P3 - Thermal Physics Flashcards
Properties of particles in solids
- The molecules are very close together and arranged in a regular pattern
- The molecules vibrate about fixed positions
Properties of particles in liquids
- The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
- The molecules are able to slide past each other
Properties of particles in gases
- The molecules are widely separated
- The molecules move about randomly at high speeds
Describe the strength of bonds in solids
- molecules held in place by strong intermolecular forces
- these prevent molecules from moving: rigid shape and fixed volume
Describe the strength of bonds in liquids
- have enough energy to make their bonds weaker
- bonds are still there but they no longer hold the molecules in place
- molecules can move around (slide) so liquid can flow
Describe the strength of bonds in gases
- The molecules have broken the bonds between them: they are widely separated with no long-range forces binding them together
- the molecules are able to move freely and so the gas can flow freely - Because of the large spaces between the molecules the gas can easily be compressed and is also able to expand
Formula for pressure
force / area (F/A)
How is temperature related to pressure?
- The hotter the gas, the faster the molecules move because of an increase in kinetic energy
- As the molecules move around, they collide with the surface of the walls of their container and each other
- Each collision applies a force across the surface area of the walls
- pressure is exerted on those walls
Momentum formula
mass x velocity
How does momentum change relate to pressure?
- When molecules collide against a wall, they bounce off, changing their momentum (right = +mv; left = -mv)
- collisions every second, resulting in a large change in momentum each second
- change in momentum each second results in a force being exerted against the wall: F = (mv-mu)/t
- Pressure exerted by the gas is equal to this force divided by the area of the wall
Brownian motion
The erratic/random motion of small particles when observed through a microscope (colloidal particles)
Kinetic Molecular Theory
The theory that all matter is composed of particles (atoms and molecules) moving constantly in random directions
What is brownian motion caused by?
- By molecules in the gas (or liquid) colliding at high speeds with the small particles
- particles change their speed and directions randomly, each time they are struck by a molecule
How can massive particles be moved by small molecules?
The small, fast-moving molecules surround the large particles in large quantities which causes them to be moved
Evaporation
A change in state of a liquid to a gas that can happen at any temperature from the surface of a liquid
What are the differences between evaporation and boiling?
- Change of state: liquid to gas (evaporation and boiling)
- Temperature: at any temp. (evaporation); only at the boiling point (boiling)
- Where: the surface (evaporation); throughout the liquid (boiling)
How is energy transferred during evaporation?
- Evaporation occurs when more energetic molecules near the surface of the liquid have enough energy to escape
- when this happens energy is lost from the liquid: - The average energy of the remaining molecules decreases: the temperature of the remaining liquid will also decrease
Factors affecting evaporation (3)
- Temperature of the liquid: at higher temperatures, more molecules have enough energy to escape
- The surface area of the liquid: with greater surface area there will be more area from which the molecules can escape
- The movement of air across the surface of the liquid: presence of a draft can help to remove less energetic molecules
How can evaporation cool things down?
If an object is in contact with an evaporating liquid, as the liquid cools the solid will cool as well
What factors increase pressure?
- Increasing the temperature of a gas (whilst keeping its volume fixed)
- Decreasing the volume of a gas (whilst keeping the temperature fixed)
Boyle’s law
If a gas has an original pressure, P1, and volume, V1, then its final pressure and volume (P2 and V2) will be given by the relationship: P1V1 = P2V2
Boyle’s law formula
P1 x V1 = P2 x V2
Thermistor
A resistor whose resistance depends on the temperature of the thermistor
Thermocouple
A thermoelectric device for measuring temperature, consisting of two wires of different metals connected at two points
- a voltage being developed between the two junctions in proportion to the temperature difference
Thermometer
An instrument used to measure temperature
How to calibrate a thermometer?
- Ice-point method (put it in pure melting ice and mark point as 0)
- Boiling-point method (put it in boiling water and mark point as 100)
Divide the remaining area into equal parts
Sensitivity
A large expansion to small stimuli or change
Range
The difference between the highest and lowest values in a set
Linearity
The thread of liquid rises and falls by a uniform amount for similar temperature increases or decreases
Specific Heat Capacity
The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C
Formula and variables of specific heat capacity
change in E = mc(change in T)
- E: thermal energy (J)
- m: mass (kg)
- c: specific heat capacity (J/kg C)
- T: temperature (C)
Convection
The transfer of thermal energy by the circulation or movement of a liquid or gas
Conduction
When two solids of different temperatures come in contact with one another, thermal energy is transferred from the hotter object to the cooler object through particle vibrations
Radiation
Energy that is radiated or transmitted in the form of rays or waves or particles (usually electromagnetic)
Thermal expansion
When a solid is heated, the molecules vibrate more, pushing each other apart
* When this happens, it is the space taken up by the molecules that increases. The molecules themselves remain the same size.
Thermal expansion in solids
Expand slightly (due to the strong bonds holding the molecules together)
Thermal expansion in liquids
Expand more than solids (due to the weaker bonds)
Thermal expansion in gases
Expand significantly (due to there being no bonds holding the molecules together)
How does a bimetallic strip respond to being heated?
