A bicycle rolls to a stop. Where did its energy go?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain that energy is not made or destroyed, only moved
Explain why some energy always ends up as wasted heat
Kofi rides along a flat road and stops pedalling. The bicycle slows down and stops. Its kinetic store is empty. Has the energy gone?
Here is one of the biggest ideas in science. Energy is never made. Energy is never destroyed. It only moves from one store to another.
The bicycle stops, but its energy has only moved. The ground, tyres and air are a little warmer.
Every time energy moves, some of it spreads out into the surroundings. These are the air, the ground and everything else nearby. The surroundings get a little warmer.
Draw it first
Draw where a bulb's energy goes
1 A small torch bulb takes in 100 J from its cell. Draw an arrow going in.
2 Draw two arrows going out. 10 J leaves as light. 90 J leaves as heat.
3 Add up the energy coming out: 10 + 90 = 100 J. It is the same as the energy that went in.
We want light from a bulb. The light is its useful energy. The heat is wasted energy. Wasted does not mean destroyed. It means the energy went where we cannot use it.
An energy flow diagram. Each arrow is as wide as its joules. The two arrows out are together as wide as the arrow in.
The wasted heat warms the air a tiny bit. It spreads out so thin that we cannot use it again. It is still there. It is wasted, not used up.
See it three ways
As a picture
In words
For every thing, the useful energy and the wasted energy add up to the energy that went in. None of it is lost. The body moves less than it takes in, and the rest warms you.
In symbols
energy in = useful out + wasted out. For the bulb: 100 J = 10 J + 90 J.
We can say how good a thing is at its job. This is its efficiency. Efficiency = useful energy out ÷ energy in × 100. We write it as a percentage, %.
A toy motor with a small cell. The numbers for the first worked example.
Worked example
A toy motor takes in 50 J. 30 J moves the toy. The rest warms the motor and the air. How much is wasted, and what is the efficiency?
Wasted energy = energy in - useful energy.
Wasted = 50 - 30 = 20 J.
Efficiency = useful ÷ in × 100.
Efficiency = 30 ÷ 50 × 100 = 60%.
Answer: 20 J wasted, efficiency 60%
A fan. It wastes energy in two places, in the motor and at the axle. It also hums, but far less than 1 J leaves as sound.
Worked example
A fan takes in 200 J. 100 J turns the blades. 70 J warms the motor. The rest warms the axle by friction. What is its efficiency?
Useful energy out = 100 J.
Axle = 200 - 100 - 70 = 30 J.
Wasted = 70 + 30 = 100 J.
Efficiency = 100 ÷ 200 × 100 = 50%.
Check: 100 + 100 = 200 J, the same as the energy in.
Answer: 50%
Try it
Rub your hands
You need: your two hands.
Be safe: Rub gently enough that your skin does not hurt.
Hold your palms against your cheeks. Notice how warm they feel.
Rub your palms together fast for 15 seconds.
Press your palms to your cheeks again. Notice the difference.
What you will see: Your hands are warmer. Your moving hands had energy in a kinetic store. Friction moved it into a thermal store. The energy did not vanish.
If you cannot try it, this is what happens.
A wrong idea many people have
Wasted energy is destroyed.
Why it feels right: We cannot use it or find it again, so it seems to have gone.
The test that breaks it: Look at the bulb. 100 J goes in. 10 J is light and 90 J is heat. Add them up and you get 100 J. Nothing is missing.
What is true: Wasted energy is still there. It has spread into the surroundings and warmed them a tiny bit. It is too thin to use.
Key words
surroundings
the air, the ground and everything else near a thing.
useful energy
the energy that does the job we want.
wasted energy
energy that spreads into the surroundings, mostly as heat.
efficiency
useful energy out ÷ energy in × 100, as a percentage.
energy flow diagram
a picture where each arrow is as wide as its joules.
Back to your guess
The answer is C: It spreads out into the ground, the tyres and the air, as a little warmth.
The energy did not vanish. Friction moved it into the thermal store of the ground, the tyres and the air. They got a tiny bit warmer, too little to feel.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
A ball rolls along the ground and stops. What has happened to its energy?
See the answer
It has moved to the thermal store of the ball, the ground and the air.
Friction moves the energy into thermal stores. The energy is still there, spread out as a tiny bit of warmth.
If you chose B: Energy is never destroyed. It only moves to another store. Go to S3.8.3.
If you chose C: The ball did not go higher, so its gravitational store did not grow. Friction warmed the ground and the ball. Go to S3.8.3.
A small lamp takes in 60 J. It gives out 6 J as light. How much energy is wasted as heat?
See the answer
54 J
Wasted = energy in - useful = 60 - 6 = 54 J.
If you chose A: You added. Wasted energy is what is left after the light, so subtract. Go to S3.8.3.
If you chose B: That is the light, which is the useful part. The wasted part is the rest. Go to S3.8.3.
If you chose C: You divided. To find what is left, subtract the light from the energy in. Go to S3.8.3.
A motor takes in 80 J and gives 20 J of useful energy. What is its efficiency?
