Energy, Year 8: more practice

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.

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.

6

提示Energy in kWh = power in kW × time in hours.

为什么3 kW × 2 h = 6 kWh.

180

提示Cost = number of units × price per unit.

为什么6 × 30 = 180 p, which is £1.80.

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.

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.

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.

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.

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.

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.