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Heat Transfer

Heat can be transferred in matter in three ways: by conduction, by convection, and by radiation.

1. Conduction is the transfer of energy from one molecule to another by direct contact. This transfer occurs when molecules hit against each other, similar to a game of pool where one moving ball strikes another, causing the second to move. Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better conductor than wood or plastic.

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 are :

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 are;

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

Convection is the process of heat transfer  in fluids (liquids and gases) by the actual motion of matter. The initial heat transfer between the object and the fluid takes place through conduction, but the bulk heat transfer happens due to the motion of the fluid.

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.

There are two types of convection, and they are:

1. Natural convection
2. Forced convection

Natural convection: When convection takes place due to buoyant force as there is a difference in densities caused by the difference in temperatures it is known as natural convection. Examples of natural convection are oceanic winds.

Forced convection: When external sources such as fans and pumps are used for creating induced convection, it is known as forced convection.

Examples of forced convection are using water heaters or geysers for instant heating of water and using a fan on a hot summer day.

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 chimney. The air above the candle gets heated and rises up the chimney causing convectional currents which carry the smoke out with them.

Applications of convection

 One of the most common applications  of natural convection is the phenomena of:.

1.Sea breeze: This phenomenon occurs during the day. The sun heats up both the sea surface and land. As the sea has a greater heating capacity, it absorbs much of the sun’s energy but gets warmed up much slower than the land. As a result, the temperature above the land rises and heats the air in the atmosphere above it. Warm air is less dense, and hence, it expands, creating a low-pressure area over the land near the coast. Meanwhile, there is relatively high pressure over the sea. The difference in air pressure causes the air to flow from sea to land. The sudden gush of wind felt due to this is known as the sea breeze.

2.Land Breeze: This phenomenon occurs during the night when the situation reverses. As the sun sets, the land and sea start cooling down. The land quickly loses heat when compared to water due to the differences in heat capacity. Consequently, the temperature of the sea is relatively higher, which creates low air pressure there. This sets up a flow of cool breeze offshore, known as the land breeze.

3.Used in car radiators.

In a car  radiator, the coolant liquid flows through the tubes inside combustion engine and absorbs the engine’s heat and in-turn gets heated itself.  The heated fluid then makes its way through a rubber hose to the radiator in the front of the car. As it flows through the thin tubes in the radiator, the hot liquid is cooled by the air stream entering the engine compartment from the grill in front of the car. Once the fluid is cooled, it returns to the engine to absorb more heat.

The air picks up heat from the coolant by convection process, i.e. heated air is blown away by the fan, and colder air replaces it.

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

Immersion heaters are used to  heat liquids that they are immersed in. A convection current will be established as water from beneath the element passes through it and rises above the element to the top of the vessel. When the warmer layer reaches the top, it curves around and heads down along the opposite side of the vessel, and then crosses the vessel again to rise beneath the element once again. Convection currents gradually spread the heat relatively evenly throughout the vessel.

4. Radiation is the transfer of heat by electromagnetic waves. When you stand in the sun, you are warmed by the electromagnetic waves, mainly infrared radiation (and to a lesser extent, visible light), that travels from the sun to Earth. In addition to the sun, light bulbs, irons, and toasters also transfer heat via radiation. Note that, unlike conduction or convection, heat transfer by radiation does not need any matter to help with the transfer. 

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.

Dewar Or Vacuum Flask

 Vacuum was developed by Sir James Ivarin 1890. Vacuum bottles are effective because they reduce heat transfer caused by conduction, convection, and radiation. The vacuum between the two walls of the glass or metal vessel greatly reduces heat transfer due to convection and conduction. A tight fitting plastic lid seals off the vessel and eliminates heat transfer by convection and evaporation. Vacuum bottles made of glass contain silvered surfaces that reflect radiant energy back into the bottle. All-steel vacuum bottles are made of polished steel that also keeps the radiant energy in the bottle.