Heat Transfer
In this section, we explore the mechanisms of heat transfer which is the movement of energy between systems owing to a temperature difference. There are three distinct modes of heat transfer:
1. Conduction
Conduction occurs when heat is transferred through direct contact between materials. This is evident when one end of a metal rod is heated, causing heat to travel through the rod to the cooler end. The rate of heat transfer by conduction is quantified using Fourier's Law, expressed as:
$$H = \frac{KA (T_C - T_D)}{L}$$
where:
- H is the rate of heat transfer,
- K is the thermal conductivity of the material,
- A is the cross-sectional area,
- T_C and T_D are the temperatures at the two ends, and
- L is the length of the rod.
Gases are generally poorer conductors than liquids and solids.
2. Convection
Convection involves the transfer of heat by the actual motion of fluid. It can be classified into:
- Natural Convection: Occurs due to differences in density within the fluid, often caused by heating from below (e.g., air rising when heated by the sun).
- Forced Convection: In this scenario, a pump or fan forces the fluid to move, enhancing heat transfer (e.g., heating systems).
3. Radiation
Radiation does not require a medium for heat transfer and occurs via electromagnetic waves. All bodies emit thermal radiation corresponding to their temperatures. The Stefan-Boltzmann law gives the rate of heat transfer due to radiation:
$$H = \sigma A T^4$$
where:
- H is the power radiated,
- \sigma is the Stefan-Boltzmann constant, and
- A is the surface area of the emitting body.
Notably, a black body has an emissivity of 1, meaning it perfectly absorbs and emits radiation.
Understanding these mechanisms is crucial in disciplines such as thermal engineering, meteorology, and environmental science.