1
Q
thermal energy
A
- electromagnetic radiation generated by the thermal motion of particles in matter
- not the same as temp (a measure of ‘hotness’) – but when the thermal energy of a substance increases, its temperature usually increases too because the higher the temp , the faster its particles move, on average
2
Q
conduction
A
- thermal energy/heat transfers from regions at higher temp to regions at lower temp
- conduction/thermal conduction is the transfer of heat from one place to another through the passing on of kinetic energy between the particles of a substance
- conduction can also transfer heat from a substance in one state to a substance in another state if the two are in contact
3
Q
thermal conduction in solids and liquids
A
- the particles in the hotter region vibrate more energetically
- over time, some of this energy is passed along to neighbouring particles, so that they also vibrate more energetically
- this process continues through the solid, so that the temperature rises even in the region farthest from the heat source
4
Q
thermal conductors and insulators
A
- good conductor=a substance through which heat transfers by conduction relatively quickly.
- best conductors: copper, silver and gold
- insulator=a substance through which heat transfers by conduction relatively slowly
- e.g. plastics, wood, gases
- heat cannot transfer through a vacuum by conduction, since a vacuum does not contain particles
5
Q
thermal conduction in gases
A
-gases are typically poorer conductors than liquids and solids because the particles in a gas are far apart relative to their size so collisions are not frequent enough to transfer kinetic energy between particles as quickly
6
Q
thermal conduction in liquids
A
- in liquids, energy is transferred via collisions between the moving particles
- liquids are typically poorer conductors than solids (but better than gases), because in a liquid the particles aren’t held tightly together, so it takes longer for kinetic energy to be transferred between particles
7
Q
conduction in metals
A
- in solid and liquid metals, there are free electrons, which can move through the lattice of metal ions
- when part of a metal becomes hotter, the ions and free electrons gain kinetic energy
- ions can only pass on kinetic energy relatively slowly, but, free electrons can transfer energy much faster, by moving through the lattice and colliding with ions and with each other
8
Q
factors affecting rate of conduction between 2 objects
A
- temp difference between objects (the higher the temp difference, the higher the rate of conduction
- distance between the two objects (the shorter the distance, the higher the rate of conduction)
- area of contact with connecting material (the greater the area of contact, the higher the rate of conduction)
9
Q
fluids density
A
- when the temp of a fluid increases, the average speed of the particles increases
- the particles collide with each other more frequently and with greater force
- if the fluid is not confined to a rigid container, its particles move further apart and the fluid expands
- therefore, the density of the fluid decreases as temp increases, because there are now fewer particles (and hence less mass) per unit volume
10
Q
convection
A
Convection occurs when a region within a fluid is heated (heat transfers from the heat source to the fluid mainly by conduction).
- the warmer fluid has lower density than the surrounding cooler fluid so it moves upwards, pushing the cooler fluid downwards
- as the warmer fluid rises, it gradually cools by conduction of heat to the cooler fluid around it,
- it becomes more dense, and tends to sink again
- if the heat source continues to supply heat, a cycle of moving fluid, called a convection current, can develop within the fluid
- If a heat source warms fluid at the top of a container, a convection current is not set up, because the warmer, less dense, fluid stays at the top
11
Q
conduction vs convection
A
- convection transfers heat through a fluid much more quickly than conduction
- both require the presence of particles
- both can occur in fluids
- conduction can occur in solids, but convection cannot
- in conduction, heat is transferred by kinetic energy being passed from one particle to the next, whereas in convection, heat is transferred by motion of a large numbers of particles which carry kinetic energy with them
12
Q
thermal/infrared radiation
A
- a type of wave that is one of the parts of the electromagnetic spectrum
- it travels at the speed of light (3x108) like all the waves in the ES
- it does not need a medium to travel (so it can transfer energy through a vacuum)
- it is not visible to the human eye
13
Q
radiation
A
- transfer of heat directly from one object to another, without heating up (or passing heat through) a medium connecting the two objects
- slower than convection and conduction
14
Q
emission of thermal radiation
A
- any substance with a temp > absolute 0 emits thermal radiation
- the higher the temp of an object, the higher the rate of thermal radiation emission (where ‘rate’ means energy per second, or power)
- when an object emits thermal radiation, thermal energy is transferred to energy of the radiation (as long as the object is not changing state)
- if an object emitted thermal radiation without any other energy transfers occurring, its temp would decrease
15
Q
absorption of thermal radiation
A
- thermal radiation can be absorbed, reflected or transmitted by an object
- when an object absorbs thermal radiation, energy of the radiation is transferred to thermal energy of the object (as long as the object is not changing state)
- if an object absorbed thermal radiation without any other energy transfers occurring, its temp would increase
16
Q
combined effect of emission and absorption
A
- If an object is at a higher temp than its surroundings, it emits thermal radiation at a higher rate than it absorbs it so there is a net loss of thermal energy from the object and its temp decreases (in the absence of other energy transfers)
- if an object is at a lower temp than its surroundings, it absorbs thermal radiation at a higher rate than it emits it so there is a net gain in thermal energy of the object and its temp increases (in the absence of any other energy transfers)
17
Q
air
A
-air absorbs thermal radiation only weakly
18
Q
shiny and small surface area
A
- poor absorbers of heat
- poor emitters of heat
19
Q
dull/matt/rough and large surface area
A
- good absorbers of heat
- good emitters of heat
20
Q
transfer of thermal energy
A
- if there is a transfer of thermal energy to or from an object (and it is not changing state), the change in the temp of the object depends on:
- the mass of the object
- the type of material it is made of
21
Q
heat capacity
A
- the transfer of thermal energy per unit change in temp of an object
- unit = J °C−1
- heat capacity is a property of a particular object/sample
22
Q
specific heat capacity
A
- specific heat capacity = thermal energy / (mass x temp change)
- J kg–1 °C–1
- specific heat capacity is a property of a type of substance
23
Q
Why does a hot air balloon rise?
A
- as the air in the balloon gets heated, it expands and flows out the bottom
- therefore the density decreases as hotter gas takes up more volume
- decreased density -> lower mass than surrounding air -> balloon rises
24
Q
evaporation
A
- the larger the surface area the easier it is for liquid to evaporate from a surface
- the warmer the liquid, the quicker the evaporation
25
black vs white clothes
- dark surfaces are better at absorbing and emitting radiation than white surfaces
- in winter, the outside air is colder than body temperature
- therefore, wearing white clothes is better in winter as less cold air is absorbed and your body heat will not be emitted
BMAT Physics
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