P3 - Thermal physics Flashcards
What is the kinetic molecular model used for
to explain the behavior of solids, liquids and gases.
Arrangement of particles and motion of particles in a solid, liquid and gas
Solid: Regular pattern, closely packed and particles in a fixed position. Vibrate within place
Liquid: Irregular pattern, closely packed and particles able to move past each other and ‘slide’ in a random motion
Gas: Irregular, widely spaced and particles move freely. Random motion yet faster movement than other states.
The kinetic theory of matter states that
all matter is made up of tiny particles that are constantly moving
Properties of solids, liquids and gases
Solids: retain a fixed shape and volume due to particles locked in place by forces of attraction between them
Not easily compressed as little space
Do not flow easily or slide pass one another
Liquids: Assume the shape of the container
Not easily compressed as little space
Flow easily because particles can slide or move pass one another
Gases: Are compressible because lots of free space
Flow easily because particles can move pass one another
Explain how a solid can change into a liquid
Particles in a fixed lattice position are heated more energy is given meaning more vibrations.
When temperature increases too much, (melting point) the particles have enough energy to break their bonds and melt into a liquid
Explain how a liquid changes into a gas
When temperature increases more and more (boiling point) particles fly apart from each other forming a gas as they have enough energy to break their weak forces of attraction.
The higher temperature of a gas the higher
kinetic energy of the gas particles
As you heat a gas you transfer more kinetic energy to the gas particles which increase the
speed of the particles
Explain pressure of gases
Gas exerts pressure onto walls of container
Gas particles collide with container each second
When colliding momentum changes and bounces off the walls
This exerts force on both particle and container wall
What happens when you cool a gas down
The particles don’t have enough energy to overcome forces and bonds will start to form between particles, condensing the gas into a liquid
What happens when you cool a liquid down
The particles don’t have enough energy to overcome forces and more bonds will start to form between particles, freezing the liquid into a solid.
In a closed system states of matter don’t change 1. ______ but do change in 2. _______
- mass
2. density
Order solid liquid and gas in terms of density
Solid - highest density
liquid
Gas - lowest density
What is the process of evaporation and why does it cause cooling
Evaporation is the process of changing states from a liquid to a gas.
Particles with more energy in a liquid break away from the surface of a liquid and form gas or vapor.
More energetic molecules escape and reduces the average energy of the remaining liquid molecules and therefore the temperature. So the evaporation causes cooling
Identify Three ways to increase the rate of evaporation and why they do
Increase temperature
More particles have more kinetic energy in which they can escape the surface of a liquid
Increase wind
Can blow away particles from the surface
Surface area increase
More area for liquid molecules to reach the surface and escape
The difference between evaporation and boiling
Evaporation can happen at any temperature.
when a liquid evaporates it loses molecules from the surface of a liquid. The molecules with the highest of energies will have left.
Boiling however is when the temperature reaches boiling point at which most of the molecules have enough energy to leave the liquid and turn into a gas.
What is Brownian motion and what does it prove
When viewed under a microscope small particles (i.e smoke particles) can be seen moving randomly.
This comes with an explanation that particles are constantly being hit by fast moving particles which are too small to see i.e air or water molecules.
Larger particles (smoke particles in this case) are moved by lighter faster particles (gas in this case). The gas molecules exert forces onto the bigger smoke particles.
It proves that small particles and atoms do exist and particle theory is true.
What causes pressure on the walls of a container
collisions made by fast moving gas molecules.
What happens when change the volume of a container but keep the temperature the same
The pressure increases as the number of molecules stayed the same but are pact more densely. More collisions with be with the walls. The average speed will stay the same as temperature is constant.
What does Boyle’s Law state
When the temperature of the gas stays constant the volume of the gas is inversely proportional to the pressure.
i.e If you halve the volume of the container the pressure will double
If you double the volume of the container the pressure will halve
Gases only follow Boyle’s Law when
The temperature is constant
The mass of the gas remains constant
What happens when there is a change of temperature at constant volume
More temperature = more kinetic energy in particles = more collisions per second in container.
