Physcis 1a

Heat energy can be tranferred by radiation, conduction or convection

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  • Created by: Heather22
  • Created on: 16-03-14 11:23

Radiation

Radiation is the transfer of heat energy by infrared radiation. Infrared radiation can be emitted bty solids, liquids and gases.

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Conduction

Conduction is the main form of heat transfer in solids. It is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles.

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Convection

Convection is the main form of heat transfer in liquids and gases. It occurs when the more energetic particles move from the hotter region to the cooler region and take their heat energy with them.

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Radiation - surface and colour

Dark, matt surfaces absorb infrared radiation falling on them much better than light, shiny surfaces. They also emit much more infrared radiation.

Light, shiny surfaces reflect a lot of the infrared radiation falling on them. Vacuum flasks have silver inner surfaces to keep heat in or out, depending on whether it's storing hot or cold liquid.

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Conductors - metals

Metals are good conductors because the electrons are free to move inside the metal. At the hot end, the electrons move faster and collide with other free electrons, transferring energy. This is why heat energy travels sofast through metals.

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Convection - Immersion heater

1. Heat energy is transferred from the heater coils to the water by conduction.

2. The particles near the coils get more energy and move around faster.

3. This means there's more distance between them, the water expands and becomes less dense.

4. This reduction in density means that the hotter water tends to rise above the denser, colder water.

5. As the hot water rises, it displaces(moves) the colder water out of the way making it sink.

6. This cold water is then heated by the coils and rises - and so it goes on. You end up with convection currents going up, roind and down, circulating the heat energy through the water.

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Radiator

To heat a room a radiator relies on convection currents. Hot, less dense air by the radiator rises and denser, cooler air flows to replace it.

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Heat energy transfer

Heat energy is radiated from the surface of an object. The bigger the surface area, the more infrared waves can be emitted from the surface. Some devices are designed to limit heat transfer, like vacuum flasks.

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Energy efficiency

The most effective methods of insulation are the ones that give you the biggest annual saving. Eventually, the money you've saved on heating bills will equal the initial cost on installing the insulation, it's called payback time.

payback time = initial cost/annual saving

Cavity wall insulation - reduces convection

Loft insulation - reduces conduction and radiation

Draught-proofing - reduces convection

Hot water tank jacket - reduces conduction and radiation

Thick curtains - reduces conduction and radiation

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Efficiency of machines

Useful devices are only useful if they can transform energy from one form to another. Usually some of the input energy is wasted or lost, often as heat. The less energy thats watsed, the more efficient the device is.

Calculating efficiency:

efficiency = useful energy/ total energy

power input and output:

efficiency = useful power/ total power

No device is 100% efficient, except electric heaters, because all the electricity is converted to useful heat. All the energy ends up as heat energy.

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Sankey diagrams

The idea of sankey diagrams is to mak it easy to see how much of the total energy in is useful or wasted.

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Cost of electricity

Kilowatt-hours are 'units' of energy:

Electrical appliances transfer electrical energy in to other forms. The amount of energy transferred depends on the power and the amount of timeits switched on. Energy is measured in joules. Power is measured in watts or kilowatts.

Equations:

Energy = Power x Time

kWh = Power(in kW) x Time

Cost = No. of Units x Price per unit

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