TOPIC SEVEN - HEAT TRANSFER

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Heat is transferred in matter through the following methods: conduction, convection and

radiation.

Conduction

This is the transfer of heat in solids. The rate of conduction depends on

1.   Amount of temperature – the higher the temperature the higher the rate of transfer.

2.   Cross-sectional area – the larger the cross-sectional area the higher the transfer.

3.   Length of material – the shorter the material the higher the rate of transfer.

4.   Type of material – different materials transfer heat at different rates.

Good and bad conductors

Conductivity is the ability of a material to conduct heat. Good conductors of heat are those

materials which are able to transfer heat easily and steadily. Bad conductors are those which do not conduct heat.

Experiment: Comparing thermal conductivity of metals

Procedure

1.   Obtain four identical rods of copper, iron, aluminium and brass.

2.   At one end of each rod attach a matchstick using paraffin wax and let it solidify.

3.   Place the rods on a tripod stand with the free ends close to one another as shown.

4.   Heat the free ends strongly with a Bunsen burner.

5.   Observe what happens.

Discussion

When done correctly and carefully the matchsticks will fall off in the following order: copper, aluminium, brass and finally iron. This shows that different metals conduct heat at different rates.

NOTE – on a cold morning a metallic chair would feel cold compared to a wooden chair at

the same temperature, this is because the metal lic chair absorbs heat from your body as opposed to wood which is a bad conductor of heat.

Applications of conductors

Good conductors

1.   They are used to manufacture cooking utensils

2.   They are used as liquids suitable for thermometers i.e. mercury

3.   Used as heat dumps (metal clips) when soldering delicate components in a circuit board i.e. transistors

Poor conductors

1.   Used as insulators in handles of cooking utensils

2.   Used in making good winter clothes i.e. wool

3.   Hot water cylinders are lagged with fibre -glass since glass is a poor conductor of heat.

4.   Houses in cold countries have double walls with air trapped in them to keep them warm.

Convection

This is the transfer of heat through fluids (liquids and gases). This occurs when part of the fluid is heated: they become less dense and rise above the cold fluid. As they move they

carry heat with them. In convection we observe streams of moving fluid called convectional currents.

Convection in air

Experiment: model chimney (smoke box)

Procedure

1.   Obtain a model chimney system or construct one as shown

2.   Place a lighted candle under one of the chimneys

3.   Place a smouldering cloth near the other chimney and observe what happens.

Discussion

Smoke will be seen going into the chimney and coming out through the other c himney. The air above the candle gets heated and rises up the chimney causing convectional currents which carry the smoke out with them.

Experiment: revolving paper-vane

Procedure

1.   Make a paper-vane by cutting a thin card as shown

2.   Put a string through the hole in the centre and hold it above a lighted Bunsen burner.

3.   Observe what happens.

Discussion

As the air above the flame gets heated convectional currents are formed and rise

upwards.as these currents brush against the paper-vane it rotates.

Convection in liquids

Experiment: heating water in a beaker

Procedure

1.   Put water in a beaker until it is three quarters full and place it on a tripod stand.

2.   Drop a crystal of potassium permanganate through a tube to settle at one corner at the bottom of the flask.

3.   Heat the water gently using a Bunsen burner and observe the movement of streams of colour.

Discussion

A stream of colour will be seen moving upwards and downwards again at the other side of the beaker. This will continue gradually until all the water becomes coloured. This shows that convectional currents also exist in liquids.

Experiment: model of hot water system

Procedure

1.   Obtain two flat bottomed flasks and set up the apparatus as shown below.

2.   Hold the flasks in place by use of clamp stands.

3.   Heat the bottom of the lower flask and observe what happens.

Discussion

When the water in the lower flask becomes hot it rises up to the upper flask. After some time the water in the upper flask will become hot due to convectional currents.

Applications of convection

1.   Brings about the land and sea breezes.

2.   Can be used to explain the weather phenomena.

3.   Used in car radiators.

4.   Used in immersion water heaters by placing them at the bottom.

Radiation

This is simply the flow of heat from one point to another by means of electromagnetic

waves.

Radiation from different surfaces

We use the Leslie cube to determine radiation of different surfaces. It is a rectangular metal

container of square base with small opening at the top. One side is coated with polished silver, another dull black (candle flame soot), the other grey and the fourth white. Experiment: Radiation from different surfaces

Procedure

1.   Place a Leslie cube on a tripod stand and attach a thermometer on each of the four sides.

2.   All thermometers should be at least 5.0 cm form the surface and should read the

same temperature.

3.   Pour hot water (about 80 0C) until it is full and note the reading of each thermometer

after 1 minute.

4.   Repeat the above procedure using boiling water (100 0C).

Discussion

The thermometer  against the black surface records the highest temperature, followed by

the one on the grey side, then the white surface while the polished side recorded the lowest

temperature. The readings when the water is boiling were higher, indicating that radiation depends on temperature. It also depends on the nature of surface.

Applications of radiation

1.   Electric kettles have a chrome coat to reduce radiation.

2.   Electric iron are silver coated to minimize radiation.

3.   Green houses use radiation (heat trap) to grow crops.

4.   Clouds reflect radiation back to the earth hence cloudy nights are warmer than clear nights.

Vacuum flask

It was developed by Sir James Ivarin 1890. It keeps a liquid hot or cold (depends on what is put in). The liquid stays at the temperature it is poured in either hot or cold. It has the following principle features;

(i)         The vacuum between the double walls (ii)        The two interior walls coated with silver (iii)       Insulating cork supports (anti-shock pads) (iv)        Insulating cork stopper at the top.

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