Watts, units, and the bill

Science · Energy · ages 14-15

Name ______________________ Date ____________

One kilowatt hour equals 3.6 million joules.
Circle one: True False
A lamp runs at 12 V and draws 2 A. Which calculation gives its power?
- 12 times 2
- 12 divided by 2
- 12 plus 2
For a resistive heater, which formula also gives power?
- Voltage divided by current
- Current squared times resistance
- Current divided by resistance
A 2 kW heater runs 3 hours a day. How many kilowatt hours does it use in one day? Type the number.
Answer: ______________
A 230 V kettle pulls 10 A. What is its power in watts? Type the number.
Answer: ______________
That heater runs 3 hours a day for a week. The unit price is 25 p. Which calculation costs the week?
- 6 times 25 p
- 42 times 25 p
- 2 times 3 p
Two heaters run the same hours. One is 1 kW, the other 3 kW. Why does the 3 kW one cost more and need thicker cable?
- It uses a higher voltage
- It runs for longer each evening
- It draws a bigger current, which heats thin wires
A learner works out a TV bill as thousands of units for one evening. What is the likely mistake?
- They used current squared instead of voltage times current
- They mixed up watts and kilowatts, skipping the divide by 1000
- They multiplied by 3.6 million

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · 1000 W for 3600 seconds multiplies out to 3.6 million joules.
12 times 2 · Power is voltage times current.
Current squared times resistance · Heating grows with current squared times resistance.
6 · Multiply kilowatts by hours: 2 times 3 is 6.
2300 · Multiply voltage by current: 230 times 10 is 2300.
42 times 25 p · 6 units a day times 7 days is 42 units, then times the unit price.
It draws a bigger current, which heats thin wires · More power at the same voltage means more current and more heating.
They mixed up watts and kilowatts, skipping the divide by 1000 · Skipping the divide by 1000 inflates the answer 1000 times.