Power, but bitesize.

Part 1Comparing two toasters

Imagine two toasters. They both toast bread. But one of them toasts the bread faster than the other. Why?

It's because they transfer energy at different rates. The fast toaster pushes more energy into the bread every second. The slow one pushes less. In physics, we say the fast toaster has a higher power.

Toaster A is more powerful because it transfers more energy per second. It doesn't matter that they're both toasters — what matters is the rate.

Part 2The proper definition

Here it is, the definition you need to know:

Power is the amount of energy transferred per second.

Two things to lock in:

1. Power is a rate. It tells you how fast energy is being transferred, not the total amount. A really powerful device can transfer a lot of energy in a tiny amount of time. A weak one takes ages to transfer the same amount.

2. Power and energy are NOT the same thing. Energy is the "stuff" being transferred (measured in joules). Power is how fast it's being transferred (joules per second).

Part 1Units: joules and watts

Two units to remember. They go together and they sound similar — don't mix them up.

Energy is measured in joules (J). A joule is a small amount of energy — about what you use to lift an apple a metre off the floor.

Power is measured in watts (W). One watt means one joule per second. So a 60 W lamp transfers 60 joules of energy every second. A 2000 W kettle transfers 2000 joules every second.

Part 2The equation

To calculate power, you divide the energy transferred by the time it took:

power = energy ÷ time

P = E ÷ t

This is just maths putting the definition into symbols. If a device transfers 500 J in 5 seconds, you divide: 500 ÷ 5 = 100 J every second. That's 100 W.

Part 1Rearranging the equation

Sometimes you're not asked for the power — you already know the power, and you need to find the energy transferred. To do that, rearrange P = E ÷ t into:

energy = power × time

E = P × t

This makes sense: if a device transfers 50 J every second, and it runs for 5 seconds, the total energy is 50 × 5 = 250 J.

Part 2The clock face method

Here's a way to picture what's happening. Imagine a clock face with one section for each second.

A 20 W device transfers 20 joules every second. If it's left on for 5 seconds, draw a clock face with 5 sections. Write "20 J" in each section. Add them up: 20 + 20 + 20 + 20 + 20 = 100 J.

It's the same calculation as E = P × t, just drawn out. Once you trust the equation, you don't need the clock — but it helps when the numbers feel abstract.

Part 3The FIFA method

To answer power calculations cleanly, use the FIFA method. Four steps, every time:

F — Formula. Write the equation.
I — Insert. Put the values into the equation.
F — Fix. Sort out any unit conversions (we'll cover these in the next topics).
A — Answer. Calculate and write the number with its unit.

Part 1What "kilo" means

"Kilo" is a prefix that means one thousand. You've already met it: a kilogram is 1000 grams, a kilometre is 1000 metres.

So:

1 kilowatt (kW) = 1000 watts (W)

1 kilojoule (kJ) = 1000 joules (J)

Kilowatts and kilojoules are used for bigger numbers, when "1000 W" would be a mouthful. A kettle's power is much easier to say as "2 kW" than "2000 W". They mean the same thing.

Part 2Converting between units

To go from kW to W (or kJ to J), multiply by 1000. To go the other way, divide by 1000.

kW × 1000 = W     W ÷ 1000 = kW

Quick examples:

· 1.2 kW = 1.2 × 1000 = 1200 W
· 3 kW = 3 × 1000 = 3000 W
· 4000 J = 4000 ÷ 1000 = 4 kJ
· 800 J = 800 ÷ 1000 = 0.8 kJ

Part 3Using kW in the equation

The power equation, E = P × t, only works if your units match. Power has to be in watts, time has to be in seconds, and you'll get energy in joules.

If a question gives you the power in kW, your first job is to convert it to W before you put it into the equation. That's the "Fix" step of FIFA.

Part 1Converting minutes to seconds

The power equation, E = P × t, only works when time is in seconds. If a question tells you the device is on for 3 minutes, you have to convert that to seconds first.

There are 60 seconds in a minute. So to convert minutes to seconds, multiply by 60.

seconds = minutes × 60

Quick examples:

· 2 minutes = 2 × 60 = 120 s
· 3 minutes = 3 × 60 = 180 s
· 5 minutes = 5 × 60 = 300 s
· 1.5 minutes = 1.5 × 60 = 90 s

Part 2Putting it all together

Now you've got all the conversions you need. A typical exam question might give you a power in kW and a time in minutes — you have to fix both.

Part 1Energy doesn't appear from nowhere

When a device transfers energy, that energy has to come from somewhere and go somewhere. This is the law of conservation of energy:

energy cannot be created or destroyed — only transferred between stores

So when an electric car drives up a hill, the energy isn't appearing from thin air. It's being transferred out of one store and into another.

The chemical energy store in the battery decreases. The kinetic energy store of the moving car increases. If the car is also going uphill, the gravitational store increases too.

The total amount of energy is the same — it's just been moved from one store to another.

Part 2Bringing it all together

An exam question on power might ask you to do the calculation and describe where the energy comes from and goes to. Both halves matter.

The calculation gives you a number (in joules). The energy stores description tells the story behind the number.