Radiation — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

Section 1 of 9Thermal radiation

Thermal radiation

Thermal radiation infrared radiation, part of the electromagnetic spectrum, emitted by all objects.

Temperature

• Temperature the hotter an object is, the more thermal radiation it emits.

No medium needed

• No medium needed radiation is the only way thermal energy can travel through a vacuum, which is how energy reaches us from the Sun.

Speed

• Speed it travels at the speed of light.

Section 2 of 9Effect of colour and texture

Surface	Absorbs	Emits	Reflects
Dull black	Very well	Very well	Poorly
White	Poorly	Poorly	Well
Dull	Reasonably	Reasonably	Poorly
Shiny	Very poorly	Very poorly	Well

Black surfaces

• Black surfaces they are the best absorbers and emitters, so black clothes feel hotter in the Sun and car radiators are painted black.

Shiny surfaces

• Shiny surfaces they reflect radiation, so they absorb little and emit little and take longer to cool.

Section 3 of 9Investigating emission from different surfaces

1

Set up four identical flasks painted black, dull grey, white and silver.

2

Fill each with the same volume of hot water at the same starting temperature.

3

Record the temperature of each flask every 30 s for 10 minutes.

4

Plot temperature (y) against time (x) for each flask.

Conclusion

• Conclusion the flask that cools fastest has the best-emitting surface, which is the dull black one.

Control variables

• Control variables same flasks, same volume and temperature of water, same time interval and same room temperature.

Why it works

• Why it works conduction and convection losses are equal for each flask, so any difference is due to radiation.

Safety

• Safety wear goggles and keep water away from electrical equipment.

Section 4 of 9Factors affecting radiation emitted

Factors affecting radiation emitted

Temperature

• Temperature hotter objects emit more thermal radiation.

Surface area

• Surface area a larger surface area emits more radiation, as in cooling fins and radiators.

Surface colour and texture

• Surface colour and texture dull black surfaces emit the most.

Section 5 of 9Thermal equilibrium

Thermal equilibrium

Thermal equilibrium

Thermal equilibrium a state in which an object transfers energy away at the same rate as it receives it, so its temperature is constant.

Receiving faster than losing

• Receiving faster than losing the temperature rises and the object emits more radiation until the rates are equal.

Losing faster than receiving

• Losing faster than receiving the temperature falls and the object emits less until the rates are equal.

Section 6 of 9Greenhouse effect

Incoming radiation

• Incoming radiation the Earth receives most of its energy as short-wavelength radiation from the hot Sun.

Outgoing radiation

• Outgoing radiation the cooler Earth emits longer-wavelength infrared radiation.

Greenhouse gases

• Greenhouse gases water vapour, methane and carbon dioxide absorb the infrared and re-emit it in all directions, some back to the surface.

Effect

• Effect the Earth is warmer than the −18 °C it would have without these gases.

More greenhouse gas

• More greenhouse gas more radiation is returned to the surface, so the surface temperature rises.

Balance

• Balance the Earth's temperature depends on the rates at which radiation is reflected, absorbed and emitted.

Section 7 of 9More from the specification

More from the specification

Thermal vibrational conduction between atoms

• Thermal vibrational conduction between atoms thermal energy transfers through non-metallic solids via vibrational collisions between adjacent lattice atoms.

Radiation

• Energy transfer by infrared waves that needs no medium; dark matt surfaces are good absorbers and emitters,
• while shiny light surfaces are poor ones and good reflectors.

Comparing surfaces

• An experiment using a Leslie cube or two metal containers of hot water shows black surfaces cool fastest.

Conduction in solids

• Energy passes through a lattice by vibrations of particles,
• and in metals also by free electrons moving through,
• so metals are good conductors.

Liquids and gases

• They are poor conductors as particles are further apart, so conduction is less effective, hence the use of insulators.

Applications

• Space shuttle tiles resist heat flow, and a shiny emergency blanket reflects radiation back to the body.

Liquid

• A state of matter with a fixed volume that takes the shape of its container.

Lattice

• Regular pattern of particles in a giant structure.

Solid

• A state of matter with a fixed shape and fixed volume.

Metal

• Element on the left of the Periodic Table.

Vibration

• Rapid backwards and forwards movement.

Lattice vibrations

• Vibrations of particles in a solid that pass energy to neighbours.

Convection current

• Circulation of heated and cooled fluid.

Section 8 of 9Key terms

Term	Meaning
Thermal radiation	Infrared radiation emitted by all objects.
Emitter	A surface that gives out thermal radiation.
Absorber	A surface that takes in thermal radiation.
Thermal equilibrium	Energy received equals energy transferred away, so the temperature is constant.
Greenhouse gas	A gas such as carbon dioxide or methane that absorbs and re-emits infrared radiation.
Infrared	Electromagnetic radiation just beyond the red end of visible light.

Section 9 of 9Exam technique

1

Saying radiation needs a medium — it travels through a vacuum.

2

Saying shiny surfaces are the best emitters — they are the worst emitters and absorbers.

3

Saying black surfaces only absorb — they are also the best emitters.

4

Saying the greenhouse effect is entirely harmful — it keeps the Earth warm enough for life, but extra gases warm it further.

5

Saying the temperature of an object in equilibrium means no energy transfer — energy is received and emitted at equal rates.