Science · Grades 6–10
Energy transfers and useful output
Identify where energy enters, is stored and is transferred out.
Understand the idea
Track the system
Identify where energy enters, is stored and is transferred out.
Conservation
Energy is conserved overall, even when useful output is less than input.
Efficiency is a ratio
Use consistent units when comparing useful output with total input.
Worked examples
A device receives 250 J and delivers 75 J as useful output. Find its efficiency as a percentage.
75/250 × 100 = 30%.
30
In a simple model of a falling object, gravitational potential energy can become
Motion increases as gravitational potential energy decreases.
kinetic energy
Try it yourself
Why is energy wasted in a device not the same as energy destroyed?
A force of 12 N acts along a displacement of 3 m. Calculate the work in J.
A machine transfers 480 J in 8 s. Calculate average power in W.
Which unit measures energy?
- second
- watt
- metre
- joule
Answers and marking notes
Answers and marking notes
- It is transferred to less useful stores or surroundings, often as thermal energy, while total energy is conserved.
Credit conservation and the usefulness distinction.
- 36
For constant aligned force, work = force × displacement.
- 60
Power = energy/time = 480/8.
- joule
The watt measures power, not energy.
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