First Law of Thermodynamics

First Law of Thermodynamics

A cup of hot tea has more internal energy than a cup of cool tea. Internal energy is the total potential and kinetic energy of the atoms in the tea. So the internal energy of a body can increase if we heat it.

If you have pumped up a bicycle tyre, you will find that the valve (small opening where air goes into the tyre) gets hot. When you pump at the air, you do work on it. This increases the internal energy.

Lets use air as our example. Consider a hollow cylinder, closed at one end, with a movable piston in the middle - like a syringe with the needle blocked. When the air inside is heated, the molecules move faster, so the internal energy increases. At same time, the air could also expand and push out the piston. This means that it does work on the piston.

Since energy is conserved, the increase U in internal energy plus the work done W must be equal to the the heat Q given to the air. This is the first law of thermodynamics: Q = U + W.

This law is also true for any body, not just air. It is true for wood, metal, water, etc. The only difference is that the work done by solids and liquids would be much smaller, since they do not expand as much as gases do.

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