential difference is 4600 V, so this is a step-up transformer.

Common Mistake

Upside-down turns ratio

If the secondary coil has more turns, the secondary voltage should be bigger. Use this as a quick check after rearranging.

Power in an ideal transformer

An ideal transformer is a transformer with 100% efficiency. That means no energy is wasted, so the input power equals the output power.

For Edexcel 13.10, use:

V_p I_p = V_s I_s

where $I_p$ is the current in the primary coil and $I_s$ is the current in the secondary coil, both in amperes (A).

This equation is also a use equation. Your job is to apply it correctly and remember that it assumes 100% efficiency.

Key Idea

Voltage up means current down

In an ideal transformer, if the voltage increases, the current decreases. The transformer does not create extra energy.

Example

Calculating current in an ideal transformer

A step-up transformer has a primary potential difference of 25 000 V and a primary current of 80 A. The secondary potential difference is 400 000 V. Calculate the secondary current, assuming 100% efficiency.

Use the ideal transformer power equation:
$V_p I_p = V_s I_s$
Rearrange to make $I_s$ the subject:
$Is=VpIpVsI_s = \frac{V_p I_p}{V_s}$
Substitute the values:
$AI_s = \frac{25000 \text{ V} \times 80 \text{ A}}{400000 \text{ V}} = 5.0 \text{ A}$
The secondary current is 5.0 A. The voltage has increased, so the current has decreased.

Common Mistake

Real transformers are not perfect

The equation $V_p I_p = V_s I_s$ assumes 100% efficiency. Real transformers waste some energy, mostly by heating, so their output power is slightly less than their input power.

The National Grid

Definition

National Grid

The National Grid is the network of power stations, transformers and cables that transfers electrical energy to homes, schools, factories and businesses.

The National Grid uses step-up transformers near power stations, high-voltage transmission cables across long distances, and step-down transformers near towns and local areas.

National Grid sequence showing power station, step-up transformer, high-voltage cables, step-down transformer and homes

Why electricity is transmitted at high voltage

Transmission cables have resistance. When current flows through them, energy is transferred to the thermal energy store of the cables, so the cables heat up.

For a fixed power transfer, increasing the potential difference reduces the current because:

P = V I

The heating loss in the cables depends strongly on current:

Ploss=I2RP_{\text{loss}} = I^2 R

So reducing the current greatly reduces the heating losses.

Key Idea

Why high voltage improves efficiency

The National Grid uses high voltage for long-distance transmission because high voltage means low current for the same power transfer, and low current means much less heating loss in the cables.

Example

Comparing heating losses in cables

A power station transfers 2.0 MW of power through cables with resistance 2.0 Ω. Compare the cable heating loss if the transmission voltage is 10 kV and if it is 400 kV.

Calculate the current at 10 kV using $P = V I$ :
$\frac{P}{V} = \frac{2.0 \times 10^6 \text{ W}}{1.0 \times 10^4 \text{ V}} = 200 \text{ A}$
Calculate the current at 400 kV:
$\frac{2.0 \times 10^6 \text{ W}}{4.0 \times 10^5 \text{ V}} = 5.0 \text{ A}$
Calculate the heating loss using $Ploss=I2RP_{\text{loss}} = I^2 R$ :
$W\begin{aligned} P_{\text{loss at 10 kV}} &= 200^2 \times 2.0 = 80000 \text{ W} \\ P_{\text{loss at 400 kV}} &= 5.0^2 \times 2.0 = 50 \text{ W} \end{aligned}$
The higher transmission voltage gives a much smaller current, so the heating loss is far smaller and the transfer is more efficient.

Common Mistake

Saying high voltage directly stops energy loss

High voltage is useful because it allows a lower current for the same power. It is the lower current that reduces heating losses in the cables.

Where step-up and step-down transformers are used

Step-up transformers are used near power stations. They increase the potential difference to a very high value for transmission across the country.

