Infra red radiation involves the transfer of Heat energy by electromagnetic raditation, also called thermal radiation.
No particles of matter are involved in the process.All objects emit and absorb infrared raditation. The hotter an object is the more infra red raditation it radiates.The amount of infrared radiation an object gives out or takes in depends on it's surface, shape and dimensions.
An object wil emit and absorb infrared radiation faster if there's a bigger difference in temperature between it and it's surroundings. Different materials transfer thermal energy at different rates.
At the same temperature dark, matt surfaces...
- emit more infrared raditation than light, shiny surfaces
- absorb more infrared radiation than light, shiny surfaces
Light, shiny surfaces are good reflectors of infrared radiation. An example of a good reflector is the 'silvering' on the inside of a vaccum flask.
Kinetic theory explains the different states and properties of matter in terms of th emovement of millions and millions of particles (atoms / molecules), the particles of gases, liquids and solids have different amounts of energy.
The atoms / molecules that make up gas are always moving. They move very quickly in random directions, colliding with each other and with the wall of the container they are in.
When the temperature increases...
- the gas molecules move faster
- the collisions become more intense
When the temperature falls...
- the gas molecules move more slowly
- the molecules move closer together
- the collisions become less frequent
- the gas begins to form a liquid.
Conduction is the transfer of energy by heating without the substance itself moving. For example, metals are good conductors of energy:
- As a metal becomes hotter the atoms vibrate more vigorously.
- This additional energy is transferred to the cooler parts of the metal by the free electrons that roam throughout the metal.
Insulators are materials that have few or no free electrons, so they can't readily transfer their energy by heating.
Convection is the transfer of energy by heating through the movement of particles.
Convection ccurs in liquids and gases, creating convection currents:
1. Particles in the liquid or gas nearest the energy source move faster, causing the substance to expand and become less dense.
2. The warm liquid or gas now rises vertically. As it does so it cools, becomes denser and eventually sinks.
3. The colder, denser liquid or gas moves into the space created (close to the heat source) and the cycle repeats
Evaporation and Condensation
Evaporation is the transfer of energy at the surface of a liquid.
In terms of kinetic theory:
- Atoms / molecules that are moving most rapidly are located at the surface of a liquid. These particles behave like a gas and escape.
- The overall energy of the atoms / molecules in the liquid then reduces. This results in a fall in it's tempertaure, i.e. it cools
When water evaporates from a surface, it becomes a gas called water vapour.
The opposite effect, where the gas or vapour returns to a liquid state at the suraface, is called condensation. This transfers energy back into the substance at the surface.
Heating and Insulating Buildings
Architects are able to calculate the amount of energy that is lost from bulidings by using U-values:
- U-values give a measure of how effective a particular material is as an insulator.
- The lower the U-value the better the material is as an insulator.
Solar panels may contains water that is heated by radiation from the Sun. This water can then be used to heat buildings or provide domestic hot water.
Specific Heat Capacity
Different materials (with identical masses and at the same temperature) can store different amounts of energy.
The quantity known as specific heat capacity is the amount of energy required to change the temperature of 1kg of a material by 1 oC. To work out the energy required use the following equation:
E = m x c x (temp change)
E= the energy transferred into joules
m= is the mass
c= is the specific heat capacity that has units of J /kg C