2. Thermal physics Flashcards
Solid - Features
- Fixed shape (can alter when forces act on them)
- Usually dense + difficult to compress
Liquid - Features
- Can flow / be poured (no fixed shape)
- Takes the shape of the bottom of the container (no fixed shape)
- Fixed volume (Can’t be compressed)
Gases - Features
- Flow very quickly (faster than a liquid)
- Spread out in a container
- Easy to compress (easier than liquid)
Deposition
Changing directly from a gas to a solid.
Sublimation
Changing directly from a solid to a gas.
Condensation
The individual particles are attracted together and the bonds between them increase in strength.
- Gas –> Liquid
Evaporation
The bonds between particles are broken and the individual particles separate and move about quickly.
- Liquid –> Gas
Freezing
The bonds between the particles increase in strength and the particles end up in fixed positions.
- Liquid –> Solid
Melting
The bonds between the particles weaken and they can flow past each other.
- Solid –> Liquid
Melting point
The temperature at which a material changes from a solid to a liquid
(Same temp as the freezing point for a liquid)
Freezing point
The temperature at which a liquid will change to a solid.
(Same temp as the melting point for a solid)
Freezing point + boiling point of water
(Only applies to pure water)
Freezes at 0C
Boils at 100C
Boiling point
The temperature at which a liquid will change to a gas.
(Same temp as the condensation point for the gas)
Condensation point
The temperature at which a gas will change to a liquid.
(Same temp as the boiling point for the liquid)
Difficulties of classifying materials into states of matter.
- Not all materials fit perfectly into one of the three states eg. Jelly.
- Materials can also contain a combination of states of matter. Eg- A sponge is solid but contains air pockets so it can be compressed.
Molecular structure of a solid
- Closely packed particles in a regular pattern
- Held in place by strong, attractive forces
- Particles vibrate around their positions
- If you try to compress a solid –> the forces holding the particles in position becomre more repulsive –> keeping the particles the same distance apart
Molecular structure of a liquid
- Molecules are still very close together.
- Arranged irregularly –> can therefore move past each other
- Weaker forces of attraction in liquids than in solids.
Molecular structure of a gas
- Particles are spread out
- Move around fast
- Irregular arrangement
- Weak forces of attraction between them (don’t attract each other –> so spread out)
- Particles far apart so can be compressed
A gas cools down
- Particles slow down
- Eventually move slow enough that forces can form between them
- Join together + material becomes a liquid.
A liquid cools down
- Particles slow down
- Particles lose enough energy to form a fixed structure
- Material becomes solid
A solid cools down
- Particles vibrate but as temp decreases, the vibrations become smaller and smaller
- Eventually the particles has the least possible kinetic energy (absolute zero)
Absolute zero
0 Kelvin (0K)
-273 C
20 C = 293 K
Gas pressure
The outward force on the walls of a container caused by the gas particles colliding with the walls.
What happens to the pressure of a gas as temperature changes?
As temperature increases –> particles move faster
–> More collisions with the surface of the container surrounding the gas –> This creates pressure.
How a hot air balloon is inflated (pressure + temperature
- Air is heated inside the balloon
–> Temperature increases so gas particles move faster + collide more with balloon material
–> Creates gas pressure
–> As prcessure increases, balloon inflates.
Increasing temperature of a gas
- Particles move faster
- Not all particles move at the same speed because collisions cause diffrent speeds etc.
- Therefore there is a range of speeds
- Temperature of gas is related to the mean/average speed.
The hotter the gas –> the higher the mean speed of particles
Momentum + pressure in gases (changing temperature)
- If temp increases –> velocity increases –> momentum increases –> force increases –> higher pressure
How to increase pressure of a gas
Double the rate of particles in balloon = double the collision = double the force on the rubber = double the force per unit area or double the pressure.
Brownian motion
- The random motion of small, microscopic particles caused by collisions with even smaller, invisible particles.
Named after Robert Brown’s observations of the movement of pollen grains in water.
