PSLE Science · Primary 5 and 6
Energy Forms and Conversion
Energy is needed for everything that happens, from a ball rolling to a plant growing. This topic, part of the Energy theme in PSLE Science, covers the different forms of energy, the Sun as the main source of energy on Earth, and how energy is converted from one form to another. It brings together ideas from light, heat, electricity and photosynthesis, so it is a good topic to revise last. Questions often ask you to trace a chain of energy conversions or explain an experiment with a ramp or a spring.
What the syllabus expects
- Recognise that energy is needed to do work and that the Sun is the main source of energy on Earth
- Identify kinetic, gravitational potential, elastic potential, chemical potential, electrical, light, heat and sound energy
- Describe the energy conversions in everyday objects and activities
- Explain that energy cannot be created or destroyed, only converted from one form to another
- Explain how plants convert light energy into chemical potential energy and how animals get energy from food
- Explain experiments on how the height of release or the amount of stretching affects how far an object moves
Forms of energy
Energy is needed to do work, such as moving, lifting or making things change. Kinetic energy is the energy a moving object has, and the faster an object moves, the more kinetic energy it has. Gravitational potential energy is stored in an object because of its height above the ground. The higher it is, the more gravitational potential energy it has. Elastic potential energy is stored in an object that is stretched or squeezed, such as a stretched rubber band or a compressed spring. Chemical potential energy is stored in food, in fuels such as petrol and in batteries. Electrical energy is carried by electricity. Light, heat and sound are also forms of energy, given out by objects such as lamps, fires and vibrating guitar strings.
The Sun and energy in living things
The Sun is the main source of energy on Earth. During photosynthesis, plants convert light energy from the Sun into chemical potential energy stored in the food they make. Animals cannot make their own food, so they get chemical potential energy by eating plants, or by eating other animals that ate plants. Inside the body, the chemical potential energy in food is converted into kinetic energy when we move and into heat energy that keeps us warm. Fossil fuels such as coal, oil and natural gas formed from the remains of plants and animals that lived a very long time ago, so the chemical potential energy in them also came from the Sun. Power stations burn fuels to generate the electrical energy we use at home.
Energy conversion
Energy cannot be created or destroyed. It can only be converted from one form to another. In a torch, chemical potential energy in the batteries is converted into electrical energy, which the bulb converts into light energy and heat energy. In an electric fan, electrical energy is converted mainly into kinetic energy, with some sound and heat energy. A solar cell converts light energy into electrical energy. When a ball is dropped, its gravitational potential energy is converted into kinetic energy as it falls. When it hits the ground, some energy is converted into sound and heat energy, so the ball cannot bounce back to its starting height, and each bounce is lower than the one before. In a catapult, the elastic potential energy of the stretched rubber is converted into kinetic energy of the stone. In almost every conversion, some energy is converted into heat or sound that is not useful for the task.
Experiments with ramps and springs
PSLE questions often test energy using a ramp, a rubber band or a spring. When a toy car is released from a higher point on a ramp, it has more gravitational potential energy at the start. More energy is converted into kinetic energy, so the car moves faster at the bottom and travels further along the floor. When the rubber band of a catapult is stretched more, more elastic potential energy is stored, so the object is launched faster and travels further. The car eventually stops because friction between its wheels and the floor converts its kinetic energy into heat and sound energy. In a fair test, change only one variable, such as the height of release, and keep the rest the same: the same car, the same ramp and the same floor. Repeat each test and take the average to make the results more reliable.
Worked example
Wei Ling released a toy car from different heights on a ramp and measured how far it travelled along the floor. From 10 cm it travelled 40 cm, and from 30 cm it travelled 120 cm. From 20 cm, three trials gave 78 cm, 80 cm and 82 cm. (a) Calculate the average distance for the 20 cm height. (b) Describe the relationship between the height of release and the distance travelled, and explain it using energy. (c) Predict the distance for a height of 25 cm. (d) Why does the car stop in the end?
- For (a), add the trials and divide by the number of trials: (78 + 80 + 82) ÷ 3 = 240 ÷ 3 = 80 cm.
- For (b), read the trend: the higher the release point, the further the car travels (10 cm gives 40 cm, 20 cm gives 80 cm, 30 cm gives 120 cm).
- Explain with energy: at a greater height the car has more gravitational potential energy. More of it is converted into kinetic energy, so the car moves faster and travels further.
- For (c), spot the pattern: each distance is 4 times the height (4 × 10 = 40, 4 × 20 = 80, 4 × 30 = 120), so for 25 cm, 4 × 25 = 100 cm.
- For (d), friction between the wheels and the floor converts the car's kinetic energy into heat and sound energy until it has no kinetic energy left.
Answer: (a) 80 cm. (b) The higher the release point, the further the car travels, because it has more gravitational potential energy, which is converted into more kinetic energy. (c) 100 cm. (d) Friction converts its kinetic energy into heat and sound energy, so it slows down and stops.
Common mistakes
- Saying energy is 'used up' or 'lost'. Energy is converted into other forms, often heat and sound that are not useful for the task.
- Calling the energy in food or batteries 'electrical energy' or just 'potential energy'. It is chemical potential energy.
- Saying a ball at the top of its bounce has the most kinetic energy. At the highest point it stops for a moment, so its energy there is mainly gravitational potential energy.
- Leaving out the Sun when tracing energy in food or fossil fuels. The chemical potential energy in both came from light energy from the Sun.
- Writing only the first and last forms of energy. Show each conversion in order, for example chemical potential energy to electrical energy to light and heat energy.
Exam tips
- Write energy conversions as a chain using 'is converted into', and name every form, including the heat and sound that are not useful.
- When a moving object slows down or stops, mention friction or air resistance converting kinetic energy into heat (and sound) energy.
- In ramp and catapult experiments, link the variable you changed to the energy stored at the start, then to kinetic energy, then to the distance travelled.
Practice questions
1. Easy: State the energy conversion that takes place in a solar cell on a calculator.
Show answer
Light energy is converted into electrical energy. The electrical energy then makes the calculator work.
2. Medium: A ball dropped from a height of 100 cm bounces back up to only 60 cm. Explain why it does not reach 100 cm.
Show answer
As the ball falls, its gravitational potential energy is converted into kinetic energy. When it hits the ground, some of this energy is converted into sound and heat energy. Less energy is left to be converted back into gravitational potential energy, so it cannot rise as high as 100 cm.
3. Harder: Trace the energy conversions from the Sun to a boy running after eating a plate of rice.
Show answer
Light energy from the Sun is converted into chemical potential energy in the rice plant during photosynthesis. The boy eats the rice and gets this chemical potential energy. When he runs, his body converts the chemical potential energy into kinetic energy, and also into heat energy.
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