Step-down transformers are used near towns, villages and local substations. They reduce the potential difference to lower values suitable for local distribution and domestic use, such as 230 V for homes.

The high voltage used on transmission lines would be far too dangerous and unsuitable for normal household appliances, so it must be stepped down before reaching homes.

Tip

The National Grid story

Power station produces AC → step-up transformer increases voltage → high-voltage cables reduce heating losses → step-down transformer lowers voltage → homes and businesses use electricity safely.

Exam technique

In the exam

For transformer explanation questions, write the full chain: AC in primary coil → changing magnetic field in iron core → induced potential difference in secondary coil → current if the secondary circuit is complete.
For calculations, choose the turns ratio equation for voltages and turns, and the power equation $V_p I_p = V_s I_s$ for currents in an ideal transformer.
For National Grid questions, always link high voltage to low current, then link low current to reduced heating losses and improved efficiency.

Self review

Check yourself

Why does a transformer need an alternating current rather than a steady direct current?
If a transformer has more turns on the secondary coil than on the primary coil, what happens to the secondary voltage?
Why does the National Grid step the voltage up near power stations and then step it down near homes?

Transformers and the national grid

What you'll learn

How an alternating current in one coil can induce a current in another coil.
How transformers step voltage up or down using different numbers of turns.
How to use the transformer turns ratio and ideal power equations.
Why the National Grid uses very high voltages for long-distance transmission.

The prerequisite ideas

Current, potential difference and power

An electric current is the flow of electric charge. Current is measured in amperes (A).

Potential difference, often called voltage, is the energy transferred per unit charge between two points in a circuit. It is measured in volts (V).

Power is the rate of energy transfer. It is measured in watts (W), where 1 watt means 1 joule transferred per second.

You already know one important electricity equation:

P = V I

This means that for the same power transfer, a higher potential difference can be paired with a lower current.

Alternating current

An alternating current, or AC, is a current that repeatedly changes direction. UK mains electricity is AC, with a typical domestic potential difference of 230 V and a frequency of 50 Hz.

A direct current, or DC, flows in one direction only.

Key Idea

Why AC matters

Transformers need a changing current, because a changing current produces a changing magnetic field. That is why transformers work with AC, not steady DC.

Electromagnetic induction

A current in a wire produces a magnetic field around the wire. If the wire is wound into a coil, the magnetic field becomes stronger and more concentrated.

Definition

Electromagnetic induction

Common Mistake

Induced p.d. versus induced current

A changing magnetic field can induce a potential difference even if the circuit is open. An induced current only flows if there is a complete circuit.

What a transformer is

Definition

Transformer

A transformer is an electrical device that uses electromagnetic induction to change the size of an alternating potential difference. It has a primary coil, a secondary coil and an iron core.

The primary coil is connected to the input supply. The secondary coil is connected to the output circuit. The two coils are not joined by a wire; the iron core links them magnetically.

Labelled transformer showing primary coil, secondary coil, iron core, AC input and induced AC output

How a transformer induces a current in another circuit

Here is the chain of events:

An alternating current flows in the primary coil.
This produces a changing magnetic field in the iron core.
The iron core carries this changing magnetic field through the secondary coil.
The changing magnetic field induces an alternating potential difference across the secondary coil.
If the secondary circuit is complete, an alternating current flows in the secondary circuit.

The iron core is usually made from iron because iron is easily magnetised, so it helps transfer the changing magnetic field from one coil to the other.

Example

Deciding whether a transformer works with steady DC

A transformer is connected to a steady DC supply instead of an AC supply. Will there be a continuous output from the secondary coil?

When the DC supply is first switched on, the current changes from zero to a steady value, so the magnetic field changes briefly.
Once the DC current is steady, the magnetic field is also steady, so there is no continuing change in magnetic field through the secondary coil.
Without a changing magnetic field, no continuous potential difference is induced in the secondary coil. The transformer needs AC for continuous operation.

