Thermodynamics

Abstract

I mentioned above that thermodynamics is concerned with changes in the total energy, E, of the system. For example, you help your friend carry a piano to the third floor. While doing this you give away mechanical work (a lot of it, in this case); we will label it as –w. You also get really hot while carrying the piano and exchange your body heat with the environment (you sweat); we label it by –q. The total energy change in your body is a sum of all the work and all the heat exchanged and we express this by the following equation: E = −w −q. As you are giving away both the work and the heat their signs are negative. If, however, someone has carried you up to the third floor and then you drank a cup of hot tea, the sign for the work done and the heat exchanged will be plus – from your point of view – and the energy equation will read E = +w + q.