What a learner can do afterwards
- Calculates the power of an appliance from its voltage and current, or from its current and resistance
- Works out energy transferred in joules and in kilowatt-hours over a stated time
- Costs a week of using an appliance given a price per kilowatt-hour
1 · Read
Power is how fast energy moves, measured in watts. In a circuit it equals voltage times current, and for a resistor that also gives current squared times resistance. A 230 V kettle pulling 10 A runs at 2300 W, all ending up heating the water.
Energy is power stretched over time. Multiply watts by seconds to get joules. Bills use a bigger unit, the kilowatt hour: 1000 W running for one hour, which is 3.6 million joules.
A 2 kW heater running 3 hours a day uses 6 units a day. Over a week that is 42 units. Multiply 42 by the price per unit on the bill to cost the week.
Read watts off the label, then divide by 1000 to get kilowatts before multiplying by hours. Forgetting that step makes the answer 1000 times too big. High power devices need thick wiring, because big currents heat thin wires.
Volts times amps gives watts, watts times hours gives units, and units times price gives the bill.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.