GCSE · Physics · Edexcel · Spec 1PH0

Advantages of high-voltage transmission

Cables carrying electricity across the country heat up, and that heat is wasted energy. How do you cut the waste without sending less power?

How much does a cable heat up?

Current in the cable (A): 5. Heating power in the cable (W): 50

Slide to 10 A, then to 5 A. What happens to the heating?

Watch out: Halving the current does not halve the heating. Heating power is P = I² × R, so it drops to a quarter.

The chain behind the pylons

Reason it through

Why does the grid send electricity along its cables at a very high potential difference?

Link 1 of 4

First link · your turn

The power to be sent is fixed, and electrical power is P = I × V. If the potential difference V goes up, what has to happen to the current I?

Locked — reveal the link above first

Following the electricity from power station to home

Step through the route. Notice that the two transformers do opposite jobs.

Power stationElectricity leaves the power station. Before it travels any distance, it goes through a transformer.
Transmission cablesThe long run happens at high potential difference and low current, so little energy is wasted heating the cables.
HomesThe electricity arrives at the lower, much safer potential difference.

Physics · Transmission

Finish the comparison

A power station sends out 12 000 W through a cable with a resistance of 2 Ω. Compare the energy wasted as heat in the cable when it transmits at 1 200 V and at 12 000 V. (All the numbers here are made up for practice.)

Current: I = P ÷ V. Heating in the cable: P = I² × R.Two equations, used one after the other. The current from the first goes into the second.
missing step

Which line is step 2?

At 1 200 V: I = 12 000 ÷ 1 200 = 10 AAt 1 200 V: I = 1 200 ÷ 12 000 = 0.1 AAt 1 200 V: I = 12 000 × 1 200 = 14 400 000 A

Exam line: Find the current with I = P ÷ V, then the heating with P = I² × R. Do both for each potential difference and compare.

Physics · Transmission

Which idea sounds like yours?

A line of pylons carries electricity across the country at a very high potential difference.

Why does the grid transmit electricity at high potential difference? Pick the one closest to what you think.

For the same power, a higher potential difference means a smaller current, and a smaller current wastes far less energy heating the cables.High voltage pushes more energy along the cable, so more arrives.The step-up transformer makes extra energy to make up for what the cables lose.Halving the current halves the energy wasted, so the saving is modest.

How sure are you?

I'm sureI think soI'm guessing

For the same power, a higher potential difference means a smaller current, and a smaller current wastes far less energy heating the cables.

This matches the evidence

That's the whole argument in one sentence. P = I × V gives the smaller current, and P = I² × R shows why a smaller current matters so much.

High voltage pushes more energy along the cable, so more arrives.

This is a common misconception

It's a tempting picture, but the power being sent is the same either way. The high potential difference doesn't add energy. It lets the same power travel with a smaller current, and that is what cuts the heating.

The step-up transformer makes extra energy to make up for what the cables lose.

This is a common misconception

The transformer only changes the potential difference, and the current changes with it. No extra energy is made. The saving comes from less energy being wasted in the cables.

Halving the current halves the energy wasted, so the saving is modest.

This is a common misconception

That treats the heating as if it followed the current in step. It depends on the current squared, so halving the current cuts it to a quarter.

The high potential difference isn't the prize. The small current it allows is.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

That one square is why electricity travels across the country at very high potential difference.

What you need to know

A power station sends out a given electrical power, and electrical power is current × potential difference (P = I × V).
So for the same power, a higher potential difference means a smaller current in the cables.
Have a goYour mate says: "Double the potential difference and you double the current." Same power, same cable. Are they right?
No. The current halves.
With the power fixed, P = I × V means the potential difference and the current move in opposite directions.
A current heats the cables because they have resistance. The heating power is P = I² × R.
The current is squared, so halving it reduces the heating to a quarter.
Have a goA cable carries 8 A and wastes 160 W as heat. The current drops to 4 A. How much does it waste now?
40 W
Halving the current quarters the heating (160 ÷ 4 = 40). Halving the heating to 80 W is the tempting slip.
Less heating means less energy wasted, so high-voltage transmission makes the National Grid more efficient.
Step-up transformers raise the potential difference, and lower the current, as electricity leaves power stations for transmission.
Have a goYou spot a transformer right where electricity leaves a power station for the cables. Does it raise or lower the potential difference?
It raises it. It's a step-up transformer.
Step-up transformers raise the potential difference, and lower the current, for transmission.
Step-down transformers lower the potential difference to a much safer level before it is supplied to homes.
Remember where the benefit comes from: the small current, not the high potential difference itself.

The big picture

Cables heat up when a current flows because they have resistance, and the heating power is P = I² × R. For a given power, P = I × V says a higher potential difference needs a smaller current. Because the current is squared, a smaller current wastes far less energy, so transmitting at high potential difference makes the National Grid more efficient. Step-up transformers raise the potential difference for transmission and step-down transformers lower it to a much safer level for homes.

Key points

1P = I × V: for the same power, a higher potential difference needs a smaller current.

2P = I² × R: the heating power in a cable depends on the square of the current.

3Halving the current reduces the cable heating to a quarter, so a small current wastes far less energy.

4Step-up transformers raise the potential difference for transmission. Step-down transformers lower it to a much safer level before homes.

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Worked example

Problem

A power of 20 000 W is sent through a cable with a resistance of 4 Ω (made-up numbers). Compare the heating in the cable at 2 000 V and at 4 000 V.

⚠ Watch out

Thinking that halving the current halves the heating. The heating depends on the current squared, so halving the current cuts it to a quarter.

🧠

Memory hook

Halve the current, quarter the heat. That one square is why the grid goes high-voltage.

✓

Check yourself

In two or three sentences, explain to a friend why a power station uses a step-up transformer before the electricity goes into the cables.

Flashcards

(10)

Fixed power, higher potential difference: what happens to the current?

The current is smaller, because P = I × V.

Why does a current heat a transmission cable?

The cable has resistance, so a current in it transfers energy by heating.

Which two equations carry the argument for high-voltage transmission, with units?

P = I × V (watts, amperes, volts) gives the current. P = I² × R (watts, amperes, ohms) gives the heating in the cable.

Current in a cable is halved. What happens to the heating?

It falls to a quarter, because the heating depends on the square of the current.

Why does reducing the current greatly reduce the energy wasted?

The heating power depends on I², so any change in current is squared.

What is the advantage of transmitting at high potential difference (low current)?

It reduces energy losses in the National Grid and makes transmission more efficient.

What does a step-up transformer do, and where is it?

It raises the potential difference (and lowers the current) where electricity leaves power stations for transmission.

What does a step-down transformer do?

It lowers the potential difference to a much safer level before electricity is supplied to homes.

Does high potential difference itself push more energy along the cables?

No. For the same power it allows a smaller current, and that is what cuts the wasted heating.

Do the grid's transformers make extra energy to cover the losses?

No. They change the potential difference (and the current with it). The saving is less energy wasted in the cables.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How much does a cable heat up?

Finish the comparison

Which idea sounds like yours?

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