Heat Transfer: Definition, Units & Mechanisms

• Heat is a form of energy associated with the kinetic energy of atoms or molecules.
• Temperature is a measure of the average kinetic energy of the particles in a substance.
• Heat transfer is energy in transit due to a temperature difference between a source and a receiver.

Units of Heat

• The SI unit of heat is the joule (J).
• Historically, the calorie (cal) was used: 1 cal is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius.
• 1 cal = 4.186 J.
• The kilocalorie (kcal) or Calorie (Cal) is also used: 1 kcal = 1000 cal.
• The British thermal unit (BTU) is commonly used in the United States: 1 BTU is the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit.

Mechanisms of Heat Transfer

• Conduction: Heat transfer through a material by direct contact.
• Convection: Heat transfer by the movement of a fluid (liquid or gas).
• Radiation: Heat transfer by electromagnetic waves.

Conduction

• Conduction occurs when there is a temperature gradient in a material.
• The rate of heat transfer by conduction is described by Fourier's law.
• Fourier's law states that the heat flux is proportional to the temperature gradient.
• The thermal conductivity (k) is a measure of a material's ability to conduct heat.
• Materials with high thermal conductivity are good conductors of heat (e.g., metals).
• Materials with low thermal conductivity are good insulators (e.g., wood, plastic).
• The formula for heat transfer through conduction is Q = (kAΔT)/d, where:
  • Q is the rate of heat transfer.
  • k is the thermal conductivity.
  • A is the cross-sectional area.
  • ΔT is the temperature difference.
  • d is the thickness of the material.

Convection

• Convection occurs when a fluid moves and carries heat with it.
• Natural convection relies on density differences due to temperature variations.
• Forced convection uses external means (e.g., fans, pumps) to move the fluid.
• The rate of heat transfer by convection is described by Newton's law of cooling.
• Newton's law of cooling states that the heat flux is proportional to the temperature difference between the surface and the fluid.
• The formula for convective heat transfer is Q = hAΔT, where:
  • Q is the rate of heat transfer.
  • h is the convective heat transfer coefficient.
  • A is the surface area.
  • ΔT is the temperature difference between the surface and the fluid.

Radiation

• Radiation is the emission of energy as electromagnetic waves.
• All objects emit thermal radiation, with the amount and spectrum depending on their temperature.
• The Stefan-Boltzmann law describes the total energy radiated by a black body.
• The Stefan-Boltzmann law: P = εσAT⁴, where:
  • P is the radiated power.
  • ε is the emissivity (0 for a perfect reflector, 1 for a black body).
  • σ is the Stefan-Boltzmann constant (5.67 x 10⁻⁸ W/m²K⁴).
  • A is the surface area.
  • T is the absolute temperature in Kelvin.
• Emissivity (ε) is a measure of how effectively a surface emits thermal radiation.
• Objects also absorb radiation from their surroundings.
• The net radiative heat transfer is the difference between emitted and absorbed radiation.

Specific Heat Capacity

• Specific heat capacity (c) is the amount of heat required to raise the temperature of 1 kilogram of a substance by 1 degree Celsius.
• Different materials have different specific heat capacities.
• Water has a high specific heat capacity.
• The heat required to change the temperature of a substance is Q = mcΔT, where:
  • Q is the heat added or removed.
  • m is the mass.
  • c is the specific heat capacity.
  • ΔT is the change in temperature.

Latent Heat

• Latent heat is the heat absorbed or released during a phase change (e.g., melting, boiling) at a constant temperature.
• Latent heat of fusion (Lf) is the heat required to change a substance from solid to liquid or vice versa.
• Latent heat of vaporization (Lv) is the heat required to change a substance from liquid to gas or vice versa.
• The heat involved in a phase change is Q = mL, where:
  • Q is the heat added or removed.
  • m is the mass.
  • L is the latent heat (Lf or Lv).

Heat Transfer Applications

• Heat exchangers: Devices designed for efficient heat transfer between two fluids.
• Insulation: Materials used to reduce heat transfer.
• Refrigeration: Processes used to remove heat from a space or substance.
• Heating systems: Systems designed to add heat to a space or substance.
• Internal combustion engines: Convert chemical energy into mechanical energy, producing heat as a byproduct.
• Power plants: Use heat to generate electricity.

Thermodynamics

• Thermodynamics is the study of heat and its relation to other forms of energy.
• The first law of thermodynamics states that energy is conserved. The change in internal energy of a system is equal to the heat added to the system minus the work done by the system: ΔU = Q - W.
• The second law of thermodynamics states that heat flows spontaneously from hot to cold and that the entropy of an isolated system tends to increase.
• The third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.

Calorimetry

• Calorimetry is the process of measuring the amount of heat released or absorbed during a chemical or physical change.
• A calorimeter is a device used to measure heat.
• The basic principle of calorimetry is that heat lost by one substance is gained by another in a closed system.
• Bomb calorimeters are used to measure the heat of combustion at constant volume.