Q: If the mean free path of a molecule in water is $$L = 10$$^{-8}$$ cm and its thermal velocity is v$$ = $$10^4 cm/s$$, what is the time required to diffuse a distance of 1 cm?

$$t = \frac{1^2$}{10$$^{-8}$$ \times 10^4$} = 10^4$ \text{ s} = 2.8 \text{ hr}$$

Question 1

Which of the following best describes the process of heat transfer by conduction in the human body?

Accepted Answer

Heat is transferred through direct contact between molecules, from the interior to the skin, following the equation $$H_c = \frac{K_c A}{L}(T_1$ - T_2$).$$

Question 2

A 70-kg man of height 1.55 m has a surface area of approximately 1.7 $$m^2. If $$his basal metabolic rate is $$40 	ext{ Cal}/m^2$\cdot hr$$, what is his total energy consumption per hour at rest?

Accepted Answer

$$68 	ext{ Cal/hr}$$

Question 3

Which statement correctly applies the first law of thermodynamics to a living animal?

Accepted Answer

Energy intake must be exactly equal to the work done and the heat lost if internal temperature and weight are constant.

Question 4

Which of the following is the correct formula for the average kinetic energy of a molecule in an ideal gas?

Accepted Answer

$$\frac{1}{2}mv^2$ = \frac{3}{2}kT$$

Question 5

A drop of colored solution is introduced into a still liquid. Over time, the color spreads throughout the liquid. What physical process is responsible for this, and what is the formula for the average distance a molecule travels after $$N$$ collisions?

Accepted Answer

Diffusion; $$S = L \sqrt{N}$$

Question 6

Which of the following best explains why contact lenses should not be worn during sleep?

Accepted Answer

Blinking is necessary to bring fresh oxygen-rich tear fluid under the lens; without blinking, the cornea is deprived of oxygen, leading to loss of transparency.

Question 7

Which equation describes the maximum ratio of work to heat input when heat flows from a region of higher temperature $$T_1 to a $$region of lower temperature $$T_2$$?

Accepted Answer

$$\frac{Work}{Heat_{input}} = 1 - \frac{T_2$}{T_1$}$$

Question 8

During moderate physical activity, a 70-kg man consumes 260 Cal/hr. If 20% of this energy is external work, how much energy is converted to heat inside the body?

Accepted Answer

$$208 	ext{ Cal/hr}$$

Question 9

Which of the following statements about the second law of thermodynamics is correct?

Accepted Answer

Spontaneous changes in a system occur from an arrangement of lesser probability to greater probability.

Question 10

Which process is primarily responsible for cooling the skin when exposed to open air, and what is the formula for the rate of heat transfer by this process?

Accepted Answer

Convection; $$H_c' = K_c' A_c (T_s - T_a)$$

Question 11

If the mean free path of a molecule in water is $$L = 10$$^{-8}$$ cm and its thermal velocity is v$$ = $$10^4 cm/s$$, what is the time required to diffuse a distance of 1 cm?

Accepted Answer

$$t = \frac{1^2$}{10$$^{-8}$$ 	imes 10^4$} = 10^4$ 	ext{ s} = 2.8 	ext{ hr}$$

Question 12

Which of the following is true regarding the regulation of body temperature in humans?

Accepted Answer

Most heat is transported from the inside of the body by blood in the circulatory system, then transferred to the skin by conduction.

Question 13

Which of the following best describes the difference between heat and work in terms of energy conversion?

Accepted Answer

Work is energy in ordered motion, while heat is energy in random motion.

Question 14

A person in a dry equatorial desert is exposed to intense solar radiation. If the intensity at the top of the atmosphere is $$1150 	ext{ Cal}/m^2$\cdot hr$$ and the person has dark skin (absorbs 80%), how much solar energy is absorbed per $$m^2$$ per hour?

Accepted Answer

$$920 	ext{ Cal}/m^2$\cdot hr$$

Question 15

Which of the following factors does NOT directly affect the rate of heat outflow from the skin in cold conditions?

Accepted Answer

The color of clothing worn

Question 16

Which formula is used to estimate the surface area of a human body given mass $$M (kg) $$and height $$H (m$$)?

Accepted Answer

$$Area = 0.202 	imes M$$^{0.425}$$ 	imes H$$^{0.725}$$

Question 17

Which of the following statements about metabolic rate and body size is correct?

Accepted Answer

Metabolic rate is proportional to $$M^{3/4}.$$

Question 18

Which process is most important for cooling a person engaged in moderate physical activity in a warm climate?

Accepted Answer

Evaporation of sweat from the skin surface

Biophysics: Transformation of Energy and Thermodynamics in the Human Body

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