If volume stays constant than pressure will increase
Pressure formula
Pressure (Pa) = Force (N)/ Area (m^2)
All materials expand in the presence of
heat
Example of how thermal expansion applies to applications and how to overcome it
On a hot day railway tracks that 1000km long can possibly expand 300 m longer. Engineers create gaps to allow for thermal expansion when designing bridges, railways and buildings. Steel reinforcements can be placed to strengthen bridges.
What is a Bimetallic strip and why does it bend
Metals expand at different rates as their temperature rises.
When strips of two metals are bound closely and warmed they bend as one metal expands more than the other.
It can be used to control temperature in heating systems i.e electric iron
Rank expansion of solids liquids and gases
Gases - expand the most
Liquids
Solids - expand the least
How does expansion occur in solids
Atoms vibrate more as temperature goes up so the atoms move slightly further apart and expands in all direction.
How does expansion occur in liquids
Atoms or molecules vibrate as they move around getting slightly further apart.
Therefore volume of liquids increase as temperature increases.
Why do liquids expand more than solids
- Forces between liquids are smaller than in solids
- Molecules are spread further apart in liquids than solids
Therefore liquids expand more than solids for same temperature change
Expansion of gases
In sealed containers gases won’t expand as the pressure will increase instead
If a gas is allowed to expand the volume will increase much more than solids or liquids and get hotter.
Temperatures can be measured using any suitable
physical property that changes with temperature
Examples of physical properties that can be used to measure temperature
Volume of a liquid - mercury in glass or alcohol in glass thermometer
Length of solid - bimetallic strip in a thermostat
Pressure of a fixed volume of gas
etc.
All thermometers need
to be calibrated
In a liquid in glass thermometer how do you calibrate it and what are the steps
A scale must be fixed into the right place in which two fixed points are needed.
There is a bulb of liquid (such as mercury or alcohol) at the base attached to a very narrow tube in which the liquid can expand.
As the liquid expands and contracts with change in temperature the length of column of liquid increases and decreases.
The two fixed points are the melting point of ice 0C and the boiling point of water 100C
To calibrate the mercury in glass thermometer it is immersed in a funnel containing melting ice and the 0C is marked.
It is then put in stem from a boiling kettle and 100C is marked.
The distance between the marks is divided into 100 equal distances each being 1 degree celsius
What are the properties of a thermometer
Sensitivity
Range
Linearity
Accuracy
What is the importance of sensitivity in thermometer
When a thermometer gives a large response to a small change in temp it gives you better chance of detecting small change in temp.
What is the importance of range in thermometer
Different thermometers read different ranges of temp.
Liquid in glass thermometer the range is limited by freezing and boiling points of liquid.
Different substances have different freezing and boiling points so range should be considered for suitable use.
What is the importance of linearity in thermometer
Thermometers need to be linear. For a liquid in glass thermometer it must contain a liquid that expands the same amount per degree rise in temperature.
What is the importance of accuracy in thermometer
So it can give correct values of temperature. The marks and details must be precise to give accurate measurements.
Sensitivity and accuracy are different
What is the Thermocouple
An electrical thermometer.
Advantages of Thermocouple
Can be made very small and responds quickly to change in temperature. Can record very high temps of up to 1000C. Useful for finding temps of inner jet engines or molten steel
How does the Thermocouple work
The two metals join together to form a junction.
When the junction is heated or cooled, a small voltage is generated in the electrical circuit of the thermocouple which can be measured, and this corresponds to temperature.
Materials that allow thermal energy to transfer through them quickly are called
thermal conductors
What is a thermal conductor
A material that allows thermal energy to transfer through it quickly
How does conduction work
If one end of a conductor is heated the atoms start to vibrate more vigorously. As the atoms are linked vibrations can be passed onto other atoms. The kinetic energy is passed on the whole material.
why are metals good thermal conductors
they have free moving electrons which move faster when heated.
