8. Thermal Physics Flashcards
What is temperature?
A measure of the kinetic energy of the molecules in a substance
What is temperature measured in (in thermal physics)?
The Kelvin scale or the absolute temperature scale
What are the units of temperature?
Kelvin (K)
What is absolute zero?
- The lowest possible temperature that any object can theoretically have (around -273˚C)
- It is given a value of zero kelvins (0 K)
How is energy transferred from hot substances to cold ones?
Via collisions between molecules
When is energy no longer transferred from hot substances to cold ones?
When thermal equilibrium is reached and all particles have the same kinetic energy
When is there no atomic movement in a substance?
When the substance is at 0 K so all the molecules have zero kinetic energy
What is thermal equilibrium?
When substances are at the same temperature and there is no net transfer of energy between them
When is heat/energy transferred faster between substances?
When there is a high difference in temperature between two substances
What is specific heat capacity?
The amount of energy needed to raise the temperature of 1kg of a substance by 1K or 1˚C (without changing state)
How does a substance’s energies change as temperature is increased?
Kinetic and potential energies will increase as a substance increases in temperature
How does a substance’s energies change as it changes state?
Kinetic energy remains constant but potential energy increases as a substance changes state
What is specific latent heat of fusion or vaporisation?
The amount of thermal energy needed to be gained or lost to change the state of 1kg of a substance without changing its temperature
What is the latent heat of fusion?
The energy gained to change a substance from a solid to a liquid is equal to the energy lost when it changes back
What is the latent heat of vaporisation?
The energy gained to change a substance from a liquid to a gas is equal to the energy lost when it changes back
What is fusion?
Melting or freezing
What is vaporisation?
Boiling or condensing
How do intermolecular bonds change when a substance changes from solid to liquid?
Some intermolecular bonds are broken
How do intermolecular bonds change when a substance changes from liquid to gas?
Almost all intermolecular bonds are broken
What is an ideal gas?
A gas that follows the three gas laws at all temperature, volumes and pressures
What are the three gas laws?
- Pressure law
- Boyle’s law
- Charles’ law
What do the three gas laws predict?
How a fixed mass of gas (some number of moles/molecules) will behave when you change its temperature, pressure or volume)
What is the pressure law?
For a fixed mass of an ideal gas, if volume remains constant then pressure and absolute temperature are directly proportional
When is pressure zero?
At absolute zero as the particles do not move so they cannot exert a force on the container
Why does increasing the temperature of an ideal gas increase the pressure?
- Increasing temperature increases the average kinetic energy of the particles and therefore the average speed
- Hence, there is a greater change of momentum and a greater frequency of collisions, meaning the rate of change of momentum is greater, therefore the force is greater, increasing the pressure
What does it mean for volume if the gradient of a pressure against absolute temperature graph is steeper? Why?
- Steeper gradient means smaller volume
- At the same temperature, the force of collisions will be the same but the pressure will be greater for a smaller volume
What is Boyle’s law?
For a fixed mass of an ideal gas, at a constant temperature, pressure is inversely proportional to volume
Why does increasing the volume of an ideal gas reduce the pressure?
- Increasing the volume increases the surface area
- Constant temperature, so average speed of particles doesn’t change
- Pressure decreases as collisions with the same force as before are spread over a larger surface area
- Frequency of collisions will also reduced as time to travel to hit container wall is greater
What happens to a pressure against volume graph as the temperature of the gas is increased?
The hotter the gas, the further away the curve moves from the origin, as the hotter gas particles would collide with more force, so for the same volume, the pressure would be higher
What is Charles’ law?
For a fixed mass of an ideal gas, at a constant pressure, volume is directly proportional to absolute temperature
How does the pressure of an ideal gas remain constant as absolute temperature increases?
- As absolute temperature increases, the average speed and therefore the average kinetic energy of the particles increases
- When the particles collide with the walls of the container, there’s a greater change in momentum per collision, hence a greater force
- The volume must increased to ensure the frequency of collisions decreases, which may also increase the surface area
- Pressure = force/area so pressure remains constant
How does the gradient of a volume against absolute temperature graph change for a gas with less pressure?
The smaller the pressure of the gas, the steeper the line will be as, at the same temperature, the force of collisions will be the same so the pressure will be greater id the container has a smaller volume
What is Avogadro’s constant?
The number of atoms in exactly 12g of carbon-12/the number of atoms in one mole of a substance
What is molar mass?
The mass of 1 mole of a substance
How does molar mass (in grams) relate to the nucleon number of a substance?
Molar mass in grams is equal to the nucleon number of a substance
What is molecular mass?
The mass of 1 mole of a molecule
What is relative molar mass?
The sum of the relative atomic masses of all the atoms making up a molecule
What is the area under a graph of pressure against volume?
The energy transferred to change the volume of a gas
What is an empirical model?
A model that can predict what will happen but it doesn’t explain why
What is a theoretical model?
A model that can predict what will happen and explain why it will happen
What is Brownian motion?
- It was observed that larger particles (e.g. pollen, smoke) seemed to follow a random, jittery, zigzag pattern
- Atoms are too small to be observed directly
- Einstein proved that this pattern was explained by the random bombardment from these smaller particles travelling at high speeds
What is the aim of kinetic theory?
To link the ‘large’ scale properties of a gas (pressure, volume and temperature) to the microscopic properties of the gas molecules
What are the five assumptions about the motion of particles?
- the molecules move rapidly
- Newtonian mechanics apply
- collisions between molecules, and molecules and the walls, are elastic
- there are no forces between molecules except during collisions (they move at constant velocity in a straight line)
- the forces that act during collisions are instantaneous
What are the three assumptions about the particles?
- the gas contains a large number of molecules
- the molecules act as point masses (the volume of the molecules is negligible compared to the volume of the container)
- all molecules of gas are identical (not always accepted)
Explain the pressure law using kinetic theory
- As the gas is heated the average kinetic energy of the particles rises causing them to travel faster
- There are more frequent collisions with the wall every second (or time between collisions decreases)
- Each collision has a greater change in momentum
- The rate of change of momentum increases
- Meaning greater force is exerted over the same area, meaning pressure increases, as pressure is force/area
Explain Boyle’s law using kinetic theory
- The kinetic energy of the particles remains the same, meaning they are travelling at the same speed
- Therefore the change of momentum of each collision remains the same
- As the volume decreases, the frequency of collisions will increase (or time between collisions decreases)
- Therefore the rate of change of momentum increases, meaning the force exerted on the walls will increase
- Therefore greater force is exerted over a smaller surface area, and as pressure is force/area, pressure increases
Explain Charles’ law using kinetic theory
- As the gas is heated the average kinetic energy of the particles rises causing them to travel faster
- Each collision has a greater change of momentum
- Pressure is equal to force/area
- So in order to maintain the same pressure, the frequency of collisions has to reduce (or time between collisions increases)
- This is achieved by increasing the volume, which in turn increases the surface area
How does speed distribution of particles change as temperature of the particles increases?
- Average particle speed increase
- Average kinetic energy of particles increases
- The distribution curve becomes more spread out
What type of energy does an ideal gas have?
Kinetic energy only, no potential energy
What are two substances at different temperatures trying to achieve?
When two substances which start at different temperatures are trying to reach thermal equilibrium, they are exchanging energy through collisions until all particles have the same MEAN kinetic energy
How is the amount of potential energy each particle has distributed?
The amount of potential energy each particles has is randomly distributed and depends on their relative positions
What is the internal energy of a body?
The sum of the randomly distributed kinetic and potential energies of all its particles
