Thermal Physics

As the temperature of the gas increases what happens to the particles?

• The average kinetic particle speed increases

• The average kinetic energy of the particles increases

• The distribution curve becomes more spread out

Define the specific latent heat of vaporisation of water

The energy required to change the state of a unit mass of water to steam when at its boiling temperature

State assumptions about the movement of molecules for an ideal gas

• The motion of molecules is random

• Collisions between molecules are elastic

• Time in collisions is negligible compared to time between collisions

Use kinetic theory of gases to explain why the pressure inside a football increases when the temperature of air inside rises. Assume that the volume of the ball remains constant

P=F/A F= rate of change of momentum

Average KE increases.

More collisions with inside surface of the football each second

Describe one way in which the motion of the molecules of air inside the bicycle tyre is similar and one way in which it is different at the two temperatures

Similar: - random motion, - range of speeds

Different: - frequency of collisions, - mean KE

Define Avagadro’s constant

The number of atoms in one mole of a substance

Outline what is meant by an ideal gas

The gas obeys Boyle’s law at all temperatures

State what is meant by the internal energy of a gas

It is the sum of the kinetic and potential energies of the particles that move at random in the gas.

Absolute zero of temperature an be interpreted in terms of the ideal gas laws or the kinetic energy of particles in an ideal gas.

Describe these two interpretations of absolute zero of temperatures

• Plotting data of volume against temperature, the plot extrapolates and crosses the temperature axis at absolute zero

• Using KE, it is the temperature at which random motion stops

State one assumption about the size of molecules

The size of the gas molecules is negligible

Pressure law

P/T = K Volume fixed

If you heat a gas, the particles gain KE. If volume is fixed the particles collide with each other and their container more often increasing pressure inside the container.

Charles’s law

V/T = K Pressure constant

When you heat a gas, the particles gain KE. At a constant pressure this means they move further apart and so the volume of the gas increases.

Boyles law

pV = K Temperature constant

If gas is compressed on what is the work done on? - Pump showing Boyle’s law

Work is done on the gas to compress it. In order to maintain a constant temperature the gas must transfer an equal amount of energy to its surroundings in the form of heat.

Explain one way in which the development of the kinetic theory of ideal gases differed from the development of the ideal gas law

Kinetic theory is theoretical and so was based on assumptions and previous theories whereas the ideal gas law is empirical and so was based on experimental observations

Explain why molecules of a gas exert a force on the walls of the container. Refer to Newtons laws of motion.

There is a change of momentum for the molecules at the wall because the direction has changed. The container exerts and equal opposite force (N3L).