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College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300

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College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300
Textbook Problem

Consider the human body performing a strenuous exercise, such as lifting weights or riding a bicycle. Work is being done by the body, and energy is leaving by conduction from the skin into the surrounding air. According to the first law of thermodynamics, the temperature of the body should be steadily decreasing during the exercise. That isn’t what happens, however. Is the first law invalid for this situation? Explain.

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

Explanation

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,

ΔUΔt=QΔt+WΔt

  • ΔU 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, QΔt and WΔt are negative. Hence, ΔUΔt 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.

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