It is a kind of interaction that would occur at the system boundaries. It can be positive or negative.
It is said to be done by a system when the ‘sole effect’ external to the system could be reduced to the raising of a weight. The word ‘sole effect’ indicates that the raising of weight should be the only interaction between the system and surroundings in order to say that there is work interaction between the system and the surroundings. The phrase ‘external to the system’ indicates that the work is a boundary phenomenon. The magnitude of work interaction depends upon the system boundary.
Figure: Work Transfer
This is illustrated with an example. Let us consider the battery and the motor in figure as a system, the motor driving a fan. The system is doing work upon the surroundings. When the fan is replaced by a pulley and a weight, the weight may be raised with the pulley driven by the motor. The sole effect on things external to the system is then the raising of a weight.
Sign Conventions for Work:
Figure: Sign Conventions
Unit for Work-