Describing brownian motion
- Cause by the impacts of molecules or atoms on larger particles (eg. pollen grains)
- Pollen grains are too small to see individually (microscopic)
- Molecules / atoms cannot be seen even with a powerful microscope –> NOT microscopic therefore.
Why brownian motion works
Small + fast-moving object = momentum of large + slow-moving object.
Therefore small molecules can still collide and have an impact on particles thousands of times larger.
Relationship between pressure of a gas and temperature - Graph
As temperature increases, pressure increases (positive correlation)
Linear graph.
Affect of volume of a gas on pressure
- Volume of a container decreases –> distance gas particles travel between collissions with walls decreases
–> more collision –> increased pressure
(Gas becomes more compressed)
Relationship between the pressure of a gas and volume - Graph
- Pressure is inversely proportional to volume
Graph eventually plateau’s because maximum compression / minimum volume and thus max pressure is reached.
ASK WHY NOT LINEAR
Pressure-volume relationship of a gas
Pressure * volume = constant
Relationship only applies if temp of gas is kept constant during compressions/ expansion.
If pressure changes, volume will also change so multiplying the values will always give teh same number (constant value)
Freezing + boiling point of water in Kelvin
Freezing point = 273 K
Boiling point = 373K
0 –> 1 K
is the same as
-273 –> -272 C
Convert from degrees celsuis to Kelvin
Add 273
Convert from kelvin to Celsius
Substract 273
Thermal expansion
Particles move faster when they have a higher temperature. Therefore states of matter increase in volume and expand as the temperature increases.
However, states of matter expand at different rates/ amounts.
Thermal expansion of different states of matter
- Solids can’t move, only vibrate
–> only a small amount of expansion can take place.
Liquids expand more than solids.
Gases will expand a lot if kept at constant pressure (if in a container the pressure will increase instead)
Thermal expansion - Safety when building
- To avoid damage, small air gaps are left between railway tracks or ends of bridges.
If the material expands during a warm day, it stops it from buckling.
Internal energy
Measure of the average kinetic energy of the particles of a substance.
Heating an object can increase its internal energy.
Factors deciding the internal energy
Mass of the object
–> Greater mass = greater amount of energy which can be stored
Temperature of the object
–> Greater termperature –> more energy stored
Material object is made from
–> Some materials require more energy to inc. their temp
Specific heat capacity
The amount of energy required to heat 1 kg of a material by 1 °C. This quantity has units
of J/kg/°C.
(This is done to make it easy to compare materials ease of storing energy.)
Specific heat capacity (formula)
Specific heat capacity = (Change in energy) / (Mass * Change in temperature)
Energy (Joules
Mass (Kg)
Temperature (degrees Celsius or Kelvin)
Units of specific heat capacity = J/(kgK) or J/(KgC)
Specific heat capacity of water
4200 J/kgC
–> Higher than most other common materials
–> Allows us to store large amounts of energy in water when it is heated
eg. radiators contain heated water.
This means it takes 4200 J of energy to increase the temperature of 1 kg of water, by 1 celsius or 1 kelvin.
Measuring the specific heat capacity of water
Heating the water with a low voltage immersion heater.
Energy provided to the heater is measured with a joulemeter or voltmeter + ammeter combination.
This value, combined with temperature rise + mass of water –> can calculate specific heat capacity.
When measuring make sure to contain water in a well-insulated container (to stop heat from dissipating)
Measuring the specific heat capacity of a solid
Boiling point - Explained
Boiling occurs when a liquid reaches its boiling point.
The liquid begins chaging to a gas throughout the entire liquid, resulting in bubbles of gas forming inside the liquid.
While boiling, the liquid doesn’t get any hotter.
Water stays at 100C until it has all boiled away.
Evaporation vs boiling
Boiling occurs at a specific temperature.
Evaporation only happens at the surface of the liquid and occurs at any temperature.
In both process liquid turns into gas.
Fluids
Gases or liquids
(substances that can flow –> no fixed shape)
State change - Graph
Heating a solid —>
Temperature increases until it hits the melting point.
Temperature stays constant during melting.
Once melted, the temperature of the liquid continues to increase.
