- The temperature difference between system and surrounding.
- This exchange of heat takes place through thermally conducting walls (boundary).
- If the temperature of a system is higher than the surrounding, heat is lost from the surrounding and heat is gained when the system has a lower temperature than the surrounding.
- So, the amount of heat that is gained or lost by the system is symbolically represented by q.
- ‘q > 0’ is positive (+ve) when system gains or absorbs heat.
- ‘q < 0’ is negative (-ve) when system loses heat.
- The energy between system and surrounding can take place in the form of work which may be a mechanical, pressure-volume or electrical.
- For example, Take a thermos flask (thermally insulated) with some water in it and note down its temp., after doing some work (mechanical) by inserting a paddle and rotating it we will notice an increase in temp. This indicates the increase in internal energy, i.e., gaining of energy by the system by doing work on it.
- The work done by the system or on the system is denoted by ‘w’.
- Work is positive (w>0) when done by the system.
- Work is negative (w<0) when done on the system.
- The pressure-volume work is also called expansion work.
DIFFERENCE BETWEEN HEAT AND WORK
- When heat is added, molecules start moving in different directions and thus, system attains randomness whereas when work is done molecules attain an order.
- Hence, work can be regarded as an organised form of energy and heat as a random form of energy.
- Work, a high-grade energy whereas Heat, is a low-grade energy.
Note- In few texts convention for work is opposite as mentioned above.
Also Read Modes of Heat Transfer