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Thermodynamics: Energy Balance for Close Systems

From the first law of thermodynamics, energy cannot be created or destroyed. This means that net energy coming in to the system minus the net energy coming out the system, whether the energy coming in or out is due to heat, work, or mass, will result in a change of the internal energy, kinetic energy, or potential energy of the system. Refer to the equation below.

General Energy Balance Equation (1)

From the above equation, the amount of heat going into the system to cause work can be calculated. Refer to the equation below.

Energy Balance between heat and work (2)

Q = Heat Energy

W = Work Energy

E = System Energy

Note equation 2 assumes that heat and work are not different from one another due to the first law of thermodynamics. However, from later sections you will learn that there is a difference between heat and work because of the second law of thermodynamics.

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