Common Mistake

Thinking the coils have to touch

The primary and secondary coils are electrically separate. Energy is transferred from one circuit to the other by the changing magnetic field in the iron core.

Step-up and step-down transformers

A turn means one complete loop of wire around the core.

A transformer can change the size of an alternating potential difference by using different numbers of turns on the primary and secondary coils.

A step-up transformer increases the potential difference: the secondary coil has more turns than the primary coil.
A step-down transformer decreases the potential difference: the secondary coil has fewer turns than the primary coil.

Key Idea

Turns decide voltage

More turns on the secondary coil means a larger secondary potential difference. Fewer turns on the secondary coil means a smaller secondary potential difference.

The turns ratio equation

For Edexcel 13.7P, you use the transformer turns ratio equation:

VpVs=NpNs\frac{V_p}{V_s} = \frac{N_p}{N_s}

where:

$V_p$ is the potential difference across the primary coil in volts (V)
$V_s$ is the potential difference across the secondary coil in volts (V)
$N_p$ is the number of turns on the primary coil
$N_s$ is the number of turns on the secondary coil

Example

Calculating secondary voltage

A transformer has 100 turns on the primary coil and 2000 turns on the secondary coil. The primary potential difference is 230 V. Calculate the secondary potential difference.

Compare the turns first: the secondary has more turns than the primary, so you should expect a step-up transformer and a larger secondary potential difference.
Rearrange the turns ratio equation to make $V_s$ the subject:
$Vs=Vp×NsNpV_s = V_p \times \frac{N_s}{N_p}$
Substitute the values:
$VV_s = 230 \text{ V} \times \frac{2000}{100} = 4600 \text{ V}$
The secondary potential difference is 4600 V, so this is a step-up transformer.

Common Mistake

Upside-down turns ratio

If the secondary coil has more turns, the secondary voltage should be bigger. Use this as a quick check after rearranging.

Power in an ideal transformer

An ideal transformer is a transformer with 100% efficiency. That means no energy is wasted, so the input power equals the output power.

For Edexcel 13.10, use:

V_p I_p = V_s I_s

where $I_p$ is the current in the primary coil and $I_s$ is the current in the secondary coil, both in amperes (A).

This equation is also a use equation. Your job is to apply it correctly and remember that it assumes 100% efficiency.

Key Idea

Voltage up means current down

In an ideal transformer, if the voltage increases, the current decreases. The transformer does not create extra energy.

Example

Calculating current in an ideal transformer

Use the ideal transformer power equation:
$V_p I_p = V_s I_s$
Rearrange to make $I_s$ the subject:
$Is=VpIpVsI_s = \frac{V_p I_p}{V_s}$
Substitute the values:
$AI_s = \frac{25000 \text{ V} \times 80 \text{ A}}{400000 \text{ V}} = 5.0 \text{ A}$
The secondary current is 5.0 A. The voltage has increased, so the current has decreased.

Common Mistake

Real transformers are not perfect

The equation $V_p I_p = V_s I_s$ assumes 100% efficiency. Real transformers waste some energy, mostly by heating, so their output power is slightly less than their input power.

The National Grid

Definition

National Grid

The National Grid is the network of power stations, transformers and cables that transfers electrical energy to homes, schools, factories and businesses.

The National Grid uses step-up transformers near power stations, high-voltage transmission cables across long distances, and step-down transformers near towns and local areas.

National Grid sequence showing power station, step-up transformer, high-voltage cables, step-down transformer and homes

Why electricity is transmitted at high voltage

Transmission cables have resistance. When current flows through them, energy is transferred to the thermal energy store of the cables, so the cables heat up.

For a fixed power transfer, increasing the potential difference reduces the current because:

P = V I

The heating loss in the cables depends strongly on current:

Ploss=I2RP_{\text{loss}} = I^2 R

So reducing the current greatly reduces the heating losses.

