Every card in Energy, Year 8: more practice
The whole deck, in order — so you can read it through before your child ever sees it.
- A 60 W bulb is on for 10 seconds. How many joules of energy does it transfer? (number only)
600
HintEnergy transferred is power multiplied by time.
Why60 W is 60 J every second, and 60 × 10 = 600 J.
- A phone charger is rated 10 W and a hairdryer 1800 W. Which transfers energy faster?
The hairdryer
HintWatts measure how quickly energy is transferred.
WhyA higher power rating means more joules every second, so the hairdryer transfers energy about 180 times as quickly.
- A 3 kW electric heater runs for 2 hours. How many kilowatt-hours of energy does it transfer? (number only)
6
HintEnergy in kWh = power in kW × time in hours.
Why3 kW × 2 h = 6 kWh.
- Electricity costs 30 p per kilowatt-hour. How much, in pence, does 6 kWh cost? (number only)
180
HintCost = number of units × price per unit.
Why6 × 30 = 180 p, which is £1.80.
- Why does a metal spoon feel colder to touch than a wooden spoon at the same temperature?
The metal conducts energy away from your hand faster
HintThink about how fast heat leaves skin.
WhyMetals are good conductors, so they carry energy away from your warm fingers quickly. Wood is an insulator, so it does not.
- Why can convection not happen in a solid?
Its particles cannot flow
HintWhat has to move for convection to carry energy?
WhyConvection needs a fluid whose particles can flow and carry energy with them. In a solid the particles only vibrate in place.
- How does energy from the Sun reach the Earth across empty space?
By radiation
HintNothing is there to be heated and passed along.
WhyRadiation does not need particles to travel through, unlike conduction and convection.
- What kind of invisible radiation do hot objects give out?
Infrared radiation
HintIt is below red in the spectrum.
WhyAll objects emit infrared. The hotter an object is, the more it emits.
- Why are houses in very hot countries often painted white?
White surfaces reflect most of the radiation
HintThink of what a light, shiny surface does with the Sun's infrared.
WhyWhite surfaces are poor absorbers, so less radiation is absorbed and the inside stays cooler.
- An emergency foil blanket is wrapped round a cold casualty with the shiny side inwards. Why?
It reflects the person's own infrared back
HintThink about what a shiny surface does with radiation.
WhyA shiny surface is a poor emitter and a good reflector of infrared, so energy radiated by the body is reflected back rather than lost.
1★ KS3-PHYS-ENE-0040
2★ KS3-PHYS-ENE-0041
3★ KS3-PHYS-ENE-0042
4★ KS3-PHYS-ENE-0043
5★ KS3-PHYS-ENE-0044
6★ KS3-PHYS-ENE-0045
7★ KS3-PHYS-ENE-0046
8★ KS3-PHYS-ENE-0047
9★ KS3-PHYS-ENE-0048
10★ KS3-PHYS-ENE-0049
Keep what you learn
Here, nothing is saved. In your child’s own sky every card is scheduled — it comes back just before they’d forget it — and the professor who wrote it is one tap away.