Revision summary

Start with the temperature difference

Net energy transfers by heating from higher to lower temperature. At thermal equilibrium, temperatures are equal and there is no net heating between the regions. Particles still move. Equal temperature does not establish equal internal energy.

Conduction

In a solid, interactions between vibrating particles transfer energy without bulk flow of the material. Mobile electrons add a mechanism in metals. Good conductors transfer energy more readily under comparable conditions; insulators slow rather than eliminate it.

Convection

Energy is carried with moving liquid or gas. With ordinary expansion, warming increases volume for the same mass, decreasing density. Less dense fluid rises while cooler, denser fluid returns. Explain a complete circulation and its conditions.

Radiation

Electromagnetic waves transfer energy without a material medium. Objects emit and absorb, so net transfer depends on both. Radiation can cross a vacuum.

Retain all three radiation comparisons

• Colour and texture: dull black is a better absorber and emitter than shiny metallic in the usual comparison of similar opaque surfaces.
• Temperature: higher surface temperature gives a greater emitted rate when finish and area are unchanged.
• Area: larger exposed area gives a greater total emitted rate at the same temperature and finish; equal illumination is needed when comparing absorption.

Hold the other conditions fixed. An emitted rate alone is not a net loss, and visible colour alone is not a universal rule for every material and wavelength.

Explain a design one route at a time

In a vacuum flask, the gap suppresses conduction and convection across it; reflective faces reduce radiation; the stopper limits conduction and fluid exchange at the opening. Neck and support connections remain conduction routes. The same design slows warming of a cold drink and cooling of a hot drink.

For faster cooling, conducting metal spreads energy to exposed surfaces, a larger area supports greater transfer, and moving air carries energy away. The intended result determines which transfers to encourage.

Use measurements to support the explanation

In a surface comparison, match material, mass, geometry, starting temperature, source distance, orientation, surroundings, sensor placement and timing. Compare changes over the same interval. A larger rise in matched samples indicates greater net energy gain under those conditions.

Record temperatures and elapsed times with units. Repeat from the same starting conditions to assess variation, and correct specific causes such as a sensor touching the wrong region or unequal lamp distances. A simple cooling test in air has not removed conduction and convection.