The first law of thermodynamics states that whenever heat is added to a
system, the heat will transform to some other form of energy in the same
A system could be anything where heat is being added such as a steam engine that will power a train or a vehicle, a steam turbine that will generate electricity, or even your body.
What happens when you add heat to a system?
This heat can either increase the internal energy of the system if it does not leave the system, do external work if it leaves the system or do a combination of both.
We can then restate the law as shown below.
Keep in mind that If the system does no work, the internal energy will keep on increasing. By the same token, the pressure inside the system will increase as well. This may become very dangerous since if the system cannot tolerate the constant increase of pressure, it may explode.
Imagine a pressure cooker built in a way so that no steam will be allowed to escape! The internal energy will get higher and higher and this may eventually create an explosion.
That is why a pressure cooker is built with a safety feature. This safety feature is usually a spring-loaded valve that will allow some of the steam to escape.
As the steam is coming out, it does some work on the valve by raising the weight of the valve. Thus, heat has been converted to mechanical energy.
Let Q = the net quantity of heat the system receivedLet ΔE = increase in internal energy or generally speaking change in internal energy.
Let W = work done by the system. We can then rewrite the 1st law of thermodynamics as shown below.Q = ΔE + w
Adding heat is not the only to increase or decrease the internal energy of a system. Let Q = 0
Work can be done on a system by compressing it.ΔE = 0 + w
An example of this is when you are using a bicycle pump. When pumping
the handle, you are doing work on the system. If you touch the pump, you
will see that it is a little hot although no heat was added to the
pump. This is because you increase the internal energy of the pump while
you were doing work on the system.