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• Revision:First Law of Thermodynamics, Adiabatic and Isothermal Changes

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Physics > First Law of Thermodynamics, Adiabatic and Isothermal Changes

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• 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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