Chapter 6: Thermal Physics

Question 1

Describe a kinetic model for solids, liquids and gases

Answer 1

1. Matter is made up of tiny particles

2. Particles tend to move about

Question 2

Describe an experiment demonstrating Brownian motion

Answer 2

Jar of smoke lit from the side; a microscope is used to view motion of smoke particles (see page 76)

Question 3

What evidence does Brownian motion provide for the movement of air molecules

Answer 3

1. They’re also moving
2. They have haphazard motion
3. They move much faster than smoke grains

Question 4

Use the kinetic model to explain the pressure exerted by gases

Answer 4

Gas molecules make elastic collision with wall, rebound with speed v
Impact imparts tiny force on wall
Pressure is total sum of these imparted forces from gas molecules/surface area of container

Question 5

Define internal energy

Answer 5

Sum of random distribution of kinetic and potential energies of molecules in the system

Question 6

Why does a rise of temperature of a body lead to an increase in its internal energy

Answer 6

Thermal energy is transferred to the molecules in a substance
This increases either the k.e. or e.p.e (at a change of state) of the molecules
Consequently this increases the internal energy of the system

Question 7

What happens at a change of state

Answer 7

1. Energy (latent heat) must be supplied to give atoms enough energy to break bonds between neighbouring atoms
2. E.p.e of atoms increases therefore internal energy increases
3. Temperature stays the same because atoms are still moving with same speed

Question 8

Describe what happens when a substance evaporates below its boiling point

Answer 8

1. Gas formed is below its boiling point = vapour
2. Some molecules in a liquid move faster than others, enough to break free from surface
3. There is a net outflow of energetic molecules

Question 9

Why does an evaporating liquid have a cooling effect

Answer 9

1. Most energetic molecules likely to leave
2. Remaining molecules have below avg. k.e.
3. Temperature is a measure of average k.e. therefore temp. decreases because avg. k.e. has decreased

Question 10

Describe the transfer of thermal energy

Answer 10

1. Energy is transferred from a region of higher temp. to lower temp.
2. When two objects are at the same temperature they are in thermal equilibrium -> no enrgy transfer between them

Question 11

Describe the basis of the Kelvin scale of temperature

Answer 11

1. Also known as the thermodynamic scale
2. Has fixed points at absolute zero and triple point of water (where ice, water and water vapour can coexist = 273.16K)
3. This range is split into 273.16 equal divisions

Question 12

Absolute zero

Answer 12

Minimal internal energy

k. e. = 0
e. p.e = minimum

Question 13

Specific heat capacity

Answer 13

The energy required per unit mass of a substance to raise the temperature by 1 K

Question 14

Describe an electrical experiment to determine the specific heat capacity of a solid or liquid

Answer 14

Diagram page 86
Measure:
1. Mass of material
2. Change in temp. of material
3. I, V and t values
Calculations:
1. E=IVt
2. c=E/m x changeT

Question 15

Describe errors and improvements for the experiment to determine specific heat capacity

Answer 15

Heat losses : Insulate vessel
Thermal capacity of vessel : Take into account in calc.
False temp. reading : Stir liquid
Temp. continues to rise after heater switched off : Measure highest value