Key Concepts in Biological Physics and Thermodynamics

Wet-Bulb Temperature

  • Defined as the temperature air reaches when blown over a wet surface.
  • Air cools by evaporation; continues until relative humidity (RH) reaches 100%.
  • Wet-bulb temperature is lower than dry-bulb temperature at low RH.

Calculating Wet-Bulb Temperature

  • Example: Evaporating 4 g of moisture in 1 kg of air at 20 °C
    • Latent heat required = 2453extkJ/kg×0.004 kg=9.81extkJ2453 ext{ kJ/kg} \times 0.004 \text{ kg} = 9.81 ext{ kJ}
    • Temperature change: ΔT=−9.81 °C\Delta T = -9.81 \text{ °C}

Psychrometric Chart

  • Tool to visualize relationships between:
    • Wet-bulb temperature
    • Dry-bulb temperature
    • Relative humidity
    • Moisture content (grams per kg of dry air)

Heat Transfer Mechanisms

  1. Conduction
    • Heat transfer through solids, liquids, and gases.
    • Depends on temperature difference and material properties.
  2. Convection
    • Heat transfer involving fluid movement (liquids & gases).
    • The rate depends on fluid velocity and other factors.
  3. Radiation
    • Heat transfer without a medium (via electromagnetic waves).
    • Governed by Stefan-Boltzmann law.

First Law of Thermodynamics

  • Expresses energy conservation:
    • For a system, ΔU=Q−W\Delta U = Q - W
    • Includes metabolic energy in the context of human physiology.

Work Efficiency in Humans

  • Efficiency η\eta defined as η=WE\eta = \frac{W}{E}
  • Typical efficiency ranges from 2-10%, can reach up to 25% under optimal conditions.
  • Factors like energy input and work done impact efficiency.

Food Energy and Metabolism

  • Various food components provide different energy values:
    • Lipid: 37 kJ/g
    • Protein & Carbohydrates: 17 kJ/g
  • Average recommended daily intake is approximately 8-10 MJ for adults.