Heat Transfer
IntermediateExplore the three mechanisms of heat transfer: conduction, convection, and radiation.
💡 Key Concepts:
- Conduction: Direct contact - metals are good conductors
- Convection: Fluid motion - hot rises, cold sinks
- Radiation: EM waves - works in vacuum (like sunlight)
- Heat always flows from hot to cold
- Higher temperature difference = faster heat transfer
- Material properties affect conduction rate significantly
Theory
Heat Transfer Fundamentals
Heat transfer is the movement of thermal energy from a region of higher temperature to a region of lower temperature. It occurs through three main mechanisms.
1. Conduction
Heat transfer through direct contact between materials. Energy is transferred through molecular collisions and electron movement.
Fourier's Law:
Q = -kA(dT/dx)
- • Q = heat transfer rate (W)
- • k = thermal conductivity (W/(m·K))
- • A = cross-sectional area (m²)
- • dT/dx = temperature gradient
Examples: Touching a hot stove, heat spreading through a metal rod
2. Convection
Heat transfer through fluid motion (liquids or gases). Warmer fluid rises while cooler fluid sinks, creating circulation patterns.
Newton's Law of Cooling:
Q = hA(Ts - T∞)
- • h = convection heat transfer coefficient
- • A = surface area
- • Ts = surface temperature
- • T∞ = fluid temperature
Types: Natural (buoyancy-driven) and Forced (external flow)
Examples: Boiling water, ocean currents, heating/cooling systems
3. Radiation
Heat transfer through electromagnetic waves. Does not require a medium and can occur through vacuum.
Stefan-Boltzmann Law:
Q = εσAT⁴
- • ε = emissivity (0 to 1)
- • σ = Stefan-Boltzmann constant (5.67×10⁻⁸ W/(m²·K⁴))
- • A = surface area
- • T = absolute temperature (K)
Examples: Sun's heat, infrared heaters, thermal imaging
Thermal Conductivity
| Material | k (W/(m·K)) | Type |
|---|---|---|
| Diamond | 2300 | Excellent conductor |
| Copper | 401 | Good conductor |
| Aluminum | 237 | Good conductor |
| Water | 0.6 | Poor conductor |
| Wood | 0.1-0.2 | Insulator |
| Air | 0.026 | Good insulator |
Applications
- Building insulation: Minimize heat loss/gain through walls and roofs
- Cooking: Conduction (pan), convection (oven), radiation (broiler)
- Electronics cooling: Heat sinks use conduction and convection
- Climate control: HVAC systems use all three mechanisms
- Solar panels: Capture radiant energy from the sun
- Thermal protection: Spacecraft heat shields during re-entry
- Industrial processes: Heat exchangers, furnaces, refrigeration