5.1 Thermal Physics

Question 1

How do you convert from Celsius to
Kelvin?

Answer 1

Add 273.
Eg. 10 degrees C = 283 K

Question 2

Why is the absolute scale used?

Answer 2

It doesn’t arbitrarily depend on the properties of a
given substance (eg. water’s melting and boiling
point for the Celsius scale).
0K (absolute zero) means that the particles have
minimum internal energy

Question 3

Describe the arrangement and energy of
particles in a solid, a liquid and a gas.

Answer 3

Solid - regular arrangement, vibrate around fixed positions.
Liquid - close together, constantly moving past each other.
Gas - spaced very far apart, free to move in all directions.

Question 4

How does Brownian motion give
evidence for the particle model of
matter?

Answer 4

Smoke particles suspended in air can be seen to
move randomly in all directions. This must be as a
result of random collisions with particles making up
the air.

Question 5

What is Internal energy ?

Answer 5

The sum of the potential and kinetic energies of a
system.

Question 6

True or false: At a given temperature, all
particles in a material have the same
kinetic energy.

Answer 6

False. The kinetic energies will be randomly
distributed around a central ‘most likely’ amount.

Question 7

How can you increase the thermal
energy of a system?

Answer 7

We can increase it by heating it up or doing work
on the object.

Question 8

Explain the energy changes that occur
during a change of state.

Answer 8

During change of state the potential energy of the
particles change but the kinetic energies don’t
change.

Question 9

What equation can be used to determine
the energy required to change the
temperature of a substance?

Answer 9

Q = mcΔ𝜭

Question 10

What is the specific heat capacity of
substance?

Answer 10

The energy required to raise the temperature of
1kg of a substance by 1K.

Question 11

Give the equation to work of the energy
for change of state?

Answer 11

Q=ml

Question 12

What is the specific latent heat of a
substance?

Answer 12

The energy required to change the state per unit
mass of a substance, while keeping the
temperature constant.

Question 13

In an experiment to find ‘c’ for water, lots
of energy input escapes to the
surroundings. Will this lead to an over or
underestimate of specific heat capacity?

Answer 13

● An overestimate.
● Specific heat capacity is calculated as: c = Q / mΔ𝜭
● The energy input will be used, but the temperature change of the
water will be lower than it should be due to the escaped energy -
therefore c will be too high.

Question 14

What is Avogadro’s constant? (in words)

Answer 14

The number of atoms there are in one mole of a
substance.

Question 15

What are the key assumptions in the
kinetic theory of gases?

Answer 15

● There are a large number of molecules in random, rapid motion.
● Particles are negligibly small compared to the total volume of the
gas.
● All collisions are perfectly elastic.
● The time taken for a collision is negligibly small compared with the
time between collisions.
● Between collisions there are no forces between particles

Question 16

What are the key assumptions in the
kinetic theory of gases?

Answer 16

● There are a large number of molecules in random, rapid motion.
● Particles are negligibly small compared to the total volume of the
gas.
● All collisions are perfectly elastic.
● The time taken for a collision is negligibly small compared with the
time between collisions.
● Between collisions there are no forces between particles

Question 17

Why do gases exert a pressure on the
container they’re in?

Answer 17

● Gas particles collide with the surfaces of the container.
● The container exerts a force on the particles to change
their direction. The particles exert an equal and opposite
force on the container.
● Pressure is force applied (in total, by all particles) per unit
area.

Question 18

What is an ideal gas?

Answer 18

A gas where:
● The gas molecules don’t interact with each
other.
● The molecules are thought to be perfectly
spheres.

Question 19

What is the ideal gas equation?

Answer 19

pV=nRT

Question 20

What is Boyle’s law?

Answer 20

Pressure is inversely proportional to volume,
providing temperature is constant.
i.e. pV = constant

Question 21

Assuming constant volume, how are the
pressure and temperature of a gas
related?

Answer 21

They’re directly proportional.
ie. P/T = constant

Question 22

Use the kinetic theory of gases to explain
why a temperature increase leads to an
increase in pressure.

Answer 22

● A temperature increase means the particles have more kinetic energy.
● More kinetic energy means a greater change in momentum during
collisions with the container. There are also more frequent collisions.
● Change in momentum is proportional to force applied, and therefore
to pressure as well.

Question 23

What equation links N, V, p, m and c?

Answer 23

pV=(1/3)Nmc^2

Question 24

What is meant by the root mean square
speed?

Answer 24

The square root of the mean of the squares of
the speeds of the molecules

Question 25

What is meant by the root mean square
speed?

Answer 25

The square root of the mean of the squares of
the speeds of the molecules

Question 26

What does the area under a
Maxwell-Boltzmann curve represent?

Answer 26

The total number of particles

Question 27

How does the Maxwell-Boltzmann curve
change if the temperature of a gas is
increased?

Answer 27

The average particle speed, and maximum
particle speed both increase (curve shifts right).
The curve becomes lower and more spread out.

Question 28

What are the units of the Boltzmann
coefficient?

Answer 28

J/K

Question 29

The average kinetic energy of a particle
in an ideal gas is equal to what?

Answer 29

1.5 kT

Question 30

True or false: ‘The internal energy of an
ideal gas is proportional to absolute
temperature’

Answer 30

True.
In an ideal gas there is no ‘potential energy’
component in the internal energy. This means the
internal energy is proportional to the kinetic energy
(which is, in turn, dependent on temperature).