Energy Transfers & Particle MotionOxford AQA IGCSE Physics: Revision notes

Conduction transfers energy through a material by particle collisions, without the particles themselves moving from place to place.

• Vibrating particles at the hot end collide with neighbouring particles, passing on kinetic energy
• In metals, free electrons move through the structure carrying energy quickly, which makes metals excellent conductors
• Insulators (like wood or plastic) have no free electrons and rigidly bonded particles, so energy transfers much more slowly through collisions alone

Convection happens in fluids (liquids and gases) when particles gain energy, move further apart, and become less dense.

• Heated particles spread out, making that region of fluid less dense
• The less dense, warmer fluid rises above the denser, cooler fluid
• This creates a convection current that circulates and transfers energy through the fluid

Convection cannot happen in solids because particles cannot move freely to circulate.

Kinetic theory explains both of these changes at the particle level:

• Evaporation: particles with the most kinetic energy escape from the surface of a liquid and become a gas, even below boiling point
• Condensation: gas particles lose energy and come close enough together to form a liquid

Factors that increase the rate of evaporation:

• Higher temperature (more particles have enough energy to escape)
• Larger surface area (more particles are at the surface)
• Air movement over the surface (removes escaped particles, keeping the concentration gradient high)
• Lower humidity of the surrounding air

The rate at which an object transfers energy by heating depends on:

• Surface area and volume — a larger surface area to volume ratio increases the rate of transfer
• Material of the object — good conductors transfer energy faster than insulators
• Nature of the surface in contact — for radiation, dark matt surfaces transfer energy faster than light shiny surfaces
• Temperature difference — the greater the temperature difference between an object and its surroundings, the faster the rate of energy transfer

Engineers and nature both exploit surface area to volume ratio to control energy transfer:

• Cooling fins on engines and electronics increase surface area, speeding up energy loss to prevent overheating
• Animals in hot climates (e.g. elephants, desert foxes) often have larger ears — this increases surface area relative to volume, helping them lose energy faster and stay cool
• Animals in cold climates tend to have smaller extremities to minimise surface area and reduce energy loss