Heat Energy Transfers

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Conductors

Materials which are good conductors of heat are: steel & copper metals

These are used in things such as, pans and irons

1. The heat energy is conducted from the hot end to the cold end.

2. It is transferred from atom to atom.

3. At the hot end, the atoms are vibrating a lot.

4. The vibration is gradually passed along to the other atoms as they bump into each other.

They are good conductors of heat because they have free moving electrons that travel through the metal transferring the energy quickly.

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Insulators

They do not have the free electrons that are found in metals. They don't conduct heat well.

Materials which are insulators: wool, polystyrene, fur and feathers

Uses of insulators: Loft insulations and quilt stuffing

Liquids are bad conductors of heat because the molecules are too far apart to transfer heat from one molecule to the next.

Air is a bad conductor of heat and a good insulator.

Foam is a good insulator because there is air trapped inside the foam. Small pockets of air prevent free movement of air molecules so preventing convection.

A baby penguin stays warm in freezing temperatures because air is trapped between the feathers. It warms up and acts as an insulator.

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Convection

The warm water rises because it has expanded and so it is less dense than the cold water. The cooler denser water falls to the bottom.

The molecules in liquids and gases are free to move about a lot which allows them to transfer heat by convection which is a much more effective process than conduction.

Convection couldn't happen in a solid because the molecules are closely packed and are not free to move.

Some clouds are held up in the air by convection currents below them. Heat is transferred from Earth to air. As air near the ground warms up. Hot air is less dense than cold air, so hot air rises and cold air falls.

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Radiation

Heat radiation is electromagnetic waves of a certain frequency. Hot objects give off radiation. The hotter they are, the more heat radiation they emit. Cooler objects will absorb this radiation
1. Travels in straight lines at the speed of light
2. Travels through a vacuum so it is the only way heat can reach us from the Sun
3. It can be very effectively reflected away by a silver surface
4. Only travels through transparent things like air, glass and water 
5. No particles are involved. It is the transfer of heat energy purely by waves 
6. Dark, matt surfaces such as black absorb heat radiation much more strongly than bright, glossy surfaces such as white and silver. They also emit heat radiation much better too. Brightly polished surfaces do not radiate much heat. 

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Real Life Examples

A fire fighters suit is white and shiny to reflect away heat that is radiated by the fire and to stop them from absorbing heat. 

Marathon runners are wrapped in silver blankets at the end of a race to reflect heat back into the body to prevent too much heat loss. It reflects body heat back on to the skin. 

Houses in hot countries are often painted white to prevent the house from getting hotter by reflecting the heat away from the house. 

Kettles are often bright, shiny silver colour to keep the heat in the kettle. 

Petrol storage tanks are painted silver to stop them absorbing heat energy from the Sun by radiation, reducing heating of petrol so less risk of explosion. 

Cooling fins on a refrigerator are black because it is better at radiating heat away to achieve maximum cooling. 

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Heat Exchangers

Car engines are cooled by convection currents in the water pipes. A pump is used to help the water circulate. (The pump is switched on by a thermostat - see later). 

Water is a very good substance to carry unwanted heat away from the engine to the radiator because it takes a lot of heat energy to raise its temperature (it has a high specific heat capacity). The radiator acts as a heat exchanger because hot water gives up its energy to the air. 

Heat is removed from the back of a refrigerator by means of a long, coiled pipe which contains the refrigeration fluid. The pipe is black because it radiates the maximum amount of heat. The pipe is long and coiled because this maximises surface area for heat exchanger to radiate heat. 

The vacuum flask is designed to minimise the rate at which heat is transferred between its contents and the environment. The flask is built with an inner & an outer container. The space between them contains a vacuum. Conduction & convection is prevented by stopper/vacuum. Radiation is prevented by the shiny surface which prevent heat being radiated out. 

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Insulating the House

Cavity Wall Insulation - Small pockets of trapped air, so no convection occurs. Air is a good insulator so no conduction. 

Loft Insulation - Small pockets of trapped air, so no convection occurs. Air is a good insulator so no conduction. 

Draught proofing on windows and doors - Reduces heat loss by convection

Double Glazing - Air is trapped between 2 layers of glass so it can't move so no convection. Air is a good insulator so no conduction. 

Jacket around hot water tank - Jacket is filled with insulation material with small pockets of trapped air, so no convection. Air is a good insulator so no conduction. 

Thick curtains - Curtain material is an insulator so less conduction. Reduces draughts so less convection. 

Floor insulation or carpet - Contain small pockets of trapped air, so no convection. Air is a good insulator so no conduction. 


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U-values

Heat lost per second (W) = U-value x surface area x temperature difference

Example - An uninsulated roof measures 10m by 10m. How much heat is lost through the roof if the temperature outside is 5 degrees celsius and inside the house is 20 degrees celsius? 

3000                     =    2          x           100m2        x       15 degrees celsius

Now calculate how much heat is lost if the roof is insulated. 

600                     =   0.4            x          100m2       x       15 degrees celsius 

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Which is better?

Why is loft insulation more effective than underfloor insulation? 

Hot air rises via convection therefore more heat loss through the roof than through the floor

Why are radiators placed near the floor rather than near the ceiling? 

Hot air rises by convection 

Houses with thick walls also lose less heat than those with thin walls. Explain why. 

Less conduction of heat through walls 

Rooms with low ceilings are cheaper to heat. Why?

Less air to heat, room heats faster and less heat is needed

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Thermostat

Thermostats are devices which control temperature. Such as, oven, iron, fridge, hot water boiler and convection heater. 

The bimetallic strip is very common. This is two thin strips of different metals bonded together. This bimetal strip is made of brass & invar. When the strip is heated the brass expands much more than the invar. This makes the strip bend. 

If the bimetal strip is cooled it would bend the other way, the brass contracts more on cooling as well as expanding more on heating. 

As the room warms, the bimetal strip bends and the two electrical contacts separate. This breaks the circuit, stopping the current flow and switches off the heater. 

As the room cools the bimetallic strip straightens or bends the other way, contact is made again, the circuit connects again and the heat is switched on. 

As the control knob is turned closer to the strip (i.e. increasing thermostat temperature) the strip has to bend more to break the contacts therefore the room has to reach a higher temperature to achieve this. 

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Mechanisms

Normal room temp rises -> Thermostat gets hot & heater switches off -> Temp falls

Normal room temp falls -> Thermostat cools down & heater switches on -> Temp rises

This is similiar to our bodies because our thermostat is the hypothalamus if e.g. detects a rise in temperature it switches on mechanisms to cool the body down. 

Other automatic feedback systems have thermostats fitted with valves. These open and close to control the flow of water to radiators. When there is high temperature in the room which closes valves so that less water can reach the radiator which means less heat is lost to the room and the room cools. 

Setting the thermostat at a slightly lower temperature saves money on fuel bills because less temperature difference with respect to outside so less heat is lost and less energy will be needed to replace the energy lost. 

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