K323 / 2027
Thermal processes overview

Topic 1 of 4

Heating and conduction

A temperature difference drives a net transfer of energy from a hotter region to a cooler one. Conduction transfers that energy through interactions between particles.

Heating is an energy transfer, measured in joules. Temperature relates to average random particle kinetic energy. Heat and cold are not substances stored inside objects.

Thermal equilibrium: equal temperatures

Place a metal block at 60°C in thermal contact with another at 20°C. Energy transfers by heating from the hotter block to the cooler one. If the pair is isolated from the surroundings, their temperatures approach a common value.

At thermal equilibrium, the two regions have the same temperature and there is no net energy transfer by heating between them. Their particles continue moving and interacting; equilibrium does not mean that particles stop.

The starting temperatures alone do not determine the final temperature. The amounts and materials also matter, so do not automatically take the average of 60°C and 20°C. Equal final temperatures also do not imply equal total internal energies.

How a solid conducts

Particles at the hotter end of a solid have greater average kinetic energy in their vibrations. Through interactions with neighbouring particles, energy is transferred along the structure towards the cooler region.

The atoms or molecules remain near their usual positions while vibrating. The solid does not conduct by sending its hot particles in a stream from one end to the other.

In a metal, mobile electrons provide an additional way to transfer energy. Electrons move through the structure and exchange energy through interactions with its particles and other electrons. Their motion in both directions produces a net transfer from the hotter region towards the cooler one.

Energy passes through the solid towards the cooler region

The large circles mark fixed mean positions. Particles at both ends vibrate; the solid does not flow along the strip.

Vibrations in a solid

Vibrations in a solid: net energy transfer from the hotter left end to the cooler right endA solid strip has its hotter end on the left and its cooler end on the right. Large particles occupy fixed mean positions in two rows. Blue double-ended arrows indicate vibration about those positions. The larger indicators at the hotter end are qualitative. Neighbouring particles interact and transfer energy; neither end has stationary particles. A separate orange arrow below the strip points right and represents net energy transfer, not the path of a particle. The models and arrow sizes are schematic.Hotter endCooler endNet energy transfer

Blue double arrows: vibration about fixed positions. Neighbouring particles transfer energy through their interactions.

An additional mechanism in a metal

An additional mechanism in a metal: net energy transfer from the hotter left end to the cooler right endA solid strip has its hotter end on the left and its cooler end on the right. Large particles occupy fixed mean positions in two rows. Small blue dots represent mobile electrons in a metal. Their blue motion arrows point in varied directions, including towards both ends. Electrons transfer energy through interactions. The large metal particles also vibrate, although their vibration indicators are omitted in this panel. The diagram does not show a one-way electric current. A separate orange arrow below the strip points right and represents net energy transfer, not the path of a particle. The models and arrow sizes are schematic.Hotter endCooler endNet energy transfer

Small blue dots and arrows: mobile electrons and their motion. They provide an additional way to transfer energy through a metal.

The orange arrows show the direction of net energy transfer. Their lengths do not measure transfer rates.

The solid particles vibrate near fixed positions, including at the cooler end. Mobile electrons add a transfer mechanism in the metal. The net energy-transfer direction is different from the individual particle or electron motion.

Conductors and insulators have different rates

With comparable dimensions and temperature differences, a good thermal conductor transfers energy more readily than a thermal insulator. An insulator slows conduction; it does not make all energy transfer impossible.

A metal pan base helps transfer energy from a hotter hob to the cooler contents. A suitable insulating handle slows conduction towards a hand. The useful material depends on which transfer the design is meant to encourage or reduce.

Worked explanation

Metal and wood at the same room temperature

A metal block and a wooden block have both reached 20°C in a room. A hand is warmer than either block.

  1. Direction: energy leaves the warmer hand and enters the cooler block in both cases.
  2. Rate: the metal conducts energy away from the contact region more readily than the wood.
  3. Observation: the hand cools faster in contact with the metal, so the metal can feel colder even though both blocks started at the same temperature.

The sensation concerns the rate at which the hand loses energy. It is not evidence that the metal began at a lower temperature.

Name the net transfer and its mechanism. In solid conduction, particle interactions transfer energy without bulk flow of the solid. In metals, include mobile electrons as well as vibrating particles.

Optional check A metal block and a wooden block have both been in a 20 degrees C room long enough to reach that temperature. A warmer hand touches each. Why can the metal feel colder?
A metal block and a wooden block have both been in a 20 degrees C room long enough to reach that temperature. A warmer hand touches each. Why can the metal feel colder?