S3.8.2 Chapter 2

Energy transfers

When a candle burns, where does its energy go?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It moves out into the air and the room, as warmth and light.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It is used up, and then it is gone.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It stays inside the candle.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Describe how energy moves when a candle burns or a ball falls
Draw an energy transfer diagram

Light a candle and watch it. The wax slowly gets shorter. The flame gives light and warmth. The energy that was in the wax has gone somewhere. Where?

When energy moves from one store to another, we call it an energy transfer. Things happen when energy moves. A ball falls. A candle shines.

We count energy in joules, J. Lifting a small apple up one metre moves about 1 J into its gravitational store. A candle gives out about 80 J every second.

Energy moves in different ways. A force moves it when something pushes, pulls or falls. Heating moves it from hot to cool things. Light, sound and an electric current also move it.

Draw it first

Draw a ball falling

1 Draw a ball held high. All its energy is in its gravitational store: 10 J.
2 Let it fall. Halfway down, 5 J has moved into its kinetic store. 5 J is still in the gravitational store.
3 Just before it lands, all 10 J is in the kinetic store. The total has stayed at 10 J.

The ball moved its energy by a force. The pull of gravity is a force. It pulled the ball down and moved the energy from one store to the other.

See it three ways

As a picture

In words

Each bar is the same length. The energy is shared between two stores. As one store gets smaller, the other gets bigger by the same amount.

In symbols

At the top: 10 J + 0 J. Halfway: 5 J + 5 J. At the bottom: 0 J + 10 J. The total is always 10 J.

Worked example

A ball has 10 J in its gravitational store when it is held high. Halfway down, 5 J is still in that store. How much energy is in its kinetic store?

The total stays 10 J.
The gravitational store holds 5 J.
The kinetic store holds the rest: 10 - 5 = 5 J.

Answer: 5 J

Now the ball lands. The ball and the ground squash a little, and they get a tiny bit warmer. So some energy moves to the thermal store of the ball and the ground. A tiny part, far less than 1 J, goes out as sound, which you hear as a thud. A ball that bounces keeps the rest in its kinetic store.

This is a transfer diagram: stores in boxes, and the way the energy moves on each arrow.

Worked example

A ball has 20 J in its kinetic store just before it lands. It bounces up with 12 J in its kinetic store. How much went into the thermal store of the ball and ground?

The total stays 20 J.
12 J is back in the kinetic store.
The thermal store took the rest: 20 - 12 = 8 J.
Check: 12 + 8 = 20 J. That is the total again.

Answer: 8 J

The numbers for the second worked example.

A burning candle moves energy out of the wax. The room gets a little warmer.

The words by heating mean energy moves from a hotter thing to a cooler one. The flame is hot, so energy moves into the cooler air around it.

Try it

Feel a candle's energy leave

You need: a candle in a dish, matches, used only by an adult, a room with no draught.

Be safe: An adult lights and blows out the candle and stays close. Tie back long hair. Keep cloth and paper away from the flame. Never hold a hand above it.

Ask an adult to light the candle on a flat table and to stay with you.
Hold your hand beside the flame, about 20 cm away. Keep your palm facing the flame.
Notice the warmth on your palm. Move your hand away and notice it fade.
Look at the wall or table near the candle. See the light on it.
Ask the adult to blow the candle out.

What you will see: You feel warmth and you see light. Both are energy leaving the wax. Hold your hand beside the flame, never above it. Hot air rises there.

If you cannot try it, this is what happens.

A wrong idea many people have

A falling ball makes new energy as it speeds up.

Why it feels right: A falling ball goes faster and faster, so it looks as though it is gaining energy.

The test that breaks it: Look at the bars. At the top the ball has 10 J. Halfway it has 5 J + 5 J. At the bottom it has 10 J. The total never grew.

What is true: The ball does not gain energy. Energy moves from the gravitational store into the kinetic store.

Key words

energy transfer: energy moving from one store to another.
joules: the units we count energy in. We write J.
by a force: a way energy moves, when things push, pull or fall.
by heating: a way energy moves, from a hotter thing to a cooler one.
transfer diagram: boxes for stores, with arrows for the way energy moves.

Back to your guess

The answer is A: It moves out into the air and the room, as warmth and light.

The energy in the wax moves out of its chemical store. It goes into the air and the room, as warmth and as light. It does not disappear.

Keep it

Copy this into your exercise book.

Energy moves from one store to another. A falling ball moves energy from the gravitational store to the kinetic store, by a force. The total stays the same.

Draw: Three boxes joined by arrows: gravitational store, kinetic store, thermal store. Write the way on each arrow.