Transfer of Thermal Energy
Section: Thermal Physics | Syllabus: Cambridge IGCSE Physics (0625)
Methods of Thermal Energy Transfer
Thermal energy transfers from a region of higher temperature to a region of lower temperature by three methods: conduction, convection and radiation.
- Conduction - transfer of thermal energy through a material, mainly in solids, without the particles themselves moving through the material
- Convection - transfer of thermal energy through a fluid (a liquid or a gas) by the movement of the fluid itself
- Radiation - transfer of thermal energy by infrared electromagnetic waves, which does not need a medium
Conduction
- a good thermal conductor allows thermal energy to pass through it quickly; a thermal insulator (a bad thermal conductor) allows thermal energy to pass through it only slowly
- metals are good thermal conductors; most non-metals (wood, plastic, air, rubber) are poor thermal conductors (insulators)
Investigating Thermal Conductors
- rods of different materials (e.g. copper, aluminium, glass, wood), of the same length and thickness, are each fixed with a ball of wax at one end, using the same amount of wax
- the free ends of all the rods are heated together, at the same time and by the same amount - for example, by standing them in a beaker of hot water or heating them over a single flame
- the rod along which the wax melts first is the best thermal conductor; the rod along which the wax takes longest to melt is the worst thermal conductor (best insulator)
Comparing thermal conductors: the wax melts fastest on the best conductor
Conduction in Solids
Extended Only
- in all solids, particles that gain thermal energy vibrate more, about their fixed positions in the atomic or molecular lattice, passing the vibrations on to neighbouring particles
- in metals, thermal energy is also transferred by free (delocalised) electrons
- free electrons near the hot end gain kinetic energy and move quickly through the metal, transferring energy to particles throughout the metal
- this is why metals, which have many free electrons, are much better thermal conductors than non-metallic solids, which rely only on lattice vibrations
Why Conduction is Poor in Gases and Most Liquids
Extended Only
- in gases, particles are far apart and collide with each other only occasionally, so thermal energy is passed on only slowly by particle collisions
- most liquids have no free (delocalised) electrons, and although their particles are closer together than in a gas, they are not held in a fixed lattice, so thermal energy is also passed on only slowly by particle collisions
A Range of Conductors
Extended Only
- thermal conductivity is not simply "good" or "bad" - many solids conduct thermal energy better than good insulators, but less well than good conductors like metals (e.g. glass and ceramics conduct thermal energy better than wood or plastic, but far less well than copper or aluminium)
Convection
- convection is an important method of thermal energy transfer in liquids and gases - it cannot occur in solids, because it relies on particles being free to flow
- when part of a fluid is heated, it expands and becomes less dense than the surrounding fluid
- the warmer, less dense fluid rises, while cooler, denser fluid sinks to take its place
- this circulation of rising warm fluid and sinking cool fluid is called a convection current
Investigating Convection
- a crystal of potassium manganate(VII) (potassium permanganate) is dropped to the bottom of a beaker of water, and the beaker is gently heated with a Bunsen burner directly below the crystal
- the crystal dissolves slowly, releasing a purple colour that traces the movement of the water
- the purple water rises above the crystal, spreads out near the surface, then sinks back down further away, showing the circulating path of a convection current
A dissolving crystal traces the path of a convection current in water heated from below
Radiation
- thermal radiation is infrared radiation - all objects emit thermal radiation, regardless of their temperature
- thermal energy transfer by radiation does not require a medium - it can travel through a vacuum, which is how thermal energy from the Sun reaches the Earth
Effect of Surface Colour and Texture
- dull, black surfaces are good emitters and good absorbers of infrared radiation
- shiny, white (or silvered) surfaces are poor emitters and poor absorbers of infrared radiation, but good reflectors of it
Investigating Emitters and Absorbers
Extended Only
- comparing emitters: a container with four different vertical surfaces (dull black, shiny black, dull white and silvered), all filled with hot water to the same temperature, is placed with a radiation detector (e.g. a thermopile connected to a meter) at the same distance from each surface in turn
- the dull black surface gives the largest reading, showing it is the best emitter; the silvered surface gives the smallest reading, showing it is the worst emitter
- comparing absorbers: two identical metal plates, one painted dull black and one painted (or polished) shiny silver, are placed the same distance from an electric heater, with a temperature sensor (or a ball of wax stuck to the back of each with a pin) on the back of each plate
- the temperature rises faster on the dull black plate (or its wax melts first), showing that a dull black surface absorbs infrared radiation better than a shiny silver surface
Comparing emitters of infrared radiation using surfaces at the same temperature
Rate of Emission
Extended Only
- the rate at which an object emits radiation depends on:
- the surface temperature of the object - a hotter surface emits radiation at a greater rate
- the surface area of the object - a larger surface area emits radiation at a greater rate
Energy Balance and Constant Temperature
Extended Only
- for an object to stay at a constant temperature, it must transfer energy away from itself at the same rate that it receives energy
- if an object receives energy at a greater rate than it transfers energy away, its temperature rises
- if an object transfers energy away at a greater rate than it receives energy, its temperature falls
- the temperature of the Earth depends on the balance between incoming radiation from the Sun and radiation emitted from the Earth's surface back into space - factors that change this balance (such as the atmosphere absorbing and re-emitting some of the outgoing radiation) change the Earth's temperature
Consequences of Thermal Energy Transfer
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