Thermal physics - CH14.CH15

Question 1

How can you convert between kelvin and Celsius

Answer 1

T(K) = T(°C) + 273

Question 2

what is thermal equilibrium and how is it reached between 2 substances of different temperatures

Answer 2

If 2 substances are in contact and one is hotter than the other there will be a net flow of energy from the hotter one to the cooler one. the hotter will cool and the other will heat until they reach equal temperature which is known as the thermal equilibrium. - when there is no net transfer of thermal energy.

Question 3

Electrostatic forces and potential energy of solids

Answer 3

Strong electrostatic forces of attraction
between molecules.
Negative electrostatic potential energy (negative
because external energy is required to separate the molecules)

Question 4

Electrostatic forces and potential energy of liquids

Answer 4

There is still electrostatic attraction, but it is weaker than in solids. The negative electrostatic potential energy value is greater (less negative).

Question 5

Electrostatic forces and potential energy of liquids

Answer 5

The electrostatic attraction between molecules is negligible, and the electrostatic potential energy is at a maximum, of 0J.

Question 6

What is Brownian motion

Answer 6

Molecules of a gas travel in random directions with random velocity.

Question 7

How can Brownian motion be observed

Answer 7

Can be seen by looking at smoke particles. the smoke particles are visible under a microscope and exhibit random motion because of their collisions with molecules in the air

Question 8

what is internal energy

Answer 8

Defined as the sum of the randomly distributed kinetic and potential energies associated with the atoms or molecules which make up the substance.

Question 9

What happens to kinetic and potential energy when a substance is heated but remains in the same state.

Answer 9

The kinetic energy of the molecules increases but the potential energy remains the same.

Question 10

What happens to kinetic and potential energy when a substance changes state

Answer 10

The potential energy increases, but the kinetic energy remains the same

Question 11

Define absolute zero

Answer 11

Absolute zero is 0K, and is the point where all molecules in a substance stop moving completely. This is also where the substance has minimal internal energy.

Question 12

Define specific heat capacity

Answer 12

The specific heat capacity, c of a substance is defined as the energy required per unit mass to increase the temperature by 1K.

Question 13

What is the method of mixtures and how can it be used to determine specific heat capacity

Answer 13

Known masses of two substance at different known temperatures are mixed together until they reach thermal equilibrium, at which point the final temperature is measured. The energy transfer from the hotter substance is the same as the energy transfer to the cooler substance, so the equation 𝐸 =𝑚c∆𝜃 can be equated for both substances. If the specific heat capacity of one of the substances is known, then it can be determined for the other substance.

Question 14

How can c be calculated using an electric heater

Answer 14

A known mass of a substance is heated by an electrical heater with known power for a given time. The initial and final temperatures of the substance are measured. as E=VIT, formula can be rearranged for v.
To increase accuracy - an insulator is used around the substance to minimise external energy transfer, increasing accuracy.

Question 14

Define specific latent heat of fusion

Answer 14

The energy required per unit mass to change the phase of a substance from solid to liquid

Question 15

Define specific latent heat of vaporization

Answer 15

The energy required per unit mass to change the
phase of a substance from liquid to gas.

Question 16

what are the assumptions made in order to model a gas as an ideal gas.

Answer 16

The gas contains a large number of atoms which move with random, rapid motion.
The volume of the gas atoms is negligible when compared to the total volume of the gas.
All collisions between atoms, with other atoms and with the walls of the container they are in, are perfectly elastic.
The time taken for atoms to collide is negligible compared to the time between collisions.
The electrostatic forces between atoms are negligible, except for when the atoms are colliding.

Question 17

Gas in a container produces pressure. The collisions between atoms and the wall of the container are …..?

Answer 17

Perfectly elastic

Question 18

what is the formula for the average force exerted on the atom by the wall

Answer 18

F=2MV/T

Question 19

Boyle’s law

Answer 19

For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume. PV=K

Question 20

Charles’ law

Answer 20

For a fixed mass of gas at a constant pressure,
the volume is directly proportional to temperature. V=KT

Question 21

value of R (molar gas constant)

Answer 21

8.31 J mol -1 k-1

Question 22

Investigating Boyle’s law

Answer 22

A sealed syringe can be filled with gas and connected to a pressure gauge.
The syringe can be used to vary the volume of the container, and the values for volume and pressure recorded.
When a graph of pressure against 1/volume is plotted, a straight line should be produced

Question 23

How to estimate absolute zero using gas

Answer 23

A sealed container of air connected to a pressure gauge placed in a water bath.
The temperature of the water bath are varied and recorded producing different values for temperature and pressure.
When pressure against temperature is plotted, a linear graph will be produced.
At absolute zero there is no kinetic energy and no gas pressure.
By extrapolating the graph back to the x intercept absolute 0 can be determined

Question 24

What is meant by RMS speed

Answer 24

The pressure exerted by a gas and the mean kinetic energy of molecules in the gas are related to the RMS speed of molecules.

Question 25

How is RMS speed determined

Answer 25

Determined by summing the square of all of the individual velocities of molecules and dividing by the number of molecules, N and then finding the square root of this value.

Question 26

What does the Maxwell-Boltzmann distribution show

Answer 26

Particles in a gas move with random velocities, in random directions. This means some of the molecules are moving very fast, whilst other molecules are barely moving at all. The Maxwell Boltzmann distribution shows the number of molecules with each speed, against speed c.

Question 27

Maxwell-Boltzmann distribution.. as the temperature increases....

Answer 27

As the temperature of the gas increases, the peak of the graph shifts to a higher speed, and the distribution becomes more spread out.

Question 28

Maxwell-Boltzmann distribution - area under graph represents

Answer 28

The area under the graph represents the total number of molecules.

Question 29

Relationship between kinetic energy and temperature

Answer 29

The kinetic energy of gas molecules in an ideal gas is proportional to the temperature

Question 30

Potential energy of an ideal gas

Answer 30

The internal energy of an ideal gas is the sum of the kinetic and potential energies. Since we assume there are no electrostatic forces between molecules in an ideal gas, there is no potential energy.

Question 31

The internal energy of an ideal gas

Answer 31

This means that for an ideal gas, the kinetic energy is equal to the total internal energy, hence the internal energy is proportional to temperature.