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Thermal energy & particle theory: 4.10
Increasing the energy store of gas - work done and temperature effects on
heating the gas
Doc Brown's Physics exam study revision notes
INDEX for my physics notes on particle model theory
explaining
state changes, latent heat, heating and cooling curves
4.10
Increasing the energy store of gas - work done and
temperature effects
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Increasing the energy store of gas
by compressing it
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When you pump air into a bicycle tyre
as energetically as you can, you can detect a rise in temperature,
particularly near the pump connection point. So, why the increase in
temperature of the gas?
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When you compress a gas by applying a
mechanical force you do work of compression on the gas.
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This work in compressing the gas
increases the internal energy and increases the temperature - increasing
the thermal energy store - the kinetic energy store is increased.
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You have to do work on the gas
because as you compress the air in the pump, the pressure rises as the
force of the particle impacts acts against you, so you
have to do work against this increased force/unit area (pressure) to get the air into
the tyre.
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By doing work on a gas in this way
the increase in the internal energy store of the gas ends up as
increased kinetic energy of the particles, which causes the temperature
rise of the
air, tyre and pump.
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This effect is used in refrigerators
where a refrigerant gas is compressed to release energy in a closed
system - this thermal energy is obtained from the refrigerant liquid
evaporating by absorbing the latent heat of vaporisation from the
interior of the fridge-freezer.
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If you compress a gas, decreasing
its volume, you increase its internal energy, increasing the average
kinetic energy of the particles and the gas gets warmer with an increase
in its temperature.
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If you expand a gas, increasing
its volume, you decrease its internal energy, decreasing the average
kinetic energy of the particles, the gas cools as the temperature is
reduced.
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These effects are due to the increase or decrease in
the intermolecular force between particles.
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Increasing the energy store of gas
by heating it
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Increasing the temperature of a gas
increases its kinetic thermal energy store.
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Increasing the temperature increases the
average speed of particles and their kinetic energy.
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In fact, the temperature of a gas is
proportional to the average kinetic energy of the particles.
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This means on heating a gas in a sealed
container there are more particle impacts and more forceful impacts on the
surface per unit area.
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Therefore heating a gas at constant
volume increases the gas pressure.
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Conversely if you cool a sealed
cylinder of gas, the pressure decreases.
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More on
gases and more on
gas calculations (both in the GCSE level chemistry notes).
INDEX of notes on Particle model theory
state changes and latent heat
Keywords, phrases and learning objectives for particle models and increasing the
internal energy of gas
Know and be able to explain how the energy store of gas is
increased by doing work on it in compression or increasing the gas
temperature.
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INDEX of notes on Particle model theory
state changes and latent heat
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