Transfer of Thermal Energy
Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)
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
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
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
- Heating a kitchen pan: the metal base and sides of a pan are good thermal conductors, so thermal energy is conducted quickly from the hob to the food; the handle is often plastic or wood (a thermal insulator), so it conducts thermal energy only slowly and stays safe to hold
- Heating a room by convection: a heater warms the air close to it, which expands, becomes less dense, and rises; cooler, denser air sinks and flows in to replace it near the heater; this circulation sets up a convection current that gradually warms the whole room
Applications Involving More Than One Transfer Method
Extended Only
- A fire burning wood or coal:
- conduction transfers thermal energy through solid objects in direct contact with the flames, such as neighbouring coals or logs
- convection carries hot gases and smoke upward above the fire, drawing in fresh air from the sides to feed the flames
- radiation is emitted directly from the flames, warming people and objects nearby without needing contact or air movement
- A radiator in a car:
- conduction transfers thermal energy from the hot engine into the coolant, and from the coolant through the metal walls of the radiator
- convection circulates the coolant between the engine and the radiator, and moves air through the radiator's fins (often helped by a fan)
- radiation is emitted directly from the hot surface of the radiator
Summary: Methods of Thermal Energy Transfer
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