Transfer of thermal energy
Good Thermal Conductors: These materials allow thermal energy to pass through them quickly. Metals (like copper and aluminium) are excellent conductors because they contain free (delocalised) electrons that move rapidly, carrying energy from the hot end to the cold end. They also have a lattice structure where vibrations transfer energy.
Thermal Insulators (Bad Conductors): These materials resist the flow of thermal energy. Examples include wood, plastic, glass, and air. In insulators, there are no free electrons to carry energy, and the particles are spaced such that lattice vibrations do not transfer energy efficiently.
Intermediate Conductors: Learning Objective 4 requires you to know that many solids (such as glass, ceramics, and some plastics) conduct thermal energy better than insulators but less well than good conductors. They lack free electrons but have a more ordered structure than gases, allowing some vibration transfer.
| Example |
|---|
| Copper, Aluminium |
| Glass, Ceramics |
| Wood, Plastic, Air |
In all solids, particles are fixed in a lattice. When heated, they vibrate more vigorously. These vibrations are passed to neighbouring particles, transferring energy along the lattice.
In metallic conductors, an additional mechanism exists: free (delocalised) electrons. These electrons are not bound to specific atoms and can move freely through the metal. They gain kinetic energy at the hot end, move rapidly to the cold end, and collide with ions/particles there, transferring thermal energy much faster than lattice vibrations alone.
Why? In gases and liquids, particles are far apart compared to solids. Because the distance between particles is large, collisions between neighbouring particles (which transfer vibrational energy) are infrequent. Therefore, thermal energy does not pass from particle to particle efficiently via conduction.
Learning Objective 5 & 6: Convection is the main method of thermal energy transfer in liquids and gases (fluids). It involves the bulk movement of particles.
How it works:
- A fluid is heated. The particles gain kinetic energy and move faster.
- The particles spread apart, causing the fluid to expand.
- This expansion decreases the density of the heated fluid (\rho = \frac{m}{V}).
- The less dense (warmer) fluid rises, while the denser (cooler) fluid sinks to take its place.
- This creates a convection current.
Experiment to Demonstrate Convection:
To see convection in water:
- Place a beaker of cold water on a tripod.
- Add a few crystals of potassium permanganate (or coloured powder) to the bottom corner.
- Gently heat the water just above the crystals with a Bunsen burner.
- Observation: The coloured water rises and moves across the surface, showing the path of the convection current.
Key Property: Unlike conduction and convection, thermal radiation does not require a medium. It can travel through a vacuum (empty space). This is how the Sun heats the Earth.
The colour and texture of a surface affect how it emits, absorbs, and reflects infrared radiation.
| Surface Property | Emission/Absorption | Reflection |
|---|---|---|
| Dull Black | Best emitter / Best absorber | Poor reflector |
| Shiny White/Silver | Poor emitter / Poor absorber | Good reflector |
Note: A good absorber is also a good emitter. A poor absorber is a poor emitter.
- Surface Temperature: The higher the temperature of the surface, the greater the rate of emission.
- Surface Area: The larger the surface area exposed to the surroundings, the greater the rate of emission.
Therefore, a large, hot, dull black object will emit thermal radiation at the highest rate.
- Constant Temperature: The object transfers energy away at the same rate it receives energy. (Net energy transfer = 0).
- Temperature Increases: The rate of receiving energy is greater than the rate of transferring energy away.
- Temperature Decreases: The rate of receiving energy is less than the rate of transferring energy away.
The Earth's Temperature:
The Earth's average temperature is determined by the balance between incoming solar radiation and infrared radiation emitted from the Earth's surface. Factors like greenhouse gases (e.g., carbon dioxide, methane) trap some of the re-emitted radiation, reducing the rate at which energy escapes into space, leading to a rise in the Earth's average temperature.
Simple Application: Heating a Room
A radiator heats the air next to it by conduction. This warm air becomes less dense and rises (convection), creating a current that circulates heat throughout the room. The radiator also emits infrared radiation (radiation) which heats objects directly.
Complex Application: A Fire Burning Wood
- Conduction: Heat travels through the wood from the burning surface to the unburnt parts.
- Convection: Hot gases and smoke rise rapidly, carrying thermal energy upwards.
- Radiation: Infrared radiation is emitted from the flames, heating people and objects nearby without needing air contact.
Complex Application: Car Radiator
- Conduction: Heat transfers from the hot engine block through the metal walls of the radiator pipes to the coolant fluid.
- Convection: The coolant circulates (forced convection by a pump) and air flows over the radiator fins (natural/forced convection) to remove heat.
- Radiation: Some heat is radiated from the hot surfaces of the radiator into the surrounding air.
Correction: In conduction, particles only vibrate about fixed positions; they do not move from place to place. Convection involves the actual movement (flow) of the fluid particles.
Correction: Thermal radiation is an electromagnetic wave and can travel through a vacuum. This is why we feel heat from the Sun.
Why examiners accept this: You must explicitly link temperature, density, and movement. Simply saying 'hot air rises' is often insufficient. You must explain why it rises (density change).
Correct phrasing example: 'The air is heated, causing the particles to spread out and the density to decrease. The less dense warm air rises above the denser cool air.'
Why examiners accept this: Examiners look for specific keywords linking surface properties to radiation types. 'Black' and 'Dull' are linked to emission/absorption. 'Shiny' and 'White' are linked to reflection.
Correct phrasing example: 'A dull black surface is a good emitter of infrared radiation, whereas a shiny silver surface is a poor emitter but a good reflector.'
- Fill two identical cans (one dull black, one shiny silver) with hot water at the same initial temperature.
- Place thermometers in each can.
- Measure the temperature drop over a set time.
- The can with the greater temperature drop is the better emitter.
- The surface temperature of the object.
- The surface area of the object.
- Conduction: The glass walls are thin or vacuum-sealed (vacuum has no particles to conduct).
- Convection: The vacuum prevents air currents.
- Radiation: The silvered surfaces reflect infrared radiation back into the flask.