These eight questions use invented devices and run from a single power-time calculation to a fair comparison. Write each answer on paper before opening the explanation.
The set covers the electricity and energy cluster. The timed practice session builder can run it against a clock.
Questions
Question 1 (2 marks). An invented heater is rated 1 500 W and runs for 2 hours. Find the energy in kWh.
Answer
Energy = 1 500 × 2 = 3 000 Wh = 3.0 kWh.
Question 2 (2 marks). A 60 W lamp runs for 45 minutes. Find the energy in Wh.
Answer
45 min = 0.75 h. Energy = 60 × 0.75 = 45 Wh.
Question 3 (3 marks). A room has a 40 W fan running 5 h and a 100 W monitor running 3 h. Find the total energy in kWh.
Answer
Fan: 40 × 5 = 200 Wh. Monitor: 100 × 3 = 300 Wh. Total = 500 Wh = 0.50 kWh.
Question 4 (2 marks). With the total from question 3 and a rate of RM0.40 per kWh, find the cost.
Answer
Cost = 0.50 × 0.40 = RM0.20, assuming the rate given.
Question 5 (3 marks). A diagram shows an input of 4.0 kJ, a useful output of 2 600 J and a wasted output of 1.4 kJ. Check that it balances and find the efficiency.
Answer
Convert: input 4 000 J, wasted 1 400 J. Balance: 2 600 + 1 400 = 4 000 J, so it balances.
Efficiency = 2 600 ÷ 4 000 × 100% = 65%.
Question 6 (2 marks). A student reports an efficiency of 115% for an invented motor. State why this cannot be right.
Answer
It would mean more useful energy came out than went in, which breaks conservation of energy. The result suggests a measurement error or an uncounted energy source.
Question 7 (3 marks). An invented heater takes in 800 J and gives 600 J of useful heat. Calculate the efficiency and the wasted energy.
Answer
Efficiency = 600 ÷ 800 × 100% = 75%. Wasted energy = 800 − 600 = 200 J.
Question 8 (5 marks). Two invented pumps run for 3 hours a day for 4 days. Pump X is 120 W and pump Y is 90 W. Compare their energy and state the assumptions.
Answer
Hours: 3 × 4 = 12 h each. Pump X: 120 × 12 = 1 440 Wh = 1.44 kWh. Pump Y: 90 × 12 = 1 080 Wh = 1.08 kWh.
Pump X uses 0.36 kWh more. Assumptions: same hours, same days, and both pumps move the same amount of water, which was not stated.
If you got these wrong
Missed 1 to 4: revisit calculating energy from a device table. Missed 5: go to interpreting an energy-flow diagram.
Missed 6 and 7: read why efficiency cannot exceed the input. Missed 8: see comparing two energy systems.
Log each error in the mistake log and paper-error review. For a teacher who marks your working, see online one-to-one Science tuition.