At boiling point, the temperature stays constant while evaporating.
Temperature of gas continues to increase.
Evaporation (How it happens)
Particles in a liquid are held together by intermolecular forces.
For particles to escape they have to be travelling quickly and be near the surface of the liquid.
The slower particles will remain behind.
Therefore the average speed of the remaining particles decreases during evaporation.
Evaporation affecting temperature
As the average energy of particles decreases, the liquid cools down.
Therefore it is an endothermic reaction.
Eg. sweat evaporating cools people down. (One way the body maintains a constant temperature)
Factors affecting the rate of evaporation
- Increasing the surface area of a liquid, increases the no. of particles near the surface –> increasing the rate of evaporation.
- Only particles with enough speed/ energy can evaporate. Increasing temp, increases speed –> rate of evaporation increases.
- Evaporated particles creates saturated vapour above the surface of the liquid –> Increased air flow above surface ‘clears way’ of evaporated particles –> easier for more particles to escape.
Conduction
A process which causes energy transfer inside a material when one part is hotter then another.
Good vs Bad conductors
Good conductors
- Metal
Bad conductors (insulators)
- Plastic + Glass
Units for conductivity
Watts per square metre
W/m²
The higher the number, the better at conducting the material is, as more power is conducted per unit of cross-sectional area.
How conduction occurs
Increased temp –> Increased vibration in solids
Vibration affects other nearby atoms –> causes them to vibrate more so the temp in that area also increases.
This process gradually causes increased vibration and increases temperature across all the material.
–> Called lattice vibration (Slow procedure bc vibrations need to be passed along by interactions between each atom)
Lattice vibration
The only thermal conduction process in non-metals. Energy is transferred as particles vibrate and cause other nearby particles to vibrate.
Why metals are good conductors
Metals have free electrons –> free to move quickly through the metal + can transfer energy faster than lattice vibration.
Particles in metals are also close together when solid (easily spread vibrations)
Why are gases + liquids very poor thermal conductors?
In gases particles are too far from each other + rarely make contact with each other to transfer energy
In liquids, particles are also slightly further apart than in solids –> conduction is more limited.
Particles with more energy don’t stay in a fixed position –> can’t transfer energy along a lattice as in a solid.
Convection
Can only happen in liquids + gases.
A process which causes energy transfer inside a fluid when one part is hotter then another. The particles of the fluid move from one place to another due to density changes.
Why can’t convection happen in solids?
When a solid is heated, the internal energy increases, and the vibration of the particles increase causing the meterial to expand.
How convention occurs –> Detailed
As liquids + gases heat –> expansion occurs. Particles move more quickly –> thus take up a larger volume –> Density of the liquid decreases.
(Less dense materials will float on top of more dense materials)
–> As the liquid is heated + becomes less dense than the liquid around it –> it floats up towards the surface.
At the surface, the liquid will begin to cool + beome denser –> causes it to sink back downwards.
Breezes - Convection - Explained
During the day, the land warms faster than the sea, so air rises above it –> drawing in cooler air from above the water.
During the night, air rises above the warmer water, pulling air from the land out to sea.
Land to sea breeze during day.
Sea to land beeze during night.
Infrared radiation (2. Thermal)
Hot objects emit infrared radiation (part of the electromagnetic spectrum) –> When the object emits radiation, it cools down –> particles inside it slow slightly, so the temp falls as the internal energy decreases.
The hotter an object is, the more radiation it will emit each.
Objects can also absorb infrared radiation, causing them to warm up. The temp increases as the particles inside speed up + their internal energy increases.
Objects at room temperature - Infrared radiation
The objects are emitting and absorbing the same amount of infrared radiation.
–> No overall change in their internal energy.
How infrared radiation travels through empty space
Earth recieves sunlight from the sun bc electromagnetic radiation can travel through the empty space (vacuum) between the sun and the earth.
Infrared radiation is part of the electromagnetic radiation –> so can also travel through a vacuum.
The temperature of the earth
- If it emits more radiation than it absorbs it will cool.
- If it absorbs more radiation than it emits it will warm up.
