Coaching Centre
Energy transformations and flow diagrams
How energy is transferred and transformed, writing energy chains, drawing Sankey (energy flow) diagrams, useful and wasted energy, and measuring the energy in food with a simple calorimeter.
When you turn on an old-style light bulb, about 95% of the electrical energy becomes heat, not light. Touch one that has been on for a minute and you'll feel it (carefully). That's why these bulbs are now banned from sale in Australia: they were really small heaters that happened to glow.
Transfer or transformation?
Energy can do two things:
- It can be transferred: moved from one object to another without changing form. When a cricket bat hits a ball, kinetic energy is transferred from the bat to the ball. When you hold a hot mug, heat is transferred to your hand.
- It can be transformed: changed from one form into another. A torch transforms chemical energy (battery) into electrical energy and then into light and heat.
Most devices are energy converters. A speaker turns electrical energy into sound. A microphone does the reverse. A solar panel turns light into electrical energy, and a car engine turns the chemical energy in petrol into kinetic energy.
Energy chains
You can show a series of transformations as an energy chain (also called an energy flow diagram), with arrows from one form to the next.
Energy chains can stretch a long way back. The music on your phone started as energy from the sun, which plants trapped by photosynthesis millions of years ago. Those plants became coal, which was burned in a power station to make electricity, which charged your battery. Almost all the energy we use on Earth can be traced back to the sun. The main exceptions are nuclear energy, geothermal energy (heat from deep inside Earth) and tidal energy (mostly from the moon's gravity).
Useful and wasted energy
No energy transformation is perfect. Whenever energy changes form, some of it ends up as forms we didn't want, usually heat and sometimes sound. We call this wasted energy (or a by-product). It isn't destroyed; it just spreads out into the surroundings where it's no use to us.
So for any device:
energy in = useful energy out + wasted energy out
A good way to show this is a Sankey diagram. The width of each arrow is drawn in proportion to the amount of energy. The useful energy goes straight on; the wasted energy bends away. Because energy is conserved, the arrows coming out must add up to the width of the arrow going in.
- Pick the old light bulb. Before you reveal, move the slider to guess what percentage of the energy becomes light.
- Compare a petrol car with an electric car. Where does most of a petrol car's energy go?
- Which device on the list is the most efficient? Why do you think heating devices do so well?
- A coal power station looks inefficient. Why might that matter even more than an inefficient light bulb?
Measuring the energy in food
Food energy is chemical potential energy. You can estimate how much is in a snack by burning it under a can of water and measuring how much the water heats up. This set-up is a simple calorimeter. It takes 4.2 J of energy to heat 1 mL of water by 1 °C, so:
energy (J) = volume of water (mL) × 4.2 × temperature rise (°C)
A 1.0 g corn chip is burned under a can holding 50 mL of water. The water warms from 20 °C to 46 °C.
46 − 20 = 26 °C
50 × 4.2 × 26 = 5460 J, or about 5.5 kJ per gram.
The packet says corn chips contain about 22 kJ per gram, so the calorimeter caught only a quarter of the energy. Much of the heat warmed the air, the can and the thermometer instead of the water, and some chip may not have burned completely.
Test yourself
What is the main energy transformation in a solar panel?
Solar panels convert light directly into electricity.
A motor takes in 200 J of electrical energy and gives out 150 J of kinetic energy. How much energy is wasted?
200 − 150 = 50 J, mostly as heat and sound.
In a Sankey diagram, the width of each arrow shows
Wider arrows mean more energy.
Which is an energy transfer rather than a transformation?
Heat moves from the pie to your hands; it stays as heat.
50 mL of water is heated from 20 °C to 30 °C by burning a nut. The energy gained by the water is
50 × 4.2 × 10 = 2100 J.
The original source of the energy in coal is
Ancient plants stored the sun's energy by photosynthesis.
Put the energy chain for a coal-fired power station in order.
A hair dryer is switched on. Sort each output.
When energy changes form it is . When it moves from one object to another it is . Energy that ends up in a form we don't want is called energy, and it is usually . In a diagram, the arrow widths show the amount of energy.
A television takes in 100 J of electrical energy each second. It gives out 20 J of light, 10 J of sound and the rest as heat. Describe how you would draw a Sankey diagram for the television, and explain why the outputs must add up to 100 J.
The heat output is 100 − 20 − 10 = 70 J. I would draw one arrow coming in labelled 100 J electrical energy. It would split into three arrows: light (20 J) and sound (10 J) going straight on as the useful outputs, and heat (70 J) bending downwards as wasted energy. Each arrow's width is in proportion to its energy, so the heat arrow is 7 times as wide as the sound arrow, and the three output arrows together are as wide as the input arrow.
The outputs must add up to 100 J because of the law of conservation of energy. Energy cannot be created or destroyed in the television; it can only be transformed into other forms.