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Physics - Infrared Radiation (AQA)
Infrared radiation is the main method of heat transfer in an electric toaster.

Physics - Infrared Radiation (AQA)

One topic covered in GCSE Science is the transfer of heat (thermal) energy. This is one of eight quizzes on that subject and it looks specifically at infrared radiation.

Infrared radiation is a part of the electromagnetic spectrum. This is a spectrum of waves that also includes light. Infrared is also called thermal radiation and is one way in which heat is transferred. Infrared is emitted (given off), absorbed (taken in) and reflected by all objects. Hotter objects give out more infrared radiation than cooler ones. It has a wavelength of just a little longer than red light waves - you may have seen or done an experiment yourself using a thermometer, prism and light source. When the visible light is dispersed (split into the colours of the rainbow) by the prism, if you hold the thermometer just beyond where the red is, the reading will increase slightly.

Since infrared is an electromagnetic wave, unlike conduction and convection, it does not require any particles to transfer it. Space is the ultimate insulator and there are too few particles in space to allow convection. Despite this, heat still reaches the Earth from the Sun. That is because it arrives in the form of infrared radiation and space is mainly transparent.

Infrared can easily be blocked. You can try this for yourself. On a sunny day, hold a piece of paper between the back of your hand and the Sun. Your hand will feel immediately cooler when the paper is casting a shadow. As well as being blocked, infrared is easily absorbed. However, some colours and surfaces absorb, emit and reflect infrared more efficiently than others. You will have learnt these in your lessons and will be expected to know them for the exam.

Infrared radiation is invisible to our eyes, but if there is enough, we can detect it with our skin as heat. Infrared sensors can detect heat from the body. They are used in security lights, burglar alarms and to automatically switch lighting on and off when someone enters or leaves a room. Thermal radiation is also used to transmit information from place to place, including remote controls for television sets, DVD players and similar appliances as well as for data links over short distances between computers or mobile phones.

Put the following in order of best to worst reflectors of infrared radiation:

1. Red, dull
2. White, shiny
3. Black, dull
4. Red, shiny
1, 3, 4, 2
2, 4, 1, 3
1, 2, 3, 4
4, 1, 3, 2
Where the colour is the same, shiny surfaces will reflect infrared better than dull ones. Dull surfaces scatter or absorb more of the incoming radiation
In sunny countries, houses are often painted white. Why?
Painting them white lowers the U-value of the walls
White paint does not conduct heat
White is a good reflector of thermal radiation
The white paint reflects the light into the narrow streets which otherwise would be dark
Infrared radiation from the Sun heats up the outside of buildings. The heat then conducts through the walls and, if the sun shines on them all day, the inside of the buildings would become unbearably hot. Using white paint reflects the infrared radiation, keeping the walls cooler so less heat is conducted to the inside
Thermal radiation can only travel through which substances?
Transparent and translucent substances
It can also travel through vacuums like space. It is very easily absorbed by opaque objects
Which are the best emitters of thermal radiation?
Black, dull surfaces
Light coloured dull surfaces
Black shiny surfaces
Light coloured shiny surfaces
They are also the best absorbers of thermal radiation
In an electric toaster, what methods of heat transfer cook the bread?
Mainly radiation with some conduction
Mainly conduction and some radiation
Conduction and convection only
Mainly convection but conduction has a part to play too
Cooking is done using red hot wires. These give out a lot of thermal radiation. This heats the surface of the bread and the heat is conducted into the bread, cooking the inside of the slice. There is a gap between the heating wires and the bread so convection will play little or no part as the hot air rises out of the toaster in a convection current
Why are car radiators often painted matt black?
So that splashes of engine oil won't show up
To make them more efficient
To make the car more attractive to buyers
It is a legal requirement
The radiator is designed to reduce the temperature of the cooling water circulating round the engine. Painting it black increases the amount of infrared radiation it gives out, making it more efficient
Thermal radiation travels at what speed?
Very slowly
The speed of sound
The speed of light
It is a member of the electromagnetic spectrum
Which one of the following emits the most infrared radiation?
1 kg of water at 95oC
1 kg of iron at 68oC
1 kg of your body at 37oC
1 kg of crude oil at 72oC
The hotter the object, the more infrared it emits
The inside container of a vacuum flask is silvered. Why?
To reduce heat loss by radiation
To completely prevent heat loss by radiation
To reduce heat loss by evaporation
To reduce heat loss by condensation
Remember, all objects give off thermal radiation so no surface will completely prevent heat loss by thermal radiation, but silver shiny surfaces radiate the least of all and therefore reduce heat loss from the flask
Which of the following does not use infrared radiation?
Security lamps
A head torch
A remote control for a TV
An intruder alarm
Not all burglar alarms use infrared detectors but many do
You can find more about this topic by visiting BBC Bitesize - Energy and heating - AQA

Author:  Kev Woodward

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