- Created by: georgieh11
- Created on: 26-05-15 10:43
Infrared radiation is energy transfer by electromagnectic waves.
All objects emit radiation - we can detect it with our skin as it makes us feel warm.
The temperature of an object affects the rate at which it emits infrared radiation.
eg. The hotter an object is, the more infrared radiation is emitted at one time.
Dark, matt surfaces are good absorbers and emitters of infrared radiation.
They will become hotter and will transfer energy and cool down quicker than a shiny white object.
States of matter
The three states of matter are solid, liquid and gas. We can make a substance change between these states by heating or cooling it.
eg. If you heat ice (a solid) it will melt and form water (a liquid).
In a solid, the particles are in a fixed position.
In a liquid, the particles are in contact but can move freely, meaning the liquid doesn't have a fixed shape and can flow.
In a gas, the particles are usually far apart and move at random very quickly. It doesn't have a fixed shape and can flow.
The density of a gas is much less than that of a solid/liquid.
Conduction occurs mainly in solids as most gases and liquids are poor conductors.
If one end of a solid is heated, the particles at the end gain kinetic energy and vibrate more. This energy is passed through the neighbouring particles. In this way, energy is transferred through the solid.
This process occurs in metals. Non-metals are poor conductors because they do not contain free electrons.
When metals are heated their free electrons gain kinetic energy and move through the metal, transferring energy by colliding with other patricles. This is why metals are good conductors.
Poor conductors are called insulators. eg. wool and fibreglass
Convection occurs in fluids (liquids and gases).
Convection is the circulation of a fluid caused by heating it.
When a fluid is heated, it expands. The fluid becomes less dense and rises. The warm fluid is replaced by the cooler denser fluid, this resulting in convection currents which transfers the energy throughout the fluid.
This process happens when heating water in a beaker.
!!! Convection cannot occur in solids !!!
Evaporation and condensation
Evaporation is when a liquid turns into a gas.
Evaporation takes place when the most energetic liquid molecules escape from the liquid and enter the air, forming a gas. Because of the loss of kinetic energy, the temperature decreases. This means that evaporation causes cooling.
Condensation is when a gas turns into a liquid. Often takes place on cold surfaces (windows and mirrors).
The rate of evaporation and condensation can in increased by increasing the surface area or temperature of the liquid. eg. Decreasing the temperature of the liquid/surface will slow down the rate of evaporation and condensation.
The higher the temperature difference there is between and object and its surroundings, the faster the rate at which energy is transferred.
The rate at which energy is transferred also depends on
-Object's shape and surface area.
-Materials the object is in contact with.
To maximise the rate of energy transfer to keep things cool we use things that are good conductors and are painted dull black. (they are good emitters) and have the air flow around them maximised.
To minimise the rate of energy transfer to keep things warm. We use things that are good insulators, white and shiny and prevent convection currents by trapping air in small pockets.
Specific heat capacity
The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1°C.
The greater the SHC, the more energy is required for each degree temperature change.
The greater the mass of substance being heated, the more energy is required for each degree temperature change. If we had 2kg of copper, we would need to transfer twice the energy needed to raise the temperature of 1kg of copper by the same amount.
E = m × c × θ
E = energy transferred (J)
m = mass (kg)
c = SHC (J/kg°C)
θ = temperature change (°C)
Heating and insulating buildings
The rate of energy transfer in our homes can be reduced. This can be done by fitting:
-fibreglass loft insulation (reduces energy transfer by conduction)
-cavity wall insulation which traps air in small pockets (reduces energy by convection)
-double glazing (reduces conduction through windows)
-draught proofing (reduces energy transfer by convection)
-aluminium foil behind radiator (reflects infrared radiation back into the room)
The U-value of a material tells us much much energy per second passes through it. The lower the U-value the better the material is as an insulator.