Energy transfers
Energy is never lost: it changes from one form into another. Almost always, part of it ends up as heat that you can no longer use.
Why
Why does a ball you drop never bounce back higher than the height you let go of it from?
Drop a ball and watch closely. After the first bounce it comes up less high, after the second even less, until it lies still. It never jumps higher on its own than where it started.
At the top, the ball had a store of energy because of its height. As it falls, that energy is turned into motion. With every bounce, part of it becomes heat and sound. That energy has not vanished, but the ball can no longer use it to jump back up.
See it
Use the slider to choose how high on the slope the ball starts and switch friction on or off. Follow the bars: gravitational potential energy, kinetic energy and heat always add up to the same total.
- gravitational energy: 0 J
- kinetic energy: 1470 J
- heat: 0 J
Description of the picture
A ball rolls down a slope and up the other side. Three bars show the gravitational potential energy, the kinetic energy and the heat. At the top almost everything is gravitational potential energy, at the bottom almost everything is kinetic energy. Without friction the ball comes back up just as high. With friction the heat bar grows and the ball comes up less high each time. Together the three bars always stay the same size.
Get it
Energy is what is needed to make something happen: to make something move, warm up, burn or grow. You cannot see energy, but you can notice what it does.
Energy comes in different forms. Anything that moves has kinetic energy. Something that is high up and can fall has gravitational potential energy. Heat, light and sound are forms of energy too. Electrical energy comes from the plug socket or a battery. Chemical energy is stored in food, fuel and batteries.
Whenever something happens, energy is transformed. A lamp turns electrical energy into light and heat. Your muscles turn chemical energy from your food into kinetic energy. A wind turbine turns the kinetic energy of the wind into electrical energy.
The most important rule: energy cannot be created out of nothing and cannot disappear. This is called the law of conservation of energy. But with every transformation, some of it goes into a form you did not want, usually heat. The efficiency tells you what share is put to useful work. An LED lamp has a much higher efficiency than an old incandescent bulb.
How much electrical energy an appliance uses depends on its power and how long it runs: E=
Energy is not lost, it is transformed. E=
Worked example
A 2000 W kettle is switched on for 3 minutes. How much electrical energy does it use in kWh?
Convert the power to kilowatts.
2000W=
2kW Convert the time to hours.
3min=
3/60= 0.05h Use the formula.
E=
P× t= 2× 0.05= 0.1 - Answer
The kettle uses 0.1 kWh.
Do it
Pick a level. You get a new exercise every time, checked straight away.
Misconceptions
Wrong: Energy runs out or disappears.
When your battery is flat, the energy has not gone. It has been turned into light, motion and above all heat in the surroundings.
Right: Energy is transformed, never destroyed. What runs out is energy in a form we can use.
Wrong: An appliance with a high power always uses a lot of energy.
A kettle has a high power but is only on for a few minutes. A fridge has a low power but runs day and night.
Right: Energy use depends on the power and on the time: E=
P× t. Wrong: With a clever machine you can get more energy out than you put in.
Then energy would come out of nothing. That has never been measured, and every machine always loses a little as heat.
Right: The efficiency is always less than 100 per cent.
Try it at home
The ball that never gets higher
What you need
- a bouncy ball or a tennis ball
- a tape measure or a ruler
- a smooth, hard floor
- a helper
- paper and a pencil
How to do it
- Ask your helper to hold the tape measure upright against a wall.
- Hold the ball at exactly 1 m high and let go, without throwing it.
- Watch how high the ball comes back up after the first bounce and write it down.
- Repeat this three times and work out the average.
- Try it on a carpet or on grass as well and compare.
Safely: Work in a room where nothing can break and nobody can trip. Do not climb on a chair to drop the ball from higher up.
What do you see, and why?
The ball never comes back higher than 1 m, and on a soft carpet even less high. With every bounce, part of the energy is turned into heat and sound, so there is less left to jump back up.
In real life
- A roller coaster car at the fair is first pulled up high. At the top it has a lot of gravitational potential energy, which turns into speed on the way down.
- The wind turbines off the coast of Ostend turn the motion of the wind into electrical energy for thousands of households.
- The energy bill at home shows how many kWh your family has used. Every kWh costs money, which is why it pays to switch off the lights.
- When you brake on your bike, the brake pads get warm. Your kinetic energy turns into heat.
History
In the nineteenth century the English brewer and physicist James Prescott Joule showed with experiments that motion can be turned into heat. This made it clear that heat is a form of energy. The unit of energy, the joule, is named after him.
At the table
Which appliances in our home turn electrical energy into heat, and which into motion?
A question to talk about together, for parents and children. There is no wrong answer.