Isobar (pressure)

An isobar in the context of thermodynamics refers to any process in which the system remains at a constant (unchanging) pressure throughout it.

When a system expands at a constant pressure, such as a piston in an internal combustion engine, its volume is increasing and this corresponds to an output of work. However the system cannot do this work without an input of heat. By analyzing the first law of thermodynamics, the amount of heat necessary to do a given amount of work without the pressure changing can be calculated by use of the system's specific heat capacity.

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