5.1 Thermal Physics

Question 1

Define thermal equilibrium (2)

Answer 1

No net transfer of thermal energy

Any objects in thermal equilibrium are at the same temperature

Question 2

What is the triple point?

Answer 2

One specific temperature at which all three phases of matter of that substance exist in thermal equilibrium

Question 3

What is the zeroth law of thermodynamics?

Answer 3

If two objects are each in thermal equilibrium with a third then all three objects are in thermal equilibrium with each other

Question 4

Kinetic model - solids (3)

Answer 4

Regular arrangement of atoms or molecules, packed closely together
Strong electrostatic forces of attraction between atoms or molecules
The atoms vibrate about their fixed positions

Question 5

Kinetic model - liquids (3)

Answer 5

Atoms or molecules are close together
More kinetic energy than in a solid
Atoms or molecules can change position and flow past each other

Question 6

Kinetic model - gases (4)

Answer 6

Atoms or molecules much further apart
More kinetic energy than liquids
Negligible electrostatic forces between atoms or molecules
Move randomly with different speeds in different directions

Question 7

What is Brownian motion?

Answer 7

The random motion of microscopic particles in a fluid as a result of their collisions with molecules in the medium

Question 8

What is the smoke cell experiment? (2)

Answer 8

Smoke is trapped in a smoke cell by a cover slip and placed under a microscope
The smoke particles move as the air molecules collide with them and reflect the light

Question 9

Define internal energy

Answer 9

The sum of the random distribution of kinetic and potential energies associated with the molecules of a system

Question 10

What is absolute zero? (2)

Answer 10

The lowest limit for temperature

The temperature at which a substance has minimum potential energy

Question 11

Why is internal energy not zero at 0K?

Answer 11

The substance still has electrostatic potential energy stored between the particles

Question 12

What happens to energy as temperature increases? (2)

Answer 12

Increases kinetic energy

Increases internal energy

Question 13

What happens during a phase change? (3)

Answer 13

Temperature remains constant
Kinetic energy remains constant
Internal energy changes

Question 14

Electrostatic potential energy - gases (2)

Answer 14

0J

Negligible electrical forces between atoms or molecules

Question 15

Electrostatic potential energy - liquids (2)

Answer 15

Negative value

Energy must be supplied to break the bonds

Question 16

Electrostatic potential energy - solids

Answer 16

Large negative value

Question 17

Define specific heat capacity

Answer 17

The energy required per unit mass to change the temperature by 1K

Question 18

How is specific heat capacity determined? (3)

Answer 18

The energy transferred by an electrical heater is equal to the energy put into the material
Record the time the heater is on for and the temperature change
Use E = IVt and E = mcΔθ to get c = (IVt)/mΔθ

Question 19

What is the method of mixtures? (2)

Answer 19

Known masses of two substances at different temperatures are mixed together
At thermal equilibrium, they are at the same temperature so the specific heat capacity of one can be determined if the SHC of the other is known

Question 20

Define specific latent heat

Answer 20

The energy required to change the phase per unit mass while at a constant temperature

Question 21

What is specific latent heat of fusion?

Answer 21

The energy required to change a substance from a solid to a liquid

Question 22

What is specific latent heat of vaporisation?

Answer 22

The energy required to change a substance from a liquid to a gas

Question 23

How is specific latent heat of fusion of ice determined? (4)

Answer 23

Use a thermometer to ensure the ice is at its melting point at the start of the experiment
Use an electrical heater to determine the input energy; E = IVt
Measure the mass of melted ice after a given time, t
Use L = E/m to determine the specific latent heat of fusion

Question 24

How is specific latent heat of vaporisation determined? (3)

Answer 24

Use an electrical heater; E = IVt
Use a condenser to collect and measure the mass of liquid that changes phase
L = IVt/m

Question 25

Why is latent heat of vaporisation usually much higher than latent heat of fusion?

Answer 25

There is a much bigger difference between the internal energies of liquids and gases, compared to solids and liquids

Question 26

What assumptions are made in the kinetic model for ideal gases? (5)

Answer 26

A gas contains a large number of atoms or molecules moving in rapid, random motion
The volume of particles in the gas is negligible compared to the volume of the container
The collisions of atoms or molecules with each other and the container walls are perfectly elastic (no KE lost)
The time taken for collisions is negligible compared to the time between collisions
Intermolecular forces between the particles are negligible except during collisions

Question 27

What is Boyle’s Law?

Answer 27

At a constant temperature, pressure is inversely proportional to volume

Question 28

What is the pressure law?

Answer 28

For a constant mass and volume, absolute temperature is directly proportional to pressure

Question 29

What is Charles law?

Answer 29

For a constant mass and pressure, absolute temperature is proportional to volume

Question 30

How can absolute zero be estimated? (3)

Answer 30

Use air sealed in a container and submerged in a water bath
Increase the temperature of the water bath and record the change in pressure of the air with the changing temperature.
Plot pressure against temperature (°C) and extrapolate the straight line back to the x-intercept as at absolute zero the particles are not moving so pressure is zero

Question 31

Why is rms speed used used instead of average velocity of particles in a gas?

Answer 31

As velocity is a vector, the average velocity would be 0 ms^-1

Question 32

How is rms speed calculated? (3)

Answer 32

Square the velocities of the particles
Take the mean average
Square root the mean

Question 33

How is the Boltzmann constant calculated?

Answer 33

R / Avogadro’s constant

Question 34

How is absolute temperature and kinetic energy of a ideal gas related?

Answer 34

Average KE of an individual molecule in an ideal gas is directly proportional to the absolute temperature of the ideal gas

Question 35

How does mass affect the speed of gas particles at the same temperature? (2)

Answer 35

Different gases at the same temperature have the same kinetic energy
This means lighter particles must be moving much faster

Question 36

What is the internal energy of an ideal gas? (2)

Answer 36

The sum of kinetic energies of the particles in the gas

There is no potential energy because the electrostatic attraction between particles is negligible