Key Idea

Why high voltage improves efficiency

The National Grid uses high voltage for long-distance transmission because high voltage means low current for the same power transfer, and low current means much less heating loss in the cables.

Example

Comparing heating losses in cables

A power station transfers 2.0 MW of power through cables with resistance 2.0 Ω. Compare the cable heating loss if the transmission voltage is 10 kV and if it is 400 kV.

Calculate the current at 10 kV using $P = V I$ :
$\frac{P}{V} = \frac{2.0 \times 10^6 \text{ W}}{1.0 \times 10^4 \text{ V}} = 200 \text{ A}$
Calculate the current at 400 kV:
$\frac{2.0 \times 10^6 \text{ W}}{4.0 \times 10^5 \text{ V}} = 5.0 \text{ A}$
Calculate the heating loss using $Ploss=I2RP_{\text{loss}} = I^2 R$ :
$W\begin{aligned} P_{\text{loss at 10 kV}} &= 200^2 \times 2.0 = 80000 \text{ W} \\ P_{\text{loss at 400 kV}} &= 5.0^2 \times 2.0 = 50 \text{ W} \end{aligned}$
The higher transmission voltage gives a much smaller current, so the heating loss is far smaller and the transfer is more efficient.

Common Mistake

Saying high voltage directly stops energy loss

High voltage is useful because it allows a lower current for the same power. It is the lower current that reduces heating losses in the cables.

Where step-up and step-down transformers are used

Step-up transformers are used near power stations. They increase the potential difference to a very high value for transmission across the country.

The high voltage used on transmission lines would be far too dangerous and unsuitable for normal household appliances, so it must be stepped down before reaching homes.

Tip

The National Grid story

Exam technique

In the exam

For transformer explanation questions, write the full chain: AC in primary coil → changing magnetic field in iron core → induced potential difference in secondary coil → current if the secondary circuit is complete.
For calculations, choose the turns ratio equation for voltages and turns, and the power equation $V_p I_p = V_s I_s$ for currents in an ideal transformer.
For National Grid questions, always link high voltage to low current, then link low current to reduced heating losses and improved efficiency.

Self review

Check yourself

Why does a transformer need an alternating current rather than a steady direct current?
If a transformer has more turns on the secondary coil than on the primary coil, what happens to the secondary voltage?
Why does the National Grid step the voltage up near power stations and then step it down near homes?

Transformers and the national grid

What you'll learn

The prerequisite ideas

Current, potential difference and power

Alternating current

Electromagnetic induction

What a transformer is

How a transformer induces a current in another circuit

Step-up and step-down transformers

The turns ratio equation

Power in an ideal transformer

The National Grid

Why electricity is transmitted at high voltage

Where step-up and step-down transformers are used

Recap questions

Transformers and the national grid

What you'll learn

The prerequisite ideas

Current, potential difference and power

Alternating current

Electromagnetic induction

What a transformer is

How a transformer induces a current in another circuit

Step-up and step-down transformers

The turns ratio equation

Power in an ideal transformer

The National Grid

Why electricity is transmitted at high voltage

Where step-up and step-down transformers are used

Recap questions

Transformers and the national grid

What you'll learn

The prerequisite ideas

Current, potential difference and power

Alternating current

Electromagnetic induction

What a transformer is

How a transformer induces a current in another circuit

Step-up and step-down transformers

The turns ratio equation

Power in an ideal transformer

The National Grid

Why electricity is transmitted at high voltage

Where step-up and step-down transformers are used

Recap questions

Transformers and the national grid Revision Guide

Transformers and the national grid

What you'll learn

The prerequisite ideas

Current, potential difference and power

Alternating current

Electromagnetic induction

What a transformer is

How a transformer induces a current in another circuit

Step-up and step-down transformers

The turns ratio equation

Power in an ideal transformer

The National Grid

Why electricity is transmitted at high voltage

Where step-up and step-down transformers are used

Recap questions

Transformers and the national grid Revision Guide