The bimetallic strip, consisting of two metals that expand at different rates, will bend upwards when heated, closing the circuit
* can be made to bend at a given temperature, forming a temperature-activated switch
Examples of solids that can buckle if the get too hot (3)
- Metal railway tracks
- Road surfaces
- Bridges
How are objects - prone to buckling in - built to prevent this?
Have gaps built in which creates space for the expansion to happen without causing damage
Internal enegry
The total energy stored inside a system by the particles that make up the system due to their motion and positions
How does convection occur when a liquid (or gas) is heated? (5 steps)
- The molecules push each other apart causing thermal expansion
- This makes the hot fluid less dense than the surroundings
- The fluid rises and the cooler surrounding fluid moves in to take its place
- Eventually the hot fluid cools, contracts and sinks back down again
- The resulting motion is called a convection current
How does convection occur when a liquid (or gas) is cooled? (5 steps)
- The molecules move together, making the fluid contract
- This makes the cold fluid more dense than the surroundings
- The cold fluid falls so the warmer fluid can move into the space created
- The warmer fluid gets cooled ans also contracts and sinks
- The resulting motion is called a convection current
Experiment for demonstrating convection currents
- Take a beaker of water and place a few crystals of potassium permanganate in it
- When the water is heated at that area the potassium permanganate will dissolve in the heated water and rise along with the warmed water
- This reveals a convection current
Why are metals the best thermal conductors?
They have a high number of free electrons and so when faster vibrating ions collide with slower vibrating ions, free electrons transfer energy through the metal
What are the two mechanisms of conduction?
- Atomic vibrations
- Free electron collisions
Describe the process of atomic vibrations when a substance is heated (4 points)
- The atoms at the hottel end of the solid will vibrate more than the atoms at the cooler end
- As they do so, they bump into each other, transferring energy from atom to atom
- These collisions transfer internal energy until thermal equilibrium is achieved throughout the substance
- This occurs in all solids, metals and non-metals alike
Describe thermal conduction in fluids
It does not happen easily and they are poor conductors of heat
* In liquids particles are close, but slide past each other
* In gases particles are widely spread apart and will not frequently collide
Relative thermal conductivity from better conductors to better insulators (7 substances)
- Copper
- Iron
- Carbon
- Water (middle)
- Air
- Glass
- Rubber
Which part of the electromagnetic spectrum is thermal radiation
Infrared
What is the only way heat can travel through a vacuum?
Through thermal radiation
Thermal equilibrium (definition and explanation
An object will remain at a constant temperature if it absorbs heat at the same rate as it loses heat
* As an object absorbs thermal radiation it will become hotter
* As it gets hotter it will also emit more thermal radiation
* Eventually and object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation
Factors effecting the amount of thermal radiation emitted by an object (3)
- The surface colour
- Texture
- Surface area (greater surface area = more area for radiation)
How does the colour of a surface influence its thermal absorbtion and emition? (4 types)
- Black: good absorber - good emitter
- Dull/dark: reasonable absorber - reasonable emitter
- White: poor absorber - poor emitter
- Shiny: very poor absorber (reflects it) - very poor emitter
Aim of the experiment for investigating thermal radiation
To investigate how the amount of infrared radition absorbed or radiated by a surface depends on the nature of that surface
Variables of thermal radiation experiment
- Independent variable = colour
- Dependent variable = temperature
- Control variables
- identical flasks (except for colours)
- same amount of hot water
- same starting temperature of water
- same time interval
Independent variable
The variable you change; unaffected by other variables
Dependent variable
The variable you measure; dependent on other variables
Control variable
The variable kept the same for accuracy
Equipment list for thermal radiation experiment (and their purpose)
- Kettle - to boil water
- 4 thermometers - to measure water temperature
- Flasks painted different colours - to investigate the heat loss of different colours
- Heatproof mat - to protect the surface and prevent heat loss from bottom of flask
- Stopwatch - to record the time it takes for water to cool
Method of thermal radiation experiment (3 steps)
- Set up for identical flasks painted black, grey, white and silver
- Fill the flasks with hot water, ensuring the measurements start from the same intial temperature
- Note the starting temperature then measure the temps. at regular intervals
In the thermal radiation experiment which flasks will cool fastest, which slowest? (black, dull grey, white, silver)
- Black - cools fastest
- Dull grey - second fastest
- White - second slowest
- Silver - cools slowest
Systematic errors
Faults in the apparatus or method which consistently affect every measurement
Random errors
Unpredictable changes in an experiment causing one measurement to differ slightly from the next
How to avoid systematic errors in the thermal radiation experiment? (2)
- Make sure the starting temperature of the water is the same for each material since this will cool very quickly
- It is best to do this experiment in pairs to coordinate starting the stopwatch and immersing the thermometer
- Use a data logger connected to a digital thermometer to get more accurate readings
How to avoid random errors in the thermal radiation experiment? (3)
- Make sure the hole for the thermometer isn’t too big, otherwise the heat will escape through the hole
- Take repeated readings for each coloured flask
- Read the values on the thermometer at eye level, to avoid parallax error
Safety considerations in thermal radiation experiment (name 3)
- Keep water away from all electrical equipment
- Make sure not to touch the hot water directly
- Do not overfill the kettle
- Make sure all the equipment is in the middle of the desk, and not at the end to avoid knocking over the beakers
- Carry out the experiment only whilst standing, in order to react quickly to any spills
