Worked example
A motor receives 100 J and transfers 40 J to useful lifting. What is its efficiency for lifting?
Answer & why
Answer: 40%
40 ÷ 100 × 100% = 40%.
Grade 9 · Science
Account for useful and transferred energy
Use a stated system boundary to calculate efficiency and account for energy transferred to other forms.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
Two motors each receive 200 J. A gives 100 J of useful output; B gives 140 J. Which is more efficient for that output?
- Neither has any useful output.
- They have equal efficiency.
- Motor B
- Motor A
A hint
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
Answer & why
Answer: Motor B
B provides 70% useful output; A provides 50%.
Question 2
A device receives 50 J and a report claims 70 J of output with no other energy source or stored-energy change. What should be questioned?
- Whether joules can measure energy.
- Whether 70 is greater than 50.
- Whether output must always be zero.
- The measurements or completeness of the energy account.
A hint
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
Answer & why
Answer: The measurements or completeness of the energy account.
The stated account supplies less energy than its claimed output.
Question 3
A heater receives 300 J and transfers 270 J to the room being heated. For this stated purpose, what is the efficiency?
- 10%
- 30%
- 90%
- 111%
A hint
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
Answer & why
Answer: 90%
270 ÷ 300 × 100% = 90%.
Question 4
A moving cart slows as friction warms its wheels and track. Which statement respects energy conservation?
- The cart's speed becomes negative energy.
- Friction creates unlimited new energy.
- Some kinetic energy is transferred into thermal energy.
- All energy ceases to exist.
A hint
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
Answer & why
Answer: Some kinetic energy is transferred into thermal energy.
The energy changes distribution and form rather than disappearing.
Question 5
A motor receives 100 J and transfers 40 J to useful lifting. What is its efficiency for lifting?
- 40%
- 4%
- 140%
- 60%
A hint
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
Answer & why
Answer: 40%
40 ÷ 100 × 100% = 40%.
Question 6
A lamp receives 80 J. It transfers 20 J as useful visible light and the remaining energy as heat. How much is transferred as heat?
- 100 J
- 4 J
- 20 J
- 60 J
A hint
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.
Answer & why
Answer: 60 J
80 − 20 = 60 J; it remains part of the energy account.