Chapter 12, Problem 3CQ

To determine

Whether the first law of thermodynamics is invalid in human body.

Explanation of Solution

Humans do work and give off energy by heat. This makes the First law of thermodynamics to be applied to living organisms to describe them.

The first law of thermodynamics is valid in a human body in work. The internal energy stored in humans goes out of the body as in the form of heat when human body performing a strenuous exercise such as lifting weights or riding a bicycle. Because the rate of change of internal energy lose as heat varies with the intensity and the duration of the work which is done by the human body, The first law of thermodynamics is given by,

$\frac{\Delta U}{\Delta t}=\frac{Q}{\Delta t}+\frac{W}{\Delta t}$

• $\Delta U$ is the change in internal energy.
• Q is the energy exchanged.
• W is the work done.

When a human body is performing a strenuous exercise, the heat is given out of the system and the work is done by the system. Therefore, $\frac{Q}{\Delta t}$ and $\frac{W}{\Delta t}$ are negative. Hence, $\frac{\Delta U}{\Delta t}$ is negative and the internal energy and body temperature should decrease if the human body were a closed system. Hence, the human body is an open system. It acquires chemical potential energy as internal energy from eating and breathing. So, the internal energy and body temperature will be  constant.

Conclusion: The first law of thermodynamics is valid in animals. Because, the human body is an open system, the internal energy will be constant by compensating the energy going out as heat by the chemical potential energy acquiring from eating and breathing.