A cyclist freewheels down a hill. Which energy conversions happen?

At the top the rider has gravitational potential energy because of the height. Going down, height falls and speed rises. Friction in the brakes and the air warms the surroundings. Potential becomes kinetic, and some of it becomes thermal through friction

A car doubles its speed from 50 to 100 km/h. By what factor does its kinetic energy grow?

Kinetic energy depends on the square of the speed. The speed has grown by a factor of 2. 2 squared is 4. By a factor of four, not two

A slice of bread supplies about 300 kilojoules. How many dietary calories is that?

One dietary calorie is about 4.2 kilojoules. Divide 300 by 4.2. About 71 calories

Energy and Heat · Grade 8

Which forms of energy are present in a kitchen while a meal is cooked?

Almost all of them at once. The gas holds chemical energy that turns into heat; the boiling pot holds thermal energy; the spinning mixer has kinetic energy and runs on electrical energy; the lamp overhead gives out light; and the rattling lid makes sound. Energy is not a material — it is the capacity to cause change, and it is measured in joules.

Learning objectives

Identify at least four different forms of energy present in a single everyday situation
Explain what kinetic energy depends on, and why a fast car carries far more of it than a slow one of the same mass
Tell stored energy apart from energy that is being transferred at this moment
Use the joule as the unit of energy and compare two quantities given in joules

Energy is a capacity to cause change

Energy is not a substance. You cannot fill a bottle with it or weigh it on its own. It is a quantity that tells you how much change a system is able to bring about — to move something, to heat something, to give out light or sound.

Kinetic energy is the energy of a moving body. It depends on mass and on speed, but on speed far more strongly. Doubling the speed multiplies the kinetic energy by four, which is why a car at 100 km/h is far more than twice as dangerous as the same car at 50.

Potential energy comes from position: a book on a shelf, a compressed spring, a stretched rubber band. Chemical energy is stored in the bonds between atoms — in food, in fuel, in a battery. Thermal energy is the total movement of the particles inside the object itself.

Stored energy and transferring energy

It helps to separate two different questions: how much energy is here, and how much energy is moving right now. A full battery in a drawer is stored energy. The same battery inside a lit torch is energy transferring from the cell to the bulb and on into the surrounding air.

The distinction matters most for heat and light. There is no such thing as a bulb that "contains light" — the light is created the moment electrical energy passes through it, and stops the moment that flow stops.

Back in the kitchen: the gas and the food are stores. The flame, the rising hot water in the pot, the rattle of the lid and the glow of the lamp are all energy in transit. Turn off the gas and the store remains while the transfer ends.

The joule: putting a number on energy

The unit of energy is the joule, written J. One joule is roughly the energy needed to lift an apple through one metre. That is a small unit, so kilojoules — a thousand joules each — are what you usually meet in practice.

The same unit covers every form, and that is precisely the point. Food, fuel, electricity and heat are all measured in joules, so they can be compared directly. A slice of bread holds about 300 kilojoules; charging a phone takes about 40.

When a food packet says "calories", it is using a different unit for the same quantity. One dietary calorie is about 4.2 kilojoules, so you can translate between the two and talk about a meal and a tank of petrol in one language.

Worked examples

A cyclist freewheels down a hill. Which energy conversions happen?
1. At the top the rider has gravitational potential energy because of the height
2. Going down, height falls and speed rises
3. Friction in the brakes and the air warms the surroundings
Answer: Potential becomes kinetic, and some of it becomes thermal through friction
A car doubles its speed from 50 to 100 km/h. By what factor does its kinetic energy grow?
1. Kinetic energy depends on the square of the speed
2. The speed has grown by a factor of 2
3. 2 squared is 4
Answer: By a factor of four, not two
A slice of bread supplies about 300 kilojoules. How many dietary calories is that?
1. One dietary calorie is about 4.2 kilojoules
2. Divide 300 by 4.2
Answer: About 71 calories

Common mistakes

Treating energy as a kind of substance: Energy does not sit in a place the way water sits in a glass. It is a quantity describing a capacity to cause change, which is why the same energy shows up in completely different forms without any material having moved.
Assuming that doubling speed doubles kinetic energy: The relation is to the square of the speed. Twice the speed means four times the energy, which is why braking distance grows far more sharply than most drivers expect.
Confusing stored energy with transferring energy: A battery in a drawer has a store but nothing is leaving it. "How much is here" and "how much is moving now" are two separate questions, and mixing them makes any energy chain impossible to follow.

What to remember

Energy is a capacity to cause change, not a material.
Kinetic energy goes with the square of the speed.
A store is not the same as a transfer.
Everything is measured in joules, so everything is comparable.