Physics 1

Infrared Radiation

Heat radation is the transfer of heat energy by inferred radiation.Infared radiation can be emitted by solids liquids and gasses. All objects are continually emitting and absorbing infared radiation. Infared radiation is emitted from the surface of an object.

Serveral factors affect the rate at which radiation is emmitted or absorbed by an object.

Temperature - an object the is warmer than its surroundings emits more radiation than it absorbs (as it cools down). Whereas an object that is cooler than its surroundings absorbs more radiation than it emits (as it warms up).

Surface colour and texture - the amount of infared radiation emitted or absorbed by a surface depends on the colour and texture not just the tempreture. Dark,matt surfaces absorb infrared radiation falling on them much better than light, shiny surfaces. they also emitt more infrared radiation. However light,shiny surfaces reflect a lot of the infrared radiation falling on them making them great for reducing the amount of energy transfered by radiation.

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Kinetic Theory

The three states of matter are solid, liquid and gas. The particles of a particular substance in each state are thesolid, liquid and gas same - only the arrangement and energy of the particles are different. The energy an object or particle has because of movement is its kinetic energy. In solids, strong forces of attraction hold the particles close together in a fixed regular arrangement. The paricles don't have much energy and so they an only vibrate about there fixed positions. Ther are weaker forces of attraction between the particles in liquids. The particles are close together but can move past each other and form irregular arangements. They have more energy than the particles in solids and so can move in random directions at slow speeds. There are almost no forces of attraction between the particles in a gas. The particles have more energy than those in a liquid and a gas and so are free to move in any direction at high speeds. When you heat a substance you give the particles more kinetic energy and so they vibrate faster this causes solids to melt and liquids to boil. Gases and liquids cn flow because ther particles can move past each other, solids can't flow as there particles are locked in a fixed position and so can't move past each other. Only gases are compresiable as there particles are very spread out, this means you can squash a gas into a smaller volume.

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Conduction

When you heat up a solid you give the particles more kinetic energy and so they vibrate more. As particles colide with their neighbouring particles they pass on some of their extra kinetic energy on to the particle they colided with. This is the process of conduction. This continues throughout the solid and so gradually some of the extra kinetic energy is passed all the way throught the solid. This causes a rise in tempreture in one end of the solid and so the amout of heat radiating from the solids surface increases. Usually conduction is faster in denser soids as the particles are closer together so they colide more often. Materials with relativly large spaces between their particles conduct geat much more slowly and so make good insulators. Metals are good conductors as they contain free electrons that are able to move inside the metal. The free electrons move faster at the hot end and collide with other free electrons, transferring the energy. These other electons pass on their extra energy to other electrons and particles in the solid. The electrons can move freely and so it is a quicker way of transfering the energy (rather than neighbouring particles bumping into each other) and so heat travels quickly through metals.

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Convection

Convection occurs when particles with more energy move from a hotter region to a cooler region. Liquids and gases can transfer heat by convection as they are free to move. Immersion heaters, kettles and convector heaters all work using convection currents.

Convection currents are all about the change in densitys. In immersion heater the water above the heater coils rises as it has a lower density however you will only get convection currents in the water above the heater as the water bellow stays cool.

Heating a room with radiators relies on convecction currents too. The hot, less dense (near the radiator) rises and the cooler, denser air flows to replace it.  

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Condensation

When a gas cools, the particles slow down as they lose kinetic energy. The space between the particles gets smaller as the attractive forces between the particles pull them closer together. If the temepreture gets cold enough the gas particles will get close enough together for condensation to take place and so the gas wil become a liquid.

If the tempreture of the gas is lower then condensation will hapen faster.

If the tempreture of the surface the gas touches is lower then condensation will happen faster.

If the concentration of the gas is higher then condensation will happen faster.

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Evaporation

Evaporation is when parrticles escape from being a liquid and form a gas, particles can evaporate fat tempretures much lower than a liquids boiling point. Near the surface particle can evaporate if they are traveing in the right direction to escape and if they have enough kinetic energy to overcome the attractive forces of other particles in the liquid. The fastest partices are most likly to evaporate from the liquid, this decreases the average particle energy and so the temperature of the liquid falls.

The rate of evaporation will be higher if:

The temperture is higher

The density of the liquid is lower

The surface area of the liquid is larger

The airflow of the liquid is greater  

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