5.1 Thermal Physics

Question 1

Define Temperature

Answer 1

Temperature is a measure of the hotness of an object on a chosen scale

Question 2

What does a temperature scale need in order to measure temperature?

Answer 2

A temperature scale needs two fixed points at defined temperatures

The temperature of other objects can then be defined as a position on this scale

Question 3

Which scale is commonly used to measure temperature?

Answer 3

The celcius scale

Question 4

Describe how the Celcius scale is marked in order to measure temperature?

Answer 4

The celcius scale marks 0°C as the melting point of pure ice, and 100°C as the boiling point of water (under atmospheric pressure)

These are the 2 fixed points at defined temperatures that the celcius scale uses in order to measure the temperature of other objects

There is nothing special about the temperatures 100°C and 0°C.

Question 5

What does it mean when two objects are in thermal equillibrium?

Answer 5

When two objects are in thermal equillibrium, it means that there is no net transfer of thermal energy between the two objects

Question 6

Explain what happens when one object is hotter than another

Answer 6

If one object is hotter than another there is a net flow of thermal energy from the hotter object into the colder one.

This increases the temperature of the colder object and lowers the temperature of the of the hotter one

Question 7

What is the triple point of a substance?

Answer 7

The triple point of a substance is a specific temperature and pressure, where the three phases of matter (solid, liquid, gas) of that substance can exist at the same time in thermal equilibrium - as in there is no net transfer of thermal energy between the phases.

Question 8

What is the problem with the celcius scale?

Answer 8

The problem with the celcius scale is that it significantly depends on the surrounding atmosheric pressure

For example, on top of a high mountain, water boils at a lower temperature (as low as 70°C)

Question 9

Describe how the thermodynamic scale / absolute temperature scale is marked in order to measure temperature?

Answer 9

The thermodynamic scale / absolute temperature scale uses the triple point of pure water and absolute zero as its fixed points

Question 10

What is the absolute zero?

Answer 10

The lowest possible temperature ( -273°C or 0K )

Question 11

What is the SI Base unit of temperature on the abssolute scale?

Answer 11

The SI Base unit of temperature on the absolute scale is called the Kelvin (K)

Question 12

What are the increments on the absolute scale similar to?

What does this mean?

Answer 12

The increments on the absolute scale are the same size as those on the Celcius scale

This means that a temperature change of 1K is the same as a change of1°C

Question 13

How many increments are there exactly between the absolute zero, and the triple point of pure water?

Answer 13

There are exactly 273.16 increments between the absolute zero (defined as 0K) and the triple point of pure water

Question 14

Give a formula linking the temperature in Celcius to Kelvins

Answer 14

Kelvin = Celcius (°C) + 273

Kelvin is always 273 higher than Celcius

Question 15

What is 0°C in Kelvin?
What is 0K in Celcius?

Answer 15

• 0°C in Kelvin is 273K
• 0K is -273°C

Question 16

What is the lowest temperature on the absolute scale?

Answer 16

0K - Temperatures in Kelvin are always positive

Question 17

What is the kinetic model?

Answer 17

The kintetic model describes how all substances are made up of atoms or molecules, which are arranged differently depending on the phase of the substance.

Question 18

Describe the simple kinetic model for solids

Answer 18

• In the solids, the atoms or molecules are arranged in a regular structure and are packed closely together in fixed positions
• This is because there are strong electrostatic forces of attraction between them.
• However, they can vibrate, as they have kinetic energy - but they cannot move out of their positions in the structure

Question 19

Explain what happens when a solid is heated, according to the kinetic model.

Answer 19

When a solid is heated, the particles gain energy and vibrate more and more vigourously until they break away from the solid structure, and become free to move aroud - a liquid.

Question 20

Describe the simple kinetic model for liquids

Answer 20

In liquids, the atoms and molecules are still close together, and they still have forces of attraction - but they have more kinetic energy than in solids, so they have no fixed shape and are free to move around and flow past each other easily.

Question 21

Explain what happens when a liquid is heated, according to the kinetic model.

Answer 21

• When a liquid is heated, some of the particles gain enough energy to break away from other particles.
• The particles which escape from the body of the liquid become a gas.

Question 22

Describe the simple kinetic model for gases

Answer 22

• In gases, the atoms are much further apart and travel at different high speeds, in different directions
• This is because they have a lot more kinetic energy than those in liquids

Question 23

Compare the volume of gases to other states of matter

Answer 23

Because the particles ina gas are so far apart, a gas with a specific mass occupies a much larger volume than a liquid with the same mass.

Question 24

Define Internal energy

Answer 24

Internal energy is defined as the sum of the randomly distributed kinetic and potential energies assosciated with the atoms or molecules within a substance.

Question 25

Describe the internal energy of a beaker of water at room temperature

