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AS90184

Demonstrate understanding of heat transfer and nuclear physics

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• What is heat?

• What is temperature?

• How does heat flow?

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Heat

• Think of two glass beakers filled with water at the same temperature (80oC). One beaker contains 100 ml of water the other is larger and contains 500ml of water.

• Which beaker would take longer to cool?

• Therefore which beaker contains more heat (energy)

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Temperature

Temperature is a measure of the heat energy contained in an object. The greater the heat energy the greater the temperature.

The molecules within some material are all moving around and have kinetic energy. Temperature is a measure of the average kinetic energy of the material.

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Scales

We are familiar with the Celcius scale, also known as the Centigrade scale.There are two fixed points on this scale0oC is melting ice and 100oC is boiling water. The range of temperatures in between are divided into 100 equal divisions.The Kelvin scale has the same size unit as Celcius, but a different zero point. It starts at absolute zero, when all molecules within some material will stop moving (this is -273oC).

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We only use the unit Kelvin (K), unlike Celcius where we use degrees Celcius (oC)We will often use K to describe a difference in temperature values.e.g. to raise the temperature of some water by 20 K is the same as raising the temperature by 20oC.

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Thermometer

A thermometer is usually constructed of glass with a bulb at one end containing a reservoir of coloured liquid. There is a thin capillary running the length of the thermometer.

Heat applied to the bulb transfers easily through the thin outside shell of the bulb and heats the liquid within. As heat energy is added to the liquid it expands and flows up the capillary. A scale marked on the side of the capillary allows us to read the value of the temperature.

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Bulb with thin wallcontaining liquid

Capillary

Scale

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Transfer of heat

There are generally three methods of heat transfer:

Conduction: Through some material which allows heat to travel through it, a thermal conductor.

Convection: Within a fluid when hot material is less dense and it rises, cooler material get heated by the source and rises up

Radiation: By being emitted from the source directly as heat energy, travels through space in all directions, by infra red light energy.

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Black or White

Which of these two colour options will warm up in the sun quickest?If we filled two flasks with hot water at the same temperature, one was black and the other white, which would cool quickest?We find that black surfaces will absorb heat more easily than white surfaces, but they will also radiate heat more easily.This effect is more pronounced when we compare a matt black surface with a shiny silver surface.

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Conductors/Insulators

What materials make good conductors of heat?

What materials make good heat insulators?

What is the key point about most good heat insulators?

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Specific Heat capacity

Consider a beaker of water containing 100ml and another containing 200ml.

If we heated both beakers up from 20oC to 30oC, which would require more energy?

The amount of energy required depends upon the amount of material to be heated. We state that the amount of energy required to heat 1 kg of material by 1oC is called theSpecific Heat Capacity of that material.

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Specific Heat Capacity of water

The specific heat capacity of pure water is 4200 J per kg per 1oC ( 4200 Jkg-1oC-1 )

We find a formula to calculate the amount of energy to heat an amount of material is given as:

Q=mcΔTWhere Q= heat energy

m = mass of materialΔT= change in temperaturec = Specific heat capacity of the

material.

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Example

How much heat energy is required to raise by 15oC 2.25kg of water.Q = mcΔTQ = 2.25 x 4200 x 15Q = 141750 JHow much does the temp rise for 3 l of water when 252000J of energy is used to heat Q = mcΔT252000 = 3 x 4200 x ΔTΔT = 252000/12600 = 20oC

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Specific heat capacities

Different materials will have different heat capacities.

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Latent heatWhen we boil a kettle of water not all the water instantly turns into steam! There is a delay as parts of the water turn into steam. This takes some time and requires a lot of energy. The larger the amount of water the more energy is required.During this process there is no change in temperature as the H2O goes from water at 100oC to vapour at 100oC.The energy required to do this is called the latent heat.Is there another heat point where this situation occurs?

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Latent heat

We call the energy to change 1 kg of a material from solid to liquid the latent heat of fusion (melting), occurs at 0oC for water.

We call the energy to change 1 kg of a material from liquid to gas the latent heat of vaporisation, occurs at 100oC for water.

For water

latent heat of fusion is 334000 Jkg-1

latent heat of vaporisation is 2260000 Jkg-1

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Temp/Energy Graph

The graph opposite shows what happens to temperature as we heat something up through it’s melting point and boiling point.If we turn the graph around as materials cool then as they condense or fuse then the temperature will remain fixed.