They transfer kinetic energy to other atoms and electrons in the metal happening quickly.
Conduction cannot occur when
there are no particles around
A vacuum is a perfect
insulator
Liquids and gases are poor heat conductors beacause
their particles are so far apart
How to test for good and bad conductors in different metals
Rods made of different metals joined together.
You light up a bunsen burner onto the middle bit so the heat starts from the end of each different metal rod.
Wax is put on the ends of each metal and a nail is stuck.
The nail that drops the first on a metal because the wax melts means that metal is the better conductor
Examples of thermal insulators
non- metals such as wood, plastics and air
Substances that do not conduct heat very well are called
thermal insulators
Convection is the main method of
thermal transfer in fluids
When a fluid is heated
energy is transferred to particles causing them to move faster and further apart.
How does a convection current show how thermal energy is transferred in fluids
- When part of a fluid is heated particles in that area gain kinetic energy.
They move away from each other so the area becomes less dense and the warm fluids rise - When the warm fluid particles rise they transfer some heat energy to nearby colder fluid particles.
The heat particles begin to sink because they are more dense and density increases - At the bottom, colder fluid particles that haven’t been heated will replace warmer particles that have risen. The colder particles are then heated and move apart and start to rise.
- This cycle creates a convection current. They are result of different densities in fluids and are created by heat.
liquids and gases cannot conduct heat well but can
transfer heat using convection currents
Examples of convection currents and how it works
Radiators - heat rooms using convection current. Warm air rises and pushes colder air down to floor level which gets heated up by radiator.
Weather - Convection currents can create wind
Air conditioners - same concept as radiators. Hot air rises and is cooled down and falls pushing up more hot air to be cooled
Suggest how lighting a fire beneath a chimney can improve ventilation in a room.
The hot gas created by the fire rises up the chimney. This gas is replaced by colder gas from the room, which is then heated by the fire, before rising up the chimney. Other gas now enters the room to replace the gas that has left through the chimney.
Radiation unlike conduction and convection
does not need particles at all
Radiation travels through a
vacuum
Radiated heat energy is carried mainly by
infrared radiation which is part of the electromagnetic spectrum
All objects absorb and give out
infrared radiation all the time
The amount of radiation given out or absorbed by objects depends on
Temperature and surface.
Infrared radiation emitted by the sun transmits
thermal energy to the earth
Infrared radiation doesn’t need a medium so can
travel through space which is a vacuum
What are examples of poor and good emitters of radiation and why
Dull black surfaces:
They are poor reflectors of radiation but good absorbers and emitters of radiation
White surfaces:
They reflect radiation and are poor absorbers and emitters of radiation
What experiment shows the properties of good and bad emitters of radiation
Leslie’s cube
It is a metal box with 4 different color sides (black, matt black, white, unpainted).
If you pour hot water into the cube, the matt (dull) black side will emit more infra-red radiation than the other sides.
The infra-red radiation emitted is measured using a thermopile (a sensitive thermometer).
What is an example of thermal transfers in everyday life and how it works
A vacuum flask.
It will keep a hot drink hot or a cold drink cold for ours by almost eliminating the flow of thermal energy in or out
Conduction is eliminated by making sure heat flowing out must travel along the glass neck of the flask. The glass is a poor conductor of heat.
Convection is eliminated as space between the inner and outer wall of the flask is evacuated so no air can form convection currents.
Radiation is greatly reduced as inner walls of flask are coated with pure aluminum which is a vacuum and reflects radiation to stop it form absorbing it.
Ways of reducing wasteful energy transfer in a house
Double glazing which reduces energy transfer by conduction.
Curtains help prevent energy transfer by radiation.
Carpets prevent thermal energy loss by conduction and convection
Roofs have loft insulation where hair is trapped so it reduces thermal energy transfer by conduction or convection.
Modern houses have cavity walls that are two single walls separated by air cavity which reduces thermal energy transfer by conduction but no convection as air is free to flow.
