Core Physics Topic 3 - Heat Transfer

Heat Flow

Objects get hot if heat energy (thermal energy) passes into them.  We know they get hot because the temperature rises.  It is important that we know the difference between temperature and heat:

• Temperature - the measure of how hot something is, measured in degrees Celsius;

• Heat - a flow of energy from a hot object to a cold object.  Heat is measured in joules (J).

Bear trap: Temperature is not heat.  Temperature represents internal energy, the energy that is the result of molecules vibrating.

Be careful.  A big heat flow does not mean a high temperature.  If we heat up a beaker of ice, there is a big heat flow, but the temperature does not rise above 0 oC until all the ice has melted.

Heat always flows from hot to cold.  Heat does not flow from a cold object to a hot object.  For example the heat from a hotplate passes into a cold pan to heat up the water in the pan.

The greater the temperature difference between the hot object and the cold object, the greater is the flow of heat.

 Question 1 You hardly feel a spark from a sparkler, which has a temperature of 3000 oC.  But if you tip a cup of coffee over yourself at 60 oC, you will be badly hurt.  Explain why.

How Heat Flows

There are three methods for heat to flow:

• Conduction;

• Convection;

Conduction

If you put a metal bar into a candle (or bunsen) flame, it gets hot quickly.  Soon you can't hold it.  If you put a glass bar into a candle, it won't get too hot to hold.  But if you touched the end that was in the flame, you would find that it was really hot!

The process in which heat passes through a solid substance is called conduction.  Metals are good conductors of heat.  Non-metals are generally bad conductors of heat.  Liquids and gases are bad conductors of heat as well.  A bad conductor of heat is called an insulator

Your duvet traps air which is a good insulator.

 Question 2 How does a duvet trap air?  Why does it keep you warm?

Conduction works like this.  You will know that atoms and molecules in a solid are in a fixed lattice like this.  They are bound with bonds (like springs).  The molecules vibrate about a central point.

If we apply heat, the molecules vibrate with bigger vibrations and set their neighbours vibrating with bigger vibrations.  These pass on the vibrations to their neighbours in turn.  The bigger the vibrations, the hotter the material.  If the vibrations are passed on easily, the material is a good conductor.  (In reality the situation is more complicated than this, but this is all you need to know at this level.)

We can demonstrate thermal conductivity with this simple experiment.

The better the conductor, the quicker the blue on the thermometer strip changes from dark blue to yellow.

Question 3

The table shows the results of a simple experiment to test the conductivity of the metals in the picture above.

 Metal Time taken (s) Aluminium 25 Brass 70 Copper 32 Steel 110

(a)  Which metal was the best conductor?

(b) How would you ensure that it was a fair test?

(c) What kind of graph would you plot to display the results?

Convection

Convection occurs only in liquids and gases.  We call liquids and gases fluids.  It cannot happen in solids.  It needs particles to be free to move about.

When a liquid is heated, the molecules at the bottom move about with bigger vibrations.  They take up more space which means that the density goes down.  The less dense fluid rises.  It gives its energy to the fluid above, and cools down.  It becomes denser and falls back to the bottom.  A convection current is set up.  You can see this happening in a pan of peas.

Animation by Oni Lukos, Wikimedia Commons

The animation shows the idea.

A radiator in a room heats up the room by convection (NOT radiation).  This is shown in the picture.

Picture by Boatbuilder, Wikimedia Commons

You can see how the hot air expands, and rises to the top of the room.  It loses its heat to the ceiling and becomes dense.  It then falls back towards the floor.  In very tall rooms, you can have a fire belting out the heat, but you still feel cold.  All the hot air is at the top of the room.

 Question 4 Answer the interactive matching question.

We have looked at the mechanism of radiation in Topic 1.  All hot objects transfer heat by radiation.  They give out infra red radiation, which we can feel with our skin, or observe using an infra-red sensitive camera.  Black objects emit and absorb infra red radiation well.  White objects reflect infra red.

 Question 5 Answer the interactive question that gets you to fill in the spaces.

How animals loose heat

These young men are competing in a running race.

Each young man's body temperature will rise as he runs.  It is important that the body does not get too hot, otherwise enzymes will stop working which could be fatal to the athlete.  The body detects the rise in temperature as the athlete runs.  To prevent that, sweat gland secrete water onto the surface of the skin.  It takes energy to evaporate water, and the outward energy flow cools the body.

If the athlete does not have enough water, the sweat glands no longer produce water, and the body can overheat.  It is vital for the athlete to keep hydrated.

The young man to the front of the picture is quite skinny.  He has a low body mass, less than 50 kg.  He also has quite a large surface area. When he stops running, he will lose heat quickly.  It is possible that he could over-cool, and risk hypothermia.  An aluminium blanket could be used to reflect the heat that he has lost through radiation.

His friend is less skinny, with a body mass of about 65 kg.  The second athlete has a slight lower surface area to volume ratio, so he will lose heat less quickly.

 Question 6 Explain how the aluminium blanket can stop the runner from getting too cold.

Let us look at how volume and surface area are related.  Consider a cube of 1 cm in every direction.

The volume is 1 cm3.  The area is 6 cm2, so the surface area to volume ratio is 6:1.  Now we double the size.

The volume is 2 × 2 × 2 = 8 cm3The area of each face is 4 cm3 so the total area is 2 × 4 = 24 cm2.  Therefore the surface area to volume ratio is 24:8 = 3:1.

 What is the surface area to volume ratio for a cube of side 3 cm?

The shape with the lowest surface area to volume ratio is a sphere.

 Question 8 Which one of these teapots would keep the tea hot for longest?  Explain your answer.

Photo Ansgar Walk, Wikimedia Commons

The shape and size of animals is important in whether they can survive in cold climates.  Big animals have a lower surface area to volume ratio, so they lose heat less than smaller animals.  The polar bear has the following features to enable it to survive in the cold Arctic:

• A large body size (it has a mass of up to 800 kg);

• Hollow hairs to enable heat to be trapped;

• Light guides in the hair to pick up what little infra-red there is;

• Black skin to absorb the heat;

• Small ears that lose little heat.

This animal is a Fennec Fox that lives in the desert.  The background is sand, not snow.

Photo by Helmut Böhm, Wikimedia Commons

 Question 9 What body features make the Fennec Fox so suited to its life in the desert?

 Summary Heat flows from hot objects to cold objects. Heat flow depends on the temperature difference. Heat is not temperature. Heat passes by conduction, convection, or radiation. Radiation is caused by infra-red radiation