Conduction, convection and radiation

Energy is transferred by ___ and ___ via the processes of

• conduction

• convection

• radiation

heating and radiation

What is thermal conduction? What about electrical conduction?

Thermal conduction is the transfer of thermal (heat) energy through a substance (usually in solids) by the vibration of the atoms within the substance

• The substance itself DOESN’T move

Electrical conduction is the movement of electric charge through matter

Conduction is the main method of energy transfer by heating in solids

What are good conductors of heat? What is the name for a bad conductor?

Metals

Insulator

Explain the process of heat energy transfer by conduction

• When a substance is heated, the atoms start to vibrate more in that area because they have more kinetic energy

• These particles containing more energy bump into neighbouring particles and transfer some of this energy to them

• The process repeats

IMPORTANT: this process takes place in all materials

What is the order of best thermal conduction between the three states of matter? Explain

Best are solids, then liquids and then gases

The particles in a liquid/gas are further apart than solids. This means that the frequency of collisions is reduced so less heat energy is being transferred to neighbouring particles in a fixed time in gases/liquids compared to solids.

• More frequent collisions increase the rate of transfer

Why are metals good conductors of heat?

• First they are solids so the particles are close together in a lattice so particles collide more frequently through vibrations

• They have delocalised electrons that absorb heat energy and so gain KE so move faster

• as they move through the metal, they collide with the ions, transferring energy much faster than atomic vibrations alone

Practical: investigate how well different metals conduct heat

Describe a method

1. Attach ball bearings to the ends of each metal strip at an equal distance from the centre, using a small amount of wax

2. The strips should then be turned upside down and the centre heated gently using a bunsen burner so that each of the strips is heated at the central point where they meet

3. When the heat is conducted along to the ball bearing, the wax will melt and the ball bearing will drop

4. Time how long this takes for each of the strips and record in a table

5. Repeat the experiment and calculate an average of each time

6. The first ball bearing to fall will be from the rod that is the best thermal conductor

Variables?

What should you do to ensure a valid test?

• Independent: type of metal

• Dependent: rate of conduction

• Control: length and thickness of metal rods, amount of wax used, identical ball bearings

Allow the rods to cool to room temperature before heating so that they all begin at the same temperature

What is convection?

Convection is the transfer of thermal energy through fluids (liquids and gases) by the upward movement of warmer, less dense regions of fluid

Convection ONLY occurs in fluids

Explain convection: what happens when a fluid is heated? What is this called?

• Heat is initially transferred through the glass wall of the beaker by conduction (if in liquid)

• The molecules push each other apart, making the fluid

• This makes the hot fluid less dense than the surroundings

• The hot fluid rises, and the cooler (surrounding) fluid moves in to take its place

• Eventually, the hot fluid cools, contracts and sinks back down again

• The resulting motion is called a convection current

Can you explain why, when a piece of card is set over the chimney that is not above the candle, the candle quickly goes out?

The rising warm air above the candle is not replaced by any air. Oxygen does not reach the flame and so it goes out.

How do temperature difference at the Earth’s surface occur?

If there are different surfaces such as land and sea

What happens during daytime? Which direction does the breeze flow?

Al revés during nighttime

• Breeze towards the shore

• During daytime, the land heats up quicker than the sea water

• Air above the land is heated, expands and begins to rise

• The rising air is replaced by cooler air drawn in from the surface of the sea

• This is why it can be cooler by the coast while further inland the temperature is higher

Explain the difference between heat and temperature

• Heat is the total amount of energy

• Temperature relates to the average kinetic energy of particles

• A 20 degree pool contains more heat than a 60 degree cup of coffee.

When you turn on a bathroom heater you feel the warmth instantly. Why is it not conduction nor convection?

• It is not conduction bc conduction doesn’t occur that fast + air is a bad conductor

• It is not convection either because it would result in heated air floating upwards on colder, denser air

How is heat energy travelling in this example? How would we know apart from the fact that it is really quick?

As infrared waves (heat waves)

• The heat is travelling from the heater in a straight line

• The design of the bathroom heater includes a specially shaped very shiny reflector, similar to the reflector behind a fluorescent light or in a torch

What is thermal radiation?

The transfer of energy by infrared waves.

Remember that we are talking about heat transfer — this is thermal radiation not nuclear radiation!

All objects ____

emit IR radiation (no matter what temperature)

Why is thermal radiation quicker than conduction?

Infrared waves have same properties as the other EM waves therefore:

• radiation transfers heat via IR waves traveling at the speed of light

• while conduction relies on slower molecular collisions. 

What is a notable difference between conduction/convection and radiation? Evidence of this?

