KS3 Energy Conduction, Convection and Radiation

Resources…

Y8 2 Heating and Cooling (Lesson resources)

Term

Definition

particle

a minute portion of matter

Internal energy

Random energy from motion of particles

temperature

heat in a substance

 Celsius

Temperature scale  based on water BP and MP

convection

Energy movement based on density changes

Joule

Unit of energy measurement

conduction

Energy transfer when particles  vibrate

Radiation

Energy transfer via Infra Red EM waves

evaporation

Fast particles escape a liquid taking kinetic energy with them

Boiling point

When a substance starts to break apart and become a gas with large spaces between

Melting point

When substance stops having a fixed shape and atoms slide over each other and can move more freely

Kinetic Energy

Movement energy of particles

Internal Energy

The movement and vibration of structures such as solids.

 

Temperature

Temperature is the result of the average total kinetic energy of particles in matter.  (movement)

Heat is transfer of thermal energy; it flows from regions of high temperature to regions of low temperature.

Thermal energy is stored as kinetic energy and, in molecules and solids, and as potential energy in the modes of vibration. We call this “Internal Energy”

  • A hot sparkler has a high temperature but low thermal energy in comparison…
  • A warm bath has a lot of thermal energy but a low temperature.

 

Thermal Energy Stores – Kinetic Theory

Theory: A metal consists of a lattice of atoms, each with a shell of electrons. This is also known as a positive ionic lattice. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a sea of electrons, This sea is a kinetic energy store  and freely moves through the metal when there is an external temperature difference.

 

Conductors and Conduction

 

The heat passes through the saucepan from the gas flame or electric element by conduction. Conduction is the way that heat is transferred in a solid.

Some solids conduct heat better than others. The best conductors by far are metals. Copper is especially good at conducting heat. Expensive saucepans often have a copper bottom to quickly conduct heat through the base to the food inside.

Many non-metals such as paper, cloth, wood and plastic are bad conductors of heat. We say they are thermal insulators.

Air is also a very bad thermal conductor. Things like duvets and ski jackets keep us warm because they are made from a padded material that traps lots of air between its fibres. This makes it a very good insulator.

We can explain conduction by thinking about what is happening to the particles in a solid.

  • The particles in a solid are close together and arranged in a regular pattern. They vibrate on the spot.
  • Heating the solid at one end transfers energy to the particles there and they vibrate more.
  • They bump into nearby particles, transferring energy to them and making them vibrate more.
  • These then bump into particles further along.
  • In this way heat energy is quickly transferred to the other end of the solid.

With the exception of mercury, ALL LIQUIDS ARE POOR HEAT CONDUCTORS.

This is because the particles in liquids are further apart than the particles

in solids. GASES are worse HEAT CONDUCTORS than liquids. They are good HEAT INSULATORS.

This is because The particles in gases are much further apart than they are in liquids.

Heat energy can move through a substance by conduction. Metals are good conductors of heat, but non-metals and gases are usually poor conductors of heat. Heat energy is conducted from the hot end of an object to the cold end.

The electrons in piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go.

 

Convection

The movement of warm and cool air creates a convection current. Birds, like this soaring bald eagle, use convection currents to rise and fall with very little effort. So do gliders which are aeroplanes without engines. An aeroplane tows a glider up into the sky. When they separate the glider uses convection currents, called thermals, to stay in the air.

A current, called a convection current becomes present in the liquid If thermal energy is supplied. The cooler liquid falls and Warmer, less dense liquid rises. The cycle continues as the heat is supplied. Energy is transferred. The Earth’s lithosphere (the crust) is cracked into a number of large pieces (tectonic plates) which are constantly moving. This is as a result of convection currents within the Earth’s mantle driven by heat released by natural radioactive processes.

In the usual water and dye experiment with a square shaped shaped pipe….

  1. The water near the flame gets hot and expands because the
  2. particles in the hot water move faster and get further apart.
  3. The water becomes less dense and so rises.
  4. Cooler, denser water moves in to take its place.
  5. This water gets heated, and so on.
  6. Soon the coloured dye has spread all around the tube and all the water is heated.

 

Have you ever wondered why smoke goes up a chimney? The smoke is hot. Hot air, as well as smoke, rises. The rising of warm air through colder air is called convection.

  • The air particles gain energy, moving faster.
  • The atoms move further apart so the density is now lower
  • the less dense air rises.
  • The colder air contracts and falls as particles lose energy.
  • Lower density air falls towards the ground

 

Infrared radiation

All objects give out and take in thermal heat which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits. Infrared radiation is a type of electromagnetic radiation that involves waves NOT particles, so it can travel through a vacuum. Black objects are strong emitters of IR radiation, meaning that they give off large amounts of IR radiation. Silver objects are  poor emitters of IR radiation, meaning that they give off small amounts of IR radiation.

Leslie’s cube is a device used in the measurement or demonstration of the variations in thermal radiation emitted from different surfaces at the same temperature.

In use, the cavity was filled with hot water; the entire cube has essentially the same temperature as the water. The IR detector showed much greater emission from the side with varnish than from any of the other three sides. (dull)

 

Vacuum Flask

A vacuum flask stops heat energy being lost by conduction, convection and radiation. As well as keeping coffee hot it can keep drinks cold.

The vacuum has no particles. It stops heat energy being lost by conduction. It stops heat energy being lost by convection. The vacuum does not stop energy being lost by heat radiation – infrared.

The shiny walls reflect the infrared radiation so it cannot escape. Shiny surfaces are poor emitters and absorbers of heat radiation.

The stopper is made of an insulating material. It stops heat energy being lost by conduction through the shiny silver surfaces. It also stops heat energy being lost by convection.

 

Evaporation and Condensation

Condensation: The particles in a gas have different energies but if a colder object touches them they may condense to form a liquid. They don’t have enough kinetic energy to remain as separate particles so the coalesce into groups to form a liquid.

EvaporationSome particles in a liquid will have enough kinetic energy to escape from the liquid and become a gas. The remaining particles in the liquid have a lower average kinetic energy than before, so the liquid cools down as evaporation happens.

The rate of evaporation increases if….

  • the temperature of the liquid is increased.
  • the surface area of the liquid is increased.
  • air is moving over the surface of the liquid.

Liquids Conduct Poorly

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