The internal energy of a system can be changed by two modes: Heat and Work.

“Heat is the energy transfer due to temperature difference between system and surroundings.”

As we have already studied heat flows from higher temperature to lower temperature.

In doing so, heat will change the temperature of the system thereby changing its internal energy. If heat flows from the surroundings to the system the temperature of the system will increase thereby increasing the internal energy of the system and vice-versa.

Another way of changing the internal energy of the system shown above is by applying external pressure on the weightless piston and compress the gas. In the process the piston moves inside and the molecules which collide with it gain energy and thus transferring energy to the gas and increasing the internal energy of the system. This mode is called work done.

Work is a fundamental physical property of Thermodynamics. Work is motion against an opposing force. For example, when we push a block and it moves we overcome friction and this results in motion and work is done by us. In the process the block also gains kinetic energy. In the same way when we push the piston from outside the molecules gain energy and work is done. In Thermodynamics work is seen as an interaction between system and surroundings. It can occur both ways. Surroundings can do work on the system and system can also do work against the surroundings.

When surroundings will do work on the system internal energy will increase and when system does work its internal energy will decrease.

Difference between HEAT and WORK 

In order to understand the difference between heat and work, let us compare the effect of adding heat and work to a gaseous system.

(i) When heat is added to a gas, its molecules start moving faster in different directions causing increase in disorder or randomness. Thus heat can be regarded as a mode which stimulates the random motion.

(ii) When work is done on a system by forcing the piston down, the initial effect is to force the molecules to move in the direction of movement of piston. Thus, work can be regarded as a mode that stimulates the organised motion.

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