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How to Calculate Heat Level from First Principles

Heat is defined as the transfer of energy within a system. A good example of a thermodynamic system is a piston. As the gas inside the piston heats up, it will expand and exert pressure on the lever, causing it to move. When the change in the internal energy of the system and the amount of work done on the system to move the lever are known, the heat level of the system can be calculated using one of the first principles of thermodynamics, known as the First Law of Thermodynamics.

Things You'll Need

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Instructions

- 1
  Set up the equation for finding the heat level of the system.The First Law of Thermodynamics is given by this equation:
  
  Q = ΔU + W
  
  Where Q is the heat, ΔU is the change in the internal energy of the system and W is the work done on the system.
- 2
  Plug in the values for ΔU and W. For example, say a piston is pushed a certain distance by the pressure of an expanding gas and that the values for ΔU and W are 150 J (Joules) and 100 J, respectively. Now, the equation will look like this:
  
  Q = 150 J + 100 J
- 3
  Finish out the math (by hand or use a calculator) to solve for Q. In this example, the value for Q is:
  
  Q = 150 J + 100 J = 250 J

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