TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Physics > First Law of Thermodynamics, Adiabatic and Isothermal Changes
- Increase in internal energy of the system = Heat transferred to the system - Work done by the system
- The internal energy of a gas only depends on its temperature. To change the temperature of a gas the internal energy must be changed by doing work on it or transferring energy to it.
- When a gas is compressed, work is done on it.
- When a gas expands, the forces due to pressure push the container walls back, so the gas is doing work. Change in W is negative (opposite of work done on it).
Adiabatic Change
- A change which occurs where no heat enters of leaves the system.
- Occurs when the work is done fast.
because no heat is transferred therefore 
Adiabatic Compression
- To reduce the volume, work is done on the gas, heat transfer does not occur so work done (
) leads to and increase in internal energy (
) which therefore leads to an increase in temperature.
Adiabatic Expansion
- When a gas expands, it does work (opposite of work done on it) -W so the work done on it is negative and therefore the internal energy decreases therefore temperature decreases.
- When a gas expands adiabatically, no heat is supplied to it so energy to do work is taken from its internal energy.
A perfectly adiabatic expansion is not actually possible. But a change is almost adiabatic when the gas is in a container which has bad conducting walls. Example of an adiabatic expansion is the escape of air when a tyre bursts.
Isothermal Process
- Takes place at a constant temperature. In an isothermal expansion or contraction,
where 
is a constant.
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