- heat is a type of energy, measured in joules
- tempreature is a measure of kinetic energy in the particles, measured in degrees celcius(°C) ,degrees fahrenheit (°F) or kelvin (K)
- 0 kelvin is absulute 0, when the particles stop moving
- -273 °C is the same as 0K
- a kelvin is the same size as a °C
- when converting kelvin into °C you + 273
- when converting °C into kelvin you - 273
- you cant have negative Kelvin tempreatures
Heat Conduction and Kinetic Theory
Kinetic theory uses the idea of induvidual particles moving to explain the properties of a substance.
heat conduction is where heat energy is passed directly from one particle to another. when the bonds between particles are strong, if one particle heats up and starts to vibrate, the particles that are bonded to this particle will start to vibrate and heat up, this then spreads as the energy is passed on.
in solids there are stronger bonds between the particles than in liquids so the heat passes on more quickly, solids are better conductors. gas is an even worse conductor because none of the particles are bonded and the particles only conduct when they colide.
metals are very good conductors because they have 'free electrons'. the atoms release an electron and become posotively charged ions and the elctrons that are released are 'free' allowing them to conduct heat very quickly.
heat can move by travelling as infa red waves. these are electromagnetic waves, like light waves, but with a longer wavelength
this means that infa red waves act like light waves.
- they can travel through a vacuum
- they can travel at 300,000,000 m/s
- they can be reflected
- they cant travel through opaque materials
black objects are the best radiators
1. plastic cap, 2. plastic container, 3. double walled glass container, 4. vacuum, 5. silvered surface, 6. plastic support,
plastic is used because it is a bad conductor, silver colour is used because its a bad radiator, a vacuum is used because nothing can conduct or convect through it, heat cannot conduct well through two seperate surfaces
Insulation at Home
- roof insulation, the insulation material has air gaps that dont conduct the air well
- a reflective suface in the roof radiates heat back into the house
- cavity walls, the air in the gap is a poor conductor
- double glazed windows have an air gap to reduce heat conduction
- insulation in the gap in the walls, stop the warm air from convecting upwards and conducting out of the roof
- insulation around the boiler to keep the heat energy in
- carpeted floors top the heat conducting throught the floor because carpet has air gaps
- draft excluders stop the warm air convecting out through the gap under the door by creating air gaps
- curtains stop the air convecting out throught the windows because the thick curtains are poor conductors
Convection and Heat sinks
- convection currents can form when liquids and gases are heated. the cold fluid sinks and the hot fluid rises because the cold fluid has a higher density than the hot fluid.
- flames go upwards because hot air is less dense than cold air, so it rises. in a smoke filled room, you should lie on the floor because the cold clean air sinks below the hot smoke.
- the element in a kettle is always at the bottom because the hot water at the bottom rises and its place is filled by cold water to be heated.
- in a fridge, the freezer compartment is always at the top because the air at the top gets cooled by the freezer and sinks, this means that the fridge needs very little cooling
a heat sink is made of very dense metal and is designed to keep electrical components cool. because the metal is dense, all of the molecules are tightly packed and because it is a metal, it is a very good conductor.
any heat from the electrical component will be conducted into the heat sink because the molecules heat will then be convected into the air around it and the component will be kept cool
how particle size effects heat loss
liquids with smaller particle size loose heat quicker because, the smaller particles can vibrate faster when they are warm, but they disperse the heat energy more quickly. this is because when the liquid is warm, the particles vibrate and some of the particles evaporate, this takes lots of energy. the smaller particles can evaporate more quickly so the energy is lost from the liquid more quickly.
ethanol, water propanol and octanol. the ethanol lost its heat the quickest, next water, then propanol, last octanol. this is because octanol has very large particles and ethanol has very small
factors that effect evaporation:
wind, the wind carries evaporated particles away so the other particles are less likely to be knocked back into the liquid. tempreature, the particles have more energy to evaporate when they are warmer. humidity, if there are lots of evaporated particles in the air already then there is more chance that the particles will be knocked back into the liquid. surface area, if more of the liquid is in contact with the air, more of the liquid is likely to evaporate
Specific Heat Capacity
the SHC of a substance is the amount of energy it takes to raise the tempreature of one kg of the substance by one °C.
SHC = energy/mass x change in tempreature. the units are J/Kg/°C
the SHC of aluminium is 1200 J/Kg/°C
the SHC of Copper is 670 J/Kg/°C
the SHC of water is 8200 J/Kg/°C
u-values are W/m²/°C the W stands for Watts or J/s
the engergy lost through a window could change because of: the tempreature outside compared to the tempreature inside and the thickness of the glass.
the U-Value of a double glased window is less than half of a standard window, this shows how much more efficient it is at keeping heat energy inside the house.