Heat is a form of energy that flows from a higher temperature body to a lower temperature body. The SI unit of heat is Joule (J), though calorie is also used (1 cal = 4.186 J).
Conduction is the transfer of heat through direct molecular contact without any bulk movement of matter. It occurs mainly in solids. Fourier's Law: Q/t = kA(ΔT/L), where k = thermal conductivity (W/mK), A = cross-sectional area, ΔT = temperature difference, L = length.
Convection is heat transfer by actual movement of a fluid (liquid or gas). Natural convection occurs due to density differences caused by temperature. Forced convection uses fans or pumps. It is the primary mechanism of heat transfer in fluids.
Radiation is the transfer of heat in the form of electromagnetic waves (infrared) without requiring any medium. Stefan-Boltzmann Law: E = σT⁴, where σ = 5.67×10⁻⁸ W/m²K⁴. All bodies above absolute zero radiate heat.
Newton's Law of Cooling: The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings: dT/dt = -k(T - T_env). This is valid for small temperature differences.
| Law | Formula |
|---|---|
| Fourier (Conduction) | Q/t = kA·ΔT/L |
| Stefan-Boltzmann | E = σT⁴ |
| Newton's Cooling | dT/dt = -k(T-T₀) |
| Specific Heat | Q = mcΔT |
CBSE/JEE Tip: Thermal conductivity of metals is high (copper: 385 W/mK) while insulators have low values (wood: 0.1 W/mK). Good conductors are bad radiators (shiny surfaces). Kirchhoff's Law: good absorbers are good emitters.