In Chap. 1 ,heat and work were seen to be quantities which have meaning only when transferred bodies. The commodity transferred is called energy mechanical energy in the case of work and thermal energy in the case of heat.
In addition to being exchanged,energy can also be stored For example,mechanical energy (work)obtained from the expansion of gas in a cylinder could be used to raise a weight.The weight is said to possess potential energy which can be stored indefinitely or used to transfer mechanical energy to another body. Similarly, thermal energy can also be stored. The addition of thermal energy (heat) to a body is often manifested as a temperature rise. If the body is then perfectly insulated, it will maintain its temperature indefinitely or cold at some later time transfer thermal energy to another body.