Radiation

- Radiation is how heat is transferred through the vacuum of space and transparent objects.

Example of Infrared Radiation:

Infrared is an electromagnetic ray.

All hot/warm objects give out heat through infrared radiation. Infrared cameras can also capture images.

Heat Radiation:

- All objects emit/absorb heat radiation. The colour of the surface determines whether it's good or bad at emitting and absorbing.

- A surface that is good at radiating heat is a good absorber. Radiation is one way of transferring heat energy. Dark-coloured surfaces are sound absorbers of radiation, so they become hot. Light can reflect radiation. Shiny surfaces will reject radiation as they're poor absorbers. Shiny silver surfaces are the worst absorbers of radiation.
- Earth absorbs radiation from the sun. In extremely hot countries, people often paint their houses white to reflect heat radiation away from the sun. It's also why space blankets are silver, as they'll keep the person warm by reflecting heat radiation back to them.

Vacuum Flask

The vacuum flask keeps things hot or cold by stopping or reducing the transfer of energy by conduction, convection, or radiation.

- Vacuum → prevents conduction and convection.
- Plastic case → prevents conduction
- Double glass bottle → prevents conduction
- Stopper → prevents convection
- Silvering → prevents radiation
- Support → prevents radiation

Conduction:

- Conduction occurs in solids.
- Direct heating causes the molecules to vibrate more quickly.
- The vibrations travel from the hot region to the cold region due to their close proximity.

- Insulators such as plastics, wood, liquids, and air are poor conductors of heat since the particles are too far apart, or they don't vibrate well.
- Metals are the best solids at conducting heat because they contain 'free electrons' that help carry the heat energy through the metal.

Ranking Conductivity of Metals:

1) Copper
2) Aluminium
3) Zinc
4) Steel
5) Brass

- Thermal conduction is the process where vibrating particles transfer energy from their KE store to the KE stores of other neighbouring particles.

- Conduction is the main form of heat transfer in solids because the molecules are relatively close together. Extra heat energy makes the molecules move more as they pass on their vibrational energy to neighbouring molecules. Metals are good conductors of heat energy as they have free electrons that can move through the solid and transfer energy. The electrons give up energy when they collide with other molecules.

- Wooden skewers are used to toast marshmallows, as metal ones are conducting heat better than the wire and would be too hot to hold. Wood isn't a proficient conductor of heat.
- It melts quicker on metal surfaces than plastic ones, as metal conducts energy better than plastic. Therefore, energy is transferred quicker on the metal surface, which is why the ice melted quicker too.

Convection

- Convection is how heat is transferred through liquids and gases.
- It works on the principle that liquids and gases become less dense when heated.
- This is because the molecules gain kinetic energy and move further apart.

The particles don't change in size.

Detailed Examples:

Air conditioner:

- The fan is positioned at the top of the room, where the cold air sinks and creates a convection current to remove the hot air from the room.

Onshore breeze:

During the day, the land heats up faster than the sea. Hot air rises from the land, and cool air moves in from the sea, producing a wind.

Other Examples:

1) Lava lamp
2) Radiator
3) Thermal flask
4) Water tank
5) Plate tectonics
6) Offshore breeze

- With convection, the heat causes the particles to move further apart, and so you get a region of less dense material. This rises away from the source of heat, where eventually it cools down and sinks. This produces a convection current. A duvet or blanket traps the air around your body, therefore convection currents form, which stops convection currents from forming.

Insulation

- There are different ways to prevent heat transfer by conduction, convection, and radiation.

Conduction:

- You can use a material that's a poor conductor, such as nonmetals, or a layer of trapped air, as it's an insulator.

Convection:

- To prevent convection currents, you need to trap the air/liquid. This can be done with a lid/blanket. The layer of trapped air will also prevent conduction.
- A vacuum will also prevent conduction and convection because it contains no particles.

Radiation:

- In order to prevent radiation from causing heat to escape, it's best to use a silver and shiny surface, as this is a bad absorber and emitter of heat radiation since it reflects it.

Preventing Heat Loss in Houses:

- Roof → Loft insulation: Fibreglass wool laid on your roof reduces conduction through the ceiling into roof space.
- Walls → Cavity wall insulation: Insulating foam between two brick walls traps pockets of air, reducing convection. The two brick walls also stop conduction.
- Floors → Carpet: Laid across a floor to reduce convection.
- Draughts → Draught-proofing: Foam/Plastic around doors and windows stops air going out, which reduces convection.
- Windows → Thick curtains: Reduce conduction and radiation through the windows.
- Double Glazing: An air gap between two layers of glass reduces conduction.

Testing Surfaces:

- Place an empty Leslie cube on a heatproof mat.
- Fill with boiling water.
- After waiting for the cube to warm up, hold the thermometer against each of the four faces. The temperature should be the same.
- Hold the IR detector set distance away from each face and record the amount of IR it detects.
- Repeat to make sure results are repeatable.