THERMALPHYSICS PART 3

HEAT TRANSFER

2020-09-14 -

Heat transfer is the flow of heat energy from a hot region to a cold region through matter. When two regions are at different temperatures heat will be move from the hot region to the cold region until the two regions are at equilibrium temperature.

When you touch an object and it feels cold it means your body temperature is higher than the object you have touched. Similarly when you touch the object and it feels hot it means you body temperature is less than the object you have touched.

There are three ways or methods by which heat moves from the hot region to the cold region and these are: conduction, convection and radiation.

Conduction

Conduction is the transfer of heat energy from the hot region to the cold region within a solid or between two solid objects in thermal contact.As explained earlier, particles in a solid are closely parked and they just vibrate to and fro. When one end of the solid is heated, they particles will gain more kinetic energy and begin to vibrate at a faster rate. As the heated particles vibrate at a faster rate, they pass energy to the neibouring particles until heat energy reaches the other end. During conduction, particles do not move. See the figure below.

Even if all solids conduct heat, the rate at which they conduct heat energy differs from one solid to the other. Solids that conduct heat energy at a faster rate are called good conductors of heat. Generally all metals are good conductors of heat energy, however, they too, conduct heat at different rates. The figure below shows an experiment to show that metals conduct heat at different rates.

When the experiment is set up as shown above, metals will conduct heat energy from the end dipped in hot water towads the ends not dipped in hot water. When heat reaches the other, the wax will melt and this will result in the pins falling off. Because metals in the above experiment donot conduct heat at the same rate, the pins will not fall at the same time.

The pin on copper will fall off first,folllowed the pin on alumnium, brass and lastly iron. Among the four metals, copper is the best conductor of heat and iron is the leat conductor of heat.

Uses of conductors of heat

Good conductors of heat are used wherever heat energy is required to move fast through something. The following are some of the uses of good conductors of heat.

• Making radiators

• Making boilers

• Making of sauce pans and pots

For the above uses mainly copper and aluminium are used. Bad conductors of heat are also used in various ways. Bad conductors of heat energy are used as insulators. Insulators are materials that do not easily allow heat energy to pass through them.Therefore, these materials are used where heat is not required to pass through easily. For example, the handle of the sauce pan and a pot is made of either plastic or wood. The following are examples of bad or poor conductors of heat.

• Wood

• Plastic

• Glass

• Air

• Convection

Convection is the flow of heat energy through liquids and gases. As earlier explained, heat energy will flow from a place of higher temperature to a place of lower temperature.

Unlike under conduction, convection invloves the movement of the particles that have been heat. The particles of a liquid or gas become less dense when they are heated and they begin to rise on top of the unheated particles. Similarly, the unheated particles sink to the bottom since they are denser than the heated particles.

The rising of the heated particles to the top of the liquid or gas and sinking of the unheated particles forms a convection currents. The formation of the convection currents ensures that heat energy is distributed throughout the entire liquid of gas. See the figures below that show convection in a liquid.

To ensure that the liquid is heated fast, the source of heat should be placed at the bottom so that the heated molecules can rise to the top of the liquid and cold particles can sink to the bottom forming convection currents. For this reason, the heating element in a gyser is placed at the bottom.

If the source of heat is placed on top of the liquid, heat will flow by conduction and because water is a poor conductor of heat, it will take a lot of time for the liquid to be heated. See the figure below that shows that an experiment to demonstrate that water is a poor conductor of heat.

When the source of heat is applied at a point as shown in the above figure, the water on top will start boiling while the water at the bottom is still cold. The ice wrapped in a wire gauze will remain for a longer period of time. In this case, heat flows from the top to the bottom by conduction and not by convection. Since water is poor conductor of heat, it will take a longer period time before heat reaches the bottom.

Convection in air occurs at the sea coast during the day and during night. During the day sea breeze occurs and during night land breeze occurs.

Sea breeze

During day time, the land is heated faster than the sea. This is because the heat capacity of the sea is much more than that of the land. Therefore, the air above the land will be heated and begin rise moving towards the sea. On the other hand, the cold air from the surface of the sea will move towards the land to replace the hot air. This movement of the hot air from the land towards the sea and cold air from the sea towards land is called sea breeze. Sea breeze occurs during the day. See the figure below.

Land breeze

At night the opposite of sea breeze happens. During the night time, the land cools faster than the sea. Therefore, warm air will be rising from the sea going towards the land. Similarly, cold air from the land will be flowing towards the sea to replace the warm air. That is the reason why when you go to the river or any water body on a very cold day in the morning, you will notice steam on the surface of water. See the diagram below that shows land breeze.

Radiation

Radiation is another method of heat transfer. Radiation is defined as the flow of heat energy from a place of high temperature to a place of low temperature by means of eletromagnetic waves. Radiation therefore does not require any medium for heat to flow from one place to the other. Note that heat energy from the sun is able to reach the surface of the earth by radiation.Radiation is emitted by all bodies above absolute temperature.

Good and bad absorbers

Certain materials absorb radiation and become hot. Such materials are called good absorbers. Generally, black or dull objects are better absorbers of radiation than white or shinny object. For example, a person wearing a black suit on hot day will feel hotter than a person wearing a white suit. The figure below shows an experiment to demonstrate that black or dull objects are good absorbers of radiation.

From the above figure, two thermometers are placed one in a dull black object and the other one in a shinny silver object. In addition, a heater is place in between the two objects at eqaul distances from the objects. Note that heat moves from the heater by radiation After some time, the thermometer placed in dull black object records a higher temperature than the one placed in a shinny silver object. This proves the fact that black objects are good absorbers of heat.

Black or dull objects are also good emitters of radiation than white or shinny objects. For this resaon, the cooling fins on heat exchangers at back of the refrigerator are painted black.

Example

1. A pupil set up the apparatus shown below to investigate heat transfer.

(a) What method of heat transfer was this pupil investigating?

(b) What could the colour of object A?

(d) State two conditions needed in order to obtain accurate resuts in the above experiment.

Solutions

(a) Radiation

(b) The colour of object A is dull black

(c) Houses in hot places are painted white because white colour is poor absorber of radiation hence inside part of such a house will be cool.

(d) The two objects should be placed at equal distances from the source of heat. In addition the two objects must be of the same size.

Example 2

2. Choose words from the list below to complete the questions that follow. Diffusion, radiation, convection, evaporation, conduction, sublimation