Conduction and convection require particles while radiation can take place in a vacuum

• That is why the Earth can be heated up by the Sun even though it is in space and so far away

• The sun radiates across the entire em spectrum but its peak output to Earth is the visible light and infrared parts, with small amounts of UV

Just like visible light, objects can ____ and ____ infrared waves.

Reflected and absorbed/emitted

Good emitters/radiators are good ___ of IR

Good reflectors are poor ___ of IR

Absorbers

Absorbers

Factors affecting radiation

• Colour:

  • black — good absorber/emitter

  • white — bad absorber/emitter

• Finish:

  • dull — reasonably good absorber/emitter

  • shiny — very poor absorber, it reflects it and also bad emitter

• Surface area:

  • high SA — good absorber/emitter

  • low SA — bad absorber/emitter

• How hot it is:

  • hotter — good emitter

  • cooler — bad emitter

The hotter an object, the ___

more infrared radiation it radiates in a given time

What changes occur as you increase the temperature of an object?

• The hotter an object, the more infrared radiation it emits per unit of time

• The nature of the EM waves also changes:

  • At low temperatures, most of the radiated energy is in the form of IR waves (invisible to the human eye).

  • As temp increases radiated waves shift to higher frequencies and shorter wavelengths, meaning more energy is emitted, moving from invisible infrared towards visible light (glow dull red, then orange, yellow, white)

  • This effect is the same as that of planets (astrophysics)

When talking about energy absorbed/emitted/reflected always say THERMAL RADIATION (not heat)

When will objects stop releasing heat?

Objects will continue to lose heat until they reach thermal equilibrium (equal temperature) with their surroundings

• For example, a mug of hot coffee will cool down until it reaches room temperature

Why would you want to reduce rate of thermal energy loss?

• It can become expensive

• Production of energy using fossil fuels produces greenhouse gases, which contribute to global warming

Conduction is the main way thermal energy dissipates to the surroundings. How can we reduce energy transfer by conduction?

What would you take into account when choosing these materials when building a house? What is a good example material to build houses with?

Insulating materials should be used

They should be strong, durable and at a sensible price

• Brick

How can we reduce energy transfer by convection?

Convection currents must be prevented from forming. Therefore the fluid that forms the currents must be prevented from moving

• When talking about convection ALWAYS mention convection currents

Insulation reduces energy transfer from both conduction and convection.

What does the effectiveness of an insulator depend upon? Explain

• Thermal conductivity of a material: the lower the conductivity, the less energy transferred

• The density: the more dense the insulator, the more conduction can occur

  • in a denser material, the particles are closer together

• The thickness: the thicker the material the better it will insulate

How is thermal energy loss reduced in houses?

• Roofs and walls are made from insulating materials reducing thermal energy lost by conduction

• Cavity wall insulation (fiberglass) prevents loss of energy by conduction and convection. This traps air between the fibres which prevents convection currents from forming in the wall cavity and transferring thermal energy from the inner layer to the outer layer.

• The panels can be covered with shiny aluminium foil which prevents loss by radiation as it reflects IR radiation

• Double glazed mirrors reduce loss by conduction (mirrors made of insulator glass) and convection by trapping air preventing convection currents from forming

• Other ways include: reduction of draughts around doors preventing air currents + carpets which prevent thermal energy loss by conduction

• They all reduce the rate of energy transfer from the home

What would happen if the layer of air between the glass panels in double glazed windows were too thick or thin?

• Too thin and its insulating property would be reduced

• Too thick and it would allow convection currents forming and transfer thermal energy from the hotter surface to the cooler one

Why do wool fleeces keep us warm? What would happen is we were to get oven mittens wet? What about wetsuits?

• Air gets trapped in the fibres preventing convection currents from forming

• Air is an thermal insulator preventing conduction

• The trapped air is heated by your body and forms an insulating layer that stops you from losing heat

• You might burn yourself because water is a better conductor of heat than air

• Same thing but traps water next to the diver’s skin

A large proportion of body heat is loss from the ___

head, wear a hat

How does wind cause rapid heat loss from out bodies? What is a term for this? How can we prevent this?

• It does this by forces convection. Makes cool air circulate close to the body surface

• Causes sweat to evaporate quicker causing rapid cooling

The cooling effects of wind contributes to the wind-chill factor. To reduce the wind-chill effect, a piece of wind-proof outer clothing should be worn

When people do loose body heat at great rate they become hypothermic. When people are rescued from e.g mountains, they are wrapped in a thin reflective blanket

Other things to watch out for:

• Heat does not rise (only hot gases or liquids rise)

• Shiny things do not reflect heat (they reflect thermal radiation)

• Black things do not absorb heat (they absorb thermal radiation)