Infrared radiation involves the tranfer of heat energy by electromagnetic radiation - thermal radiation. No particels of matter involved in the process.
All objects emit and absorb I.R
Hotter an object is, more I.R. it radiates
Amount of I.R. an object gives out or takes in depends on surface, shape & dimensions
Object will emit & absorb I.R. faster if bigger difference in temperature between it & its surroundings. Different materials transfer thermal energy at different rates
At same temperature, dark matt surfaces:
- emit more I.R. than light, shiny surfaces
- absorb more I.R. than light, shiny surfaces
Light, shiny surfaces good reflectors of I.R.
K.T. explains different staes & properties of matter in terms of movement of millions & millions of particles. Particles of gases, liquids & solids have different amounts of energy.
Atoms/ molecules that make up a gas are always moving. Move very quickly in random directions, colliding w each other & with the walls of container they are in.
When temperature increases:
- gas molecules move faster
- collision beome more intense
When temperature falls:
- gas molecules move more slowly
- molecules move closer together
- collisions become less frequent
- gas begins to form a liquid
Eventuallu liquid becomes solid. Atoms/molecules in solid can only vibrate about fixed position, form regular & orderly pattern
Energy Trasnfer by Heating
Energy transfer by heating involves movement of particles. Can be undertaken in variety of mechanisms including:
Conduction - transfer of energy by heating w/out substance itself moving. e.g. metals good conductors of energy:
- as metal becomes hotter atoms vibrate more vigorously
- additional energy is transferred to cooler parts of metal by free electrons that roam throughout the metal
Insulators - materials that have few or no free electrons, so can't readily trasnfer their energy by heating
Convection - tranfer of energy by heating through movement of particles.
Convection occurs in liquids & gases, creating convection currents:
1. Particles in liquid or gas nearest the energy source move faster, causing the substance to expand & become less dense
2. Warm liquid or gas now rises vertically. As it does so it cools, becomes denser & eventually sinks
3. The colder, denser liquid or gas moves into the space created & the cycle repeats
Evaporation and Condensation
Evaporation - tranfer of energy at surface of a liquid
In terms of K.T:
- Atoms/molecules that are moving more rapidly are located at surface of a liquid - particles behave like a gas & escape
- Overall energy of the atoms/molecules in liquid then reduces - results in fall in temperature
When water evaporates from a surface - becomes gas (water vapour)
Opposite effect, where gas or vapour returns to liquid state at the surface, is called condensation. Transfers energy back into substance at the surface
Rate of Energy Transfer
Under similar conditions different materials will transfer energy by heating at very different rates. Rate at which a material transfers energy depends on:
- surface area & volume
- type of material its made from
- nature of surface with which the material is in contact
- its temperature
the bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred
Heating and Insulating Buildings
Architects are able to calculate the amount of energy that is lost from buildings 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 panesls may contain 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 materiasl can store different amounts of energy. Quantity known as specific heat capacity is the amount of energy required to change the temperatuee of energy required to change the temperature of 1kg of a material by 1 degree celsius.