Energy, Year 8: more practice：全部卡片
整套按顺序列出——孩子看到之前，您可以先通读一遍。
- A 60 W bulb is on for 10 seconds. How many joules of energy does it transfer? (number only)
600
提示Energy transferred is power multiplied by time.
为什么60 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
提示Watts measure how quickly energy is transferred.
为什么A 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
提示Energy in kWh = power in kW × time in hours.
为什么3 kW × 2 h = 6 kWh.
- Electricity costs 30 p per kilowatt-hour. How much, in pence, does 6 kWh cost? (number only)
180
提示Cost = number of units × price per unit.
为什么6 × 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
提示Think about how fast heat leaves skin.
为什么Metals 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
提示What has to move for convection to carry energy?
为什么Convection 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
提示Nothing is there to be heated and passed along.
为什么Radiation does not need particles to travel through, unlike conduction and convection.
- What kind of invisible radiation do hot objects give out?
Infrared radiation
提示It is below red in the spectrum.
为什么All 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
提示Think of what a light, shiny surface does with the Sun's infrared.
为什么White 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
提示Think about what a shiny surface does with radiation.
为什么A shiny surface is a poor emitter and a good reflector of infrared, so energy radiated by the body is reflected back rather than lost.
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