Topic 1 How is heat transferred?

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

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.

How Heat Flows

There are three methods for heat to flow:

• Conduction;

• Convection;

• Radiation.

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.

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.)

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.

A radiator in a room heats up the room by convection (NOT radiation).

Radiation

Radiation passes heat on as an electromagnetic wave called infra-red radiation.  All the heat from the Sun reaches us as electromagnetic radiation.  (We will look at the electromagnetic spectrum in another topic.)

Our eyes cannot see infra red, but a digital camera can.  Here is a picture of a hot plate that appears much brighter than it actually is because of the infra red radiation.

All objects can absorb or emit radiation.

Now answer Question 1

The hotter the object, the more the radiation it emits.  A heat sensitive camera can show hot spots.  The cat's nose in the picture below is the hottest part.

For further reading see http://imagers.gsfc.nasa.gov/ems/infrared.html

Dark (especially black) surfaces absorb infra-red well.  White and silver surfaces reflect radiation.  You may have done simple experiments like this with your teacher.  Dark coloured cars get hot in the sunshine.

Now answer Question 2

The shape of an object affects the rate at which it absorbs or emits heat.  The heat-sink in an electronic circuit is:

• made of metal so that it conducts well;

• fin-shaped so that the area for cooling by convection is increased;

• painted black so that heat can pass by radiation.

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 weighs 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.

Now try the Topic Quiz

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