btec_appliedscience_2017_ppt_5_9.pptx

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BTEC Nationals

Applied Science

Unit 5:

Principles and Applications of Science II:

Physics

Isothermal change for ideal gas

• An isothermal change takes place at a constant temperature: ΔU = 0.

• From the ideal gas equation, if ΔU = 0, pV is constant.

• The equation for the First Law of Thermodynamics becomes: Q = W.

  • If the gas expands and does work, heat must be supplied.

  • If the gas contracts and heat is produced, the heat must be allowed to leave the gas.

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Isothermal changes

A gas can only be changed isothermally if:

• it is in a good conducting, thin-walled container

• it is surrounded by a constant temperature reservoir

• the change occurs very slowly.

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What is required for isothermal gas changes?

A

Good conducting, thin-walled container

B

Constant temperature reservoir

C

Gas in an insulated container

D

Change occurs very slowly

Adiabatic change for ideal gas

• During an adiabatic change, no heat enters or leaves the gas: Q = 0.

• The equation for the First Law of Thermodynamics becomes: ΔU = −W.

  • If the gas expands and does work, the temperature of the gas falls.

  • If the gas is compressed and work is done on it, the temperature of the gas rises.

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Adiabatic changes

• True adiabatic expansion is not possible. However, rapid expansion of gas in a thick-walled, badly conducting vessel is close to this.

• To find the work done by one stroke of a petrol engine, it is assumed that the gas expands and is compressed adiabatically.

• If an adiabatic process is reversible, then it can also be called isentropic.

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Ideal Otto cycle for a four stroke engine (1)

In an engine:

1. A piston moves up in a cylinder to compress gas.

2. A spark heats the gas.

3. The heat causes the gas to expand, moving the piston down.

This process is shown by the diagrams and graph on the following slide.

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Ideal Otto cycle for a four stroke engine (2)

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Ideal Otto cycle for a four stroke engine (3)

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