Example from Physics, 12.2 First law of Thermodynamics: Thermal Energy and Work

Calculating Change in Internal Energy

Suppose 40.00 J of energy is transferred by heat to a system, while the system does 10.00 J of work. Later, heat transfers 25.00 J out of the system, while 4.00 J is done by work on the system. What is the net change in the system’s internal energy?

Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.

Strategy
You must first calculate the net heat and net work. Then, using the first law of thermodynamics, $Δ U = Q - W,$ find the change in internal energy.
Solution
The net heat is the transfer into the system by heat minus the transfer out of the system by heat, or

$Q = 40 .00 J - 25 .00 J = 15 .00 J.$
12.7

The total work is the work done by the system minus the work done on the system, or

$W = 10 .00 J - 4 .00 J = 6 .00 J.$
12.8

The change in internal energy is given by the first law of thermodynamics.

$Δ U = Q - W = 15 .00 J - 6 .00 J = 9 .00 J$
12.9

0 of 2 steps open

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Strategy

Solution

Strategy

Solution