3.6.2 Thermal Physics

Question 1

Define avogadro Constant

Answer 1

The number of atoms in 12g of carbon-12 (or the number of molecules in one mole of substance)

Question 2

What does Boyle’s law state?

Answer 2

​The pressure of an ideal gas is inversely proportional to its volume when held at constant temperature.

Question 3

What is brownian motion?

Answer 3

The ​random motion of larger particles in a fluid​ caused by ​collisions​ with surrounding particles (e.g. smoke and air)

Question 4

What does Charles’ law state?

Answer 4

The volume of an ideal gas is directly proportional to its absolute temperature when held at constant pressure.

Question 5

Define internal energy of a body

Answer 5

The sum of the randomly distributed kinetic energies and potential energies of the particles in a body.

Question 6

What does the pressure law state?

Answer 6

​The pressure of an ideal gas is directly proportional to its absolute temperature, when the volume is fixed.

Question 7

Define Specific Heat Capacity

Answer 7

The amount of energy required to increase the temperature of 1kg of a substance by 1 Kelvin without changing its state

Question 8

Define Specific Latent Heat

Answer 8

The amount of energy required to change the state of 1kg of a substance without a change of temperature.

Question 9

What are the energy changes during a state change?

Answer 9

the (mean/total) potential energy of the system is changing but the (mean/total) kinetic energy is not

Question 10

2 ways to increase internal energy

Answer 10

• Do work on the system to transfer energy to it
• Heat (increase the temperature) of the system

Question 11

How to observe Brownian motion

Answer 11

By looking at smoke particles under a microscope

Question 12

What was Brownian motion used as evidence for?

Answer 12

Existence of atoms

Question 13

5 assumptions for kinetic theory model

Answer 13

• No intermolecular forces act on the molecules
• Duration of collisions in negligibly in comparison to time between collisions
• Collisions are perfectly elastic
• Molecules move in straight lines between collisions
• Size of gas molecules is negligible

Question 13

How much potential energy does an ideal gas have?

Answer 13

None (as potential energy is associated with intermolecular forces)

Question 14

Fundamental difference between gas laws and kinetic theory model?

Answer 14

The gas laws are empirical in nature whereas the kinetic theory model arises from theory

Question 15

Boyle’s law explanation using kinetic theory model

Answer 15

• If you ​increase the volume​ of a fixed mass of gas, its molecules will move further apart
• so collisions will be less frequent
• so force per unit area on surface on container decreases
• therefore ​pressure decreases​

Question 16

Charle’s law explanation using kinetic theory model

Answer 16

• When the ​temperature of a gas is increased​, the molecules mean kinetic energy increases
• so the molecules mean speed increases
• and because pressure is kept constant (therefore frequency of collisions is constant) the molecules ​move further apart and volume is increased​.

Question 17

Pressure law explanation using kinetic theory model

Answer 17

• When the ​temperature of a gas is increased​, the mean kinetic energy of the particles increases
• meaning their mean speed increases
• as volume is constant, the frequency of collisions between molecules and their container increases and they collide at higher speeds
• the rate of change of momentum (force exerted on container) increases
• pressure is force / area
• therefore pressure is increased​.

Question 18

Describe how the motion of the atoms changes when a gas’ temperature is increasing

Answer 18

The mean speed/kinetic energy increases

Question 19

Gas law interpretation of absolute zero

Answer 19

Temperature at which the volume/pressure of a gas extrapolates to zero

Question 20

Kinetic theory model interpretation of absolute zero

Answer 20

it is the temperature at which the kinetic energy of the particles can be extrapolated to be zero

Question 21

What do you talk about when using the kinetic theory to talk about P/V in/decreasing?

Answer 21

(Mean/Total) Momentum change!
Rate of collisions!

Question 22

What is meant by an ideal gas?

Answer 22

• A gas that obeys the ideal gas laws at all temperatures and pressures

Question 23

What is represented by the area under a pressure volume graph?

Answer 23

The work done to the system

Question 24

Define the molar mass of a substance

Answer 24

The mass in grams of 1 mole of the substance

Question 25

How does the arrangement of the atoms change when changing from solid to liquid?

Answer 25

• Some of the atomic bonds are broken
• Allowing atoms to change from vibrating around fixed positions to being able to slide around each other

Question 26

Does the mark scheme refer to mean or average kinetic energy?

Answer 26

mean

Question 27

Define internal energy of an ideal gas

Answer 27

The sum of the randomly distributed kinetic energies of the particles in a gas.