Forms of Energy and Energy Transformations — Answers
Answer sheet with model answers and success criteria for the 8th Grade energy, light, sound & electricity worksheet.
Success TutoringWorksheetsAnswer sheet8th Grade · Science · Energy, Light, Sound & ElectricityForms of Energy and Energy Transformations — Answers
Part A: Forms of energy
Energy is measured in joules (J). It comes in several forms:
Kinetic energy is the energy of a moving object. Gravitational potential energy is stored in an object because of its height above the ground. Elastic potential energy is stored in something stretched or squashed, such as a spring. Chemical energy is stored in food, fuels and batteries. Electrical energy is carried by an electric current. Thermal (heat) energy comes from the movement of particles in a substance. Light and sound energy travel outwards from their sources.
The law of conservation of energy says that energy cannot be created or destroyed. It can only be transferred from one object to another or transformed from one form into another. The total amount of energy stays the same.
Devices rarely turn all of their energy into the form we want. The rest is wasted energy, usually thermal energy and sound that spread into the surroundings. Efficiency = useful energy output ÷ total energy input × 100%.
- A rolling soccer ball
- A book sitting on a high shelf
- A stretched rubber band
- Food, fuel or a battery
- A hot cup of soup
- Current flowing through a wire
- Kinetic energy
- Gravitational potential energy
- Chemical energy
- Electrical energy
- Thermal energy
- Elastic potential energy
Answer: A rolling soccer ball → Kinetic energy; A book sitting on a high shelf → Gravitational potential energy; A stretched rubber band → Elastic potential energy; Food, fuel or a battery → Chemical energy; A hot cup of soup → Thermal energy; Current flowing through a wire → Electrical energy
Part B: Energy transformation chains
Write the main energy transformation for each, using arrows. Example: a wood fire: chemical → thermal + light.
Answer: Chemical → electrical → light (with some thermal energy wasted)
Answer: Electrical → thermal (with some sound)
Answer: Light → electrical
Answer: Gravitational potential → kinetic
Answer: Electrical → kinetic (with some sound and thermal energy wasted)
Answer: Chemical (from food) → kinetic → gravitational potential (with thermal energy as the person warms up)
Answer: Electrical → kinetic (the vibrating cone) → sound
Answer: Elastic potential → kinetic
Part C: Conservation of energy calculations
Show your working. Remember: total energy in = total energy out.
Answer: 85 J, mostly thermal energy.
- Energy in = energy out
- 100 J − 15 J = 85 J wasted, mainly as thermal energy
Answer: 15%
- Efficiency = useful output ÷ total input × 100%
- 15 ÷ 100 × 100% = 15%
Answer: 60 J of thermal energy; 40% efficient.
- 100 J − 40 J = 60 J
- 40 ÷ 100 × 100% = 40%
Answer: 350 J; 70% efficient.
- 500 J − 150 J = 350 J useful
- 350 ÷ 500 × 100% = 70%
Answer: 20 J. All of the gravitational potential energy is transformed into kinetic energy.
Answer: About 2 J was transformed into thermal energy (and a little sound) by air resistance as the ball pushed through the air. The energy was not destroyed.
Part D: Check your understanding
Write T for true or F for false, or circle the best answer.
Answer: False
Energy is never destroyed. The chemical energy in the fuel or battery is transformed into kinetic energy, thermal energy and sound.
Answer: True
Heat flows from hot to cold until both reach the same temperature.
Answer: True
At the lowest point it is moving fastest. At the highest points it stops for an instant, so its kinetic energy is zero and its gravitational potential energy is greatest.
- ADevice A: 200 J in, 50 J useful out
- BDevice B: 100 J in, 30 J useful out
- CDevice C: 400 J in, 80 J useful out
- DDevice D: 50 J in, 20 J useful out
Answer: Device D: 50 J in, 20 J useful out
A = 50 ÷ 200 = 25%; B = 30 ÷ 100 = 30%; C = 80 ÷ 400 = 20%; D = 20 ÷ 50 = 40%. Device D is the most efficient.
Part E: Explain
Model answer: As the ball falls, gravitational potential energy is transformed into kinetic energy. When it hits the floor and squashes, kinetic energy is stored as elastic potential energy, which is transformed back into kinetic energy as the ball springs up, and then into gravitational potential energy as it rises. At each bounce, some energy is transformed into thermal energy (the ball and floor warm slightly) and sound, and some is lost to air resistance. Energy is not destroyed, but less of it is left as gravitational potential energy each time, so each bounce is lower.
Success criteria:
- Describes gravitational potential → kinetic energy as the ball falls
- Mentions elastic potential energy when the ball squashes
- Identifies thermal energy and sound as wasted energy at each bounce
- States that the total energy is conserved, not destroyed
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
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