Thermal Physics

Question 1

Define internal energy

Answer 1

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

Question 2

When is the internal energy of a system increased

Answer 2

When energy is transferred to it by heating or when work is done on it (& visa versa)
Eg: a qualitative treatment of the first law of thermodynamics

Question 3

What happens to the particle ensemble during a change in state

Answer 3

The potential energies of the particle ensemble are changing but not the kinetic energies

Question 4

What is the formula for a change in state

Answer 4

Q = ml
Where L is the specific latent heat.

Question 5

What is the proof for existence of atoms

Answer 5

Brownian motion

Question 6

What is potential energy in particles due to

Answer 6

The separation between the molecules

Question 7

What is the internal energy of a system determined by

Answer 7

Temperature

Random motion of molecules

The phase of matter - gases have the highest internal energy, solids have the lowest.

Intermolecular forces between particles
(Greater intermolecular forces, higher potential energy)

Question 8

State the First Law of Thermodynamics

Answer 8

The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa)

Question 9

1st Law: what happens when a gas expands?

Answer 9

WORK is done BY the gas ON the surroundings
Decreasing the internal energy of the gas

Question 10

What happens when a gas is compressed

Answer 10

Work is done ON gas BY surroundings

Question 11

Define specific heat capacity

Answer 11

The amount of thermal energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K) without a change of state

Question 12

How can the specific heat capacity of a fluid be found?

Answer 12

By a CONTINUOUS-FLOW CALORIMETER

Question 13

Describe the use of a continuous flow calorimeter

Answer 13

A fluid flows continuously over a heating element where energy is transferred to the fluid
- It is assumed that the heat transferred from the apparatus to the surroundings is constant

A fluid flows through an electrical heating wire. The rise in temperature of the fluid is measured using the electric thermometers and is calculated by:
Δθ = T2 – T1

For mass of fluid: flow rate recorded, multiplied bu time taken to give mass of fluid that flows (M1)

The current I and potential difference V are also recorded
The flow rate is then altered to give a mass m2 and the potential difference of the power supply is changed so the temperature difference, Δθ stays the same
The specific heat capacity is found by assuming the thermal losses to the surroundings are constant for both flow rates

Question 14

Define latent heat

Answer 14

The thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

Question 15

What are the 2 types of latent heat

Answer 15

Specific latent heat of fusion (melting)
Specific latent heat of vaporisation (boiling)

Question 16

Define the specific latent heat of fusion

Answer 16

The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

Question 17

Define the specific latent heat of vaporisation

Answer 17

The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

Question 18

What does latent heat of fusion apply to

Answer 18

Melting a solid
Freezing a liquid

Question 19

What does latent heat of vaporisation apply to

Answer 19

Vaporising a liquid
Condensing a gas

Question 20

What happens during a change in state

Answer 20

• no change in temperature
• the potential energies of the molecules change, but not their kinetic energies

Question 21

What does the heat in melting and boiling cause the molecules to do

Answer 21

Move further apart by overcoming the intermolecular forces of attraction

In freezing and condensation molecules move closer together and intermolecular forces of attraction become stronger

As

Kinetic energy is proportional to temp
If no change in tempo, no change in kinetic energy either

Question 22

Define absolute zero

Answer 22

The temperature at which the molecules in a substance have zero kinetic energy

Question 23

What does boule’s law state

Answer 23

pressure is inversely proportional to the volume of a gas

Question 24

State charles’s law

Answer 24

the volume is proportional to the temperature of a gas
At constant pressure

Question 25

State pressure law

Answer 25

pressure is proportional to the temperature

Question 26

What is an ideal gas

Answer 26

An ideal gas is one that obeys the relation:
pV ∝ T

Question 27

How does the temperature of a gas relate to the average speed of the molecules

Answer 27

The hotter the gas, the faster the molecules move
Hence the molecules collide with the surface of the walls more frequently

Since force is the rate of change of momentum:
Each collision applies a force across the surface area of the walls
The faster the molecules hit the walls, the greater the force on them

Since pressure is the force per unit area
Higher temperature leads to higher pressure
If the volume V of the box decreases, and the temperature T stays constant:
There will be a smaller surface area of the walls and hence more collisions
This also creates more pressure

Since this equates to a greater force per unit area, pressure in an ideal gas is therefore defined by:
The frequency of collisions of the gas molecules per unit area of a container

Question 28

Define the 5 qualities of an ideal gas

Answer 28

1. Has molecules with negligible volume
2. Collisions which are elastic
3. Cannot be liquified
4. Has no interactions between the molecules (except during collisions)
5. Obeys the (ideal) gas laws (Boyles law, Charles’ law and Pressure law)

Question 29

Equation for ideal gas

Answer 29

Question 30

Define an ideal gas

Answer 30

A gas which obeys the equation of state pV = nRT at all pressures, volumes and temperatures

Question 31

Work done by a gas formula

Answer 31

W = pΔV

Where:
W = work done (J)
p = external pressure (Pa)
V = volume of gas (m3)

Question 32

Derive work done by a gas formula

Answer 32

Derivation
The volume of gas is at constant pressure. This means the force F exerted by the gas on the piston is equal to :
F = p × A

Where:
p = pressure of the gas (Pa)
A = cross-sectional area of the cylinder (m2)
The definition of work done is:
W = F × s

Where:
F = force (N)
s = displacement in the direction of force (m)
The displacement of the gas d multiplied by the cross-sectional area A is the increase in volume ΔV of the gas:
W = p × A × s

This gives the equation for the work done when the volume of a gas changes at constant pressure:
W = pΔV

Where:
ΔV = increase in the volume of the gas in the piston when expanding (m3)

This is assuming that the surrounding pressure p does not change as the gas expands
This will be true if the gas is expanding against the pressure of the atmosphere, which changes very slowly
When the gas expands (V increases), work is done by the gas
When the gas is compressed (V decreases), work is done on the gas

Question 33

Define molar mass

Answer 33

The molar mass of a substance is the mass, in grams, in one mole
Its unit is g mol-1

Question 34

What is Boltzmann and molar gas constant formula

Answer 34

Where:
R = molar gas constant
NA = Avogadro’s constant