- If it emits and absorbs the same amount of radiation it will stay at a constant temperature.
Greenhouse effect (thermal)
1) Radiation travels through empty space from the Sun to Earth.
2) Radiation passes through the atmosphere.
3) Some of the radiation warms the surface of the Earth.
4) Some of the radiation escapes to space and some is reflected by to earth.
5) The earth reemits the radiation at shorter wavelengths.
???? 5)
During the past two centures extra CO² in the atmosphere from burning fossil fuels has changed the atmosphere + more radiation is being relfected back to earth –> gradually increasing the temp –> climate change
Which factors affect how good the surface is at absorbing emitting + infrared radiation
Surface colour + surface texture
Surface colours affecting absorption + emission of radiation.
- Silvered surfaces are poor absorbers of radiation; they reflect it instead.
- Black surfaces are good absorbers of radiation.
(Brown bread absorbs more radiation initially –> so toasts faster)
When cooling…
A black surface is a much better emitter of radiation than a silver surface.
Black objects will cool more quickly than lighter coloured objects.
Surface textures affecting absorption + emission of radiation.
Shiny surface –> increases its reflectiveness –> its not as good at absorbing radiation as a rough surface.
Dull coloured surfaces are better at absorbing radiation than shiny ones.
Surface area affecting the amount of infrared radiation emitted.
A bigger object (larger surface aea) emits more radiation than a smaller object at the same temperature.
Objects with large surface areas will also absorb more radiation than those with small surface areas.
–> Small animals often huddle together to preserve energy. –> Reduce overall surface area while keeping overall volume chonstant –> Absorb radiation that each of them emits.
How a vacuum flask works
Stopper of the flask is filled with an air + polastic foam (insulators –> reducing conduction)
Small air gap in container reduces conduction from outer + inner surfaces.
Vacuum within air gaps in the walls prevents conduction or convection bc there are no particles inside to vibrate or flow.
Silver surfaces on the glass in the air gaps reduce emission or absorption of infrared radiation.
Insulated supports are made of poor conductor/ insulator such as plastic. This reduces conduction to the outer casing.
Double layer of glass inside the flask walls.
Temperature control in buildings.
- Walls made of poor conductors to reduce conduction losses. (eg. brick + wood)
- Thick carpets on floor
- Double glazed windows to reduce heat loss bc thinner than the wall.
- Lofts insulated with glass fibre/ wool –> reduces air flow + energy loss due to convection currents (bc hot air rises)
- Building painted in light colours –> Reduces emission + absorption of radiation.
Kitchen pans - Thermal energy transfer
Good conductors (aluminium/ copper –> Metals) used in kitchen pans
Handles made of wood bc its a good insulator.
Heating a room - Thermal energy transfer
Rooms are heated using radiators/ convector heater.
Air cloes to radiators or convector heaters rises because it becomes less dense.
Colder air moves in to take the place + warms + rises + etc.
(Convection current set up, allowing thermal energy to be transferred throughout the room.)
Why is thermal energy transfer needed to cool cars?
Car engines burn fuel for power –> creates massive heating –> engine could quickly overheat + fail
–> Conduction + radiation processes used to transfer energy away from the enegine + keep it running.
spe
- Engine mostly metal –> conducts energy away from cylinders to outer surfaces.
- Water pumped throguh metal pipes to cool hot parts of the engine
- Energy conducted into the water as the metal heats up –> engine thus cools.
- Water is pumped into radiator. Hot water heats the rest of the car + water gets cooled as energy is released.
- Cool air surrounding air as car travels, so energy is transferred to that by conduction.
- Cooler water pumped back to the engine + cycle continues.
Why are dark colours better absorbers of infrared radiation?
The colours we see is the reflected radiation.
White light contains all the colours of the rainbow. Therefore white surfaces reflect all colours, causing them to absorb less radiation.
Black surfaces however, absorb all infrared radiation and emit very little (reflect no colour)
What is Brownian motion evidence of?
The kinetic particle model/theory.
Shows that all particles above absolute zero move (and technically at bc particles always has some amount of motion)
