3 Conduction, convection and radiation

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Learning outcomes

At the end of this lesson, you should be able to:1. Explain conduction through materials2. Describe convection currents3. Explain the transfer of energy by radiation.Conduction• Consider a pan of water on a gas hob. How does the

energy get from the hot flame into the water?• Process is called thermal conduction – conduction

of heat.• Energy conducts into bottom of saucepan from the

hot gases.• Energy conducts through the metal base of

saucepan.• Energy conducts out of metal base into the water.

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Conduction • Solids are better conductors of heat than liquids,

and much better than gases.• In hot substances particles move faster than in

cooler substances – they have more energy.• More energetic particles pass on some energy

when they jostle against slower neighbouring particles.

• This transfer of energy by jostling is thermal conduction.

• Particles in a solid are held closely together – pass on their energy easily.

• Solids are good conductors.• Gas particles only pass on energy during collisions

– poor conductors (good insulators).3

Conductors • Why are metals good conductors of heat?• Stone conducts heat quite well – jostling atoms

pass on their energy efficiently.• Metals are better conductors because they

contain free electrons.• Electrons move through the metal easily.• Electrons gain kinetic energy from collisions with

‘hot’ atoms and pass on energy when they collide with ‘cold’ atoms.

• This transfers heat more quickly.• Explains why metals are good conductors of heat

(and electricity).

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Convection

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‘Heat rises’ is the everyday description of convectionLook at the photograph, you can see the hot smoke risingAir near the flame is heated by conduction and then rises in convection currents.

Convection happens in all fluids (liquids and gases), but what makes a hot fluid rise and a cold fluid fall?

Consider a hot air balloon

Burner fills balloon with gas that is much hotter than surrounding air.

Convection • Hot gas is much less dense than cold air

surrounding the balloon.• Because gases are in Earth’s gravity field, gas in

the balloon weighs much less than the upthrust force on it.

• Due to cold air it displaces.• Resultant upward force makes the balloon move

upwards, just like a bubble rising in water.• Any hot fluid moves upwards in the same way –

weighs less than upthrust it experiences from the colder fluid around it.

• Colder surrounding fluid falls.

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Heat radiation

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Thermal image shows hot and cold areas of a face.Hottest parts are white, coolest parts are blue/purple.

Photograph is taken with an infra-red camera, infra-red (IR) radiation is also called heat radiation.

Given off by all hot objects

IR radiation is part of the electromagnetic spectrum – radiation travels in small packets called photons.

IR photon energies similar to energies needed to excite atoms in matter.

Infra-red radiation• When IR photon strikes matter, it can be absorbed.• Energy of absorbed photon is transferred into

random energy in a atom of matter.• Internal energy of matter increases – it gets hotter• In opposite process, excited atom can emit an IR

photon – becomes less excited.• Internal energy of matter falls – it gets cooler.• IR and visible light are similar – are adjacent in EM

spectrum.• Its why matt black objects (good absorbers and

emitters of visible light) are also good absorbers and emitters of IR.

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Black bodies• Usual meaning of ‘black’ is ‘absorbs visible light’.• In Physics, a black body is one that absorbs all

radiation that falls on it, at all wavelengths.• It is a perfect absorber.• A black body is also a perfect emitter or perfect

radiator.• It emits energy in all regions of the

electromagnetic spectrum.• One familiar ‘black body’ looks far from black: our

Sun!

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Black bodies and temperature

Every black body gives off different amounts of different wavelengths, depending on its temperature.The graph shows the spectrum for a number of black bodies.

Radiant intensity is the energy per second (W) given out by each square metre of surface, at different wavelengths.

Black bodies and temperature• Grey line is for a black body at 9000 K. This is the

surface temperature of a blue-white star Procyon.• The aqua-blue line is for a black body with a surface

temperature of 6000 K – our Sun.• Procyon is much hotter than our Sun – gives out

greater intensity of radiation so line is higher.• Peak of curve for Procyon occurs at a shorter

wavelength than for our Sun.• The peak wavelength is connected to temperature by:Peak wavelength (m) x Temperature (K) = 0.0029 (mK)

• or λmax .T = 0.0029 m K (Wein’s law)

• For Sun, peak wavelength is close to middle of visible spectrum – looks yellow-white. For Procyon peak is near the blue end – looks blue.

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Colour and temperature• Example 1 – The star Antares has a surface

temperature of 3000 K. Calculate the peak wavelength in its spectrum, and explain why Antares looks red.

• Using Wein’s law, λmax .T = 0.0029 m K

• So λmax x 3000 K = 0.0029 m K

• Therefore λmax = 0.0029 m K / 3000 K

• = 9.7 x 10-7 m = 970 nm• Visible spectrum is between 400 nm and 700 nm,

so 970 nm is in IR region.• Means spectrum of Antares will look very red,

because it will have some red but hardly any blue wavelengths.

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Questions

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1. The stars Betelgeuse and Rigel are similar in size, but Betelgeuse appears reddish to the eye, while Rigel is blue-white. What does this tell you about the surface temperatures of the two stars?

2. The metal tungsten which is used in the filament of light bulbs, has a melting point of 3683 K. you can assume that the filament in the light bulb is at a lower temperature than that, at 3000 K, and that the filament gives off electromagnetic radiation as a black body. calculate the peak wavelength of the radiation given out by a light bulb. How does this explain why the glass of light bulbs gets so hot in use?

3. Cosmology theory suggests that the entire Universe is a black body which has cooled down from the Big Bang and is now at 2.7 K. Use Wein’s law to calculate the peak wavelength of the radiation predicted to fill the Universe.

4. The hottest stars have surface temperatures of about 1 x 107 K. Show that these stars emit X-rays?