Temperature, pressure and volume
Brownian motion:
- Gas molecules move rapidly and randomly
- This is due to collisions with other gas molecules
- Massive particles may be moved by light, fast-moving molecules
- The temperature of a gas is related to the average kinetic energy of the molecules.
The higher the temperature, the greater the average kinetic energy
and so the faster the average speed of the molecules.
- Gases exert pressure on a container due to collisions between gas molecules and the wall. When
the molecules rebound off the walls, they change direction so their velocity and therefore
momentum changes. This means they exert a force because force is equal to the change in
momentum over time.
At a constant volume, if the temperature increases, the pressure increases because the molecules move
faster so they collide harder and more frequently with the walls - creating a force
At a constant temperature, if the volume increases, the pressure decreases because the molecules collide
less frequently with the walls.
- For a gas at fixed mass and temperature, 𝒑𝑽 = 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕 or P1V1=P2V2
(p is the pressure in Pascals and V is the volume in m3)
- It is inversely proportional (one decreases, the other increases)
, Evaporation
- Evaporation is the escape of molecules with higher energy from the surfaces of liquids.
- After they escape, the remaining molecules have a lower average kinetic energy which
means the temperature is lower (i.e. evaporation cools the liquid).
To increase the rate of evaporation:
- Increased Temperature: more higher energy molecules at the surface
- Surface Area: more molecules at the surface
- Draught: molecules are removed before returning to the liquid
Evaporation cools a body in contact with an evaporating liquid (i.e. skin with sweat on it)
because the liquid absorbs energy from the body so that it can continue to evaporate.
Thermal expansion
When something is heated, it expands because the molecules take up more space:
- When a solid is heated, the molecules vibrate more but stay in place, so the order of magnitude of
the expansion is small.
- When a liquid is heated, it expands for the same reason as a solid, but the
intermolecular forces are less so it expands more.
- When a gas is heated, the molecules move faster and further apart, so the relative
The order of magnitude of the expansion is the greatest.
Some applications and consequences of thermal expansion include:
- Railway tracks having small gaps so that they don’t buckle when they expand
- The liquid in a thermometer expands with temperature and rises up the glass
Prevent consequences
- leave gap for concrete roads, won’t crack
- Use then glass to hold hot water, won’t crack
- Put telephone wire sag high, won’t reach the floor
Thermal capacity
When the temperature of a body rises, its internal energy increases and its molecules vibrate more.
- The specific heat capacity is the amount of energy required to raise the temperature of 1kg of a
substance by 1°C.
𝑐h𝑎𝑛𝑔𝑒 𝑖𝑛 𝑡h𝑒𝑟𝑚𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 = 𝑚𝑎𝑠𝑠 × 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 h𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 ×𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐h𝑎𝑛𝑔𝑒
∆𝐸 = 𝑚𝑐∆𝑇
where ΔE is the change in thermal energy inJ
c is the specific heat capacity in J/kg°C
m is the mass in kg
ΔT is the change in temperature in °C.
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