Heat Transfer and Energy Concepts for Students

Energy: Units and Types

• Unit of energy: Joule (J)
• Conservation of Energy: Energy cannot be created or destroyed, only transferred or transformed.
• Kinetic Energy: Energy of motion.
• Potential Energy: Stored due to position (e.g., at the top of a pendulum).

Conduction

• Occurs mainly in solids.
• Particles transfer energy by vibrating and passing it to neighbors.
• Metals are good conductors.
• Poor conductors (insulators), e.g., air and layers of clothing.

Convection

• Happens only in fluids (liquids and gases).
• Heated fluid becomes less dense → rises.
• Cooler fluid becomes more dense → sinks.
• Creates convection currents.
• Example: Hot water in a pan rises to the top.

Radiation

• Transfer by infra-red waves.
• Only method that works in a vacuum (space).
• Best absorber/emitter: dull black.
• Best reflector: shiny white.

Temperature

• Measures average thermal energy of particles—not total energy.
• Example: A cup of hot coffee has a higher temperature than the ocean but less total energy because it has fewer particles.
• Evaporation
  • Slow
  • Happens below boiling point
  • Fastest particles escape, cooling the remaining liquid
• Boiling
  • Fast
  • Happens at the boiling point
  • Needs a heat source

Why objects don’t bounce back to the same height

• Some energy is dissipated to the surroundings (sound, heat, air resistance).

Pendulum Energy Transfer

• Highest point: Maximum potential energy
• Lowest point: Maximum kinetic energy
• Energy constantly transfers between the two.

Why layers of clothes keep you warm

• They trap air, which is a good insulator—prevents heat loss.

Why a wood-stove pipe goes upward

• Hot gases rise due to convection.

Why heat sinks are black

• Dull black surfaces emit heat efficiently.

Why conduction is best in solids

Particles are close together, so energy passes quickly.

Liquids and gases have particles farther apart, so conduction is slower.

Three heat transfer methods & where they occur

• Conduction → solids
• Convection → liquids & gases
• Radiation → vacuum (no particles needed)
• Why water on skin feels cold
• High-energy particles evaporate, removing energy from your skin → skin cools.