In any thermodynamic process, when heat Q is added to a system, this energy appears as an increase in the internal energy ΔU stored in the system plus the work W done by the system on its surroundings.
when heat is added to a system there is an increase in the internal energy due to the rise in temperature, an increase in pressure or change in the state. if at the same time, a substance is allowed to do work on its environment by expansion, the heat Q required will be the heat necessary to change the internal energy of the substance from U1 in the first state to U2 in the the second state plus the work W done on the environment.
Thus Q= (U2-U1) +W
or Q= ΔU + W ………. (1)
thus the change in internal energy ΔU = U2 – U1 is defined as Q-W . since it is the same for all processes concerning the state, the first law of thermodynamics.
a bicycle pump provides a good example. when we pump on the handle rapidly, it becomes hot due to mechanical work done on the gas, raising their by its internal energy. one such simple arrangement is shown in figure.
it consist of a bicycle pump with a blocked outlet allows the air temperature to be monitored. when piston is rapidly pushed, thermometer shows a temperature rise due to increase of internal energy of the air. the push force does work on the air, thereby, increasing its internal energy, which is shown, by the increase in temperature in the air.
human metabolism also provides an example of energy conservation. human beings and other animals do work when they walk, run, or move heavy objects. work requires energy. energy is also needed for growth to make new cells and to replace old cells that have died. energy transforming processes that occur with in an organisms are named as metabolism. we can apply the first law of thermodynamics as : ΔU = Q-W
to an organism of the human body. work W done will result in the decrease in internal energy of the body. consequently the body temperature or in other words internal energy is maintained by the food we eat.