Heat Transfer

Heat transfer is the movement of thermal energy from a warmer place to a cooler place. It explains why a metal spoon gets hot in soup, why air circulates in a room, and how sunlight warms Earth through space. The three main mechanisms are conduction, convection, and radiation, and each works in a different physical way.

Conduction transfers energy through direct particle collisions, usually in solids. Convection transfers energy by the bulk motion of fluids such as liquids and gases, often driven by density differences caused by temperature changes. Radiation transfers energy by electromagnetic waves, so it does not require matter and can occur through empty space.

Key Facts

Heat flows spontaneously from higher temperature to lower temperature until thermal equilibrium is reached.
Conduction in one dimension can be modeled by Q/t = kA(DeltaT)/L.
Convection is often described by Q/t = hA(T_surface - T_fluid).
Thermal radiation power is given by P = e sigma A T^4.
Net radiative transfer can be written as P_net = e sigma A(T_hot^4 - T_cold^4).
Good conductors such as metals transfer heat quickly, while insulators such as foam, wood, and air transfer heat slowly.

Vocabulary

Conduction: Heat transfer by direct contact and particle collisions within a material or between materials touching each other.
Convection: Heat transfer caused by the bulk movement of a fluid such as air or water.
Radiation: Heat transfer by electromagnetic waves, which can travel through empty space.
Thermal conductivity: A material property that measures how easily heat moves through the material.
Thermal equilibrium: The state in which objects in contact have the same temperature and no net heat flows between them.

Common Mistakes to Avoid

Confusing heat with temperature, because temperature measures average particle energy while heat is energy transferred due to a temperature difference.
Assuming convection happens in solids, because convection requires bulk motion of a fluid and solid particles do not flow that way.
Thinking radiation needs air or another medium, because thermal radiation is electromagnetic energy and can travel through a vacuum.
Using Celsius directly in the Stefan-Boltzmann law, because radiation formulas with T^4 require absolute temperature in kelvin.

Practice Questions

1 A wall has thermal conductivity k = 0.80 W/m K, area A = 12 m^2, thickness L = 0.20 m, and a temperature difference of 15 K across it. Find the conduction heat transfer rate Q/t.
2 A hot plate with emissivity e = 0.90 and area A = 0.50 m^2 is at 500 K in a room at 300 K. Using sigma = 5.67 x 10^-8 W/m^2 K^4, find the net radiative power P_net.
3 A metal pan on a stove heats its handle, boiling water circulates in the pot, and your hand feels warmth from the burner without touching it. Identify which parts are conduction, convection, and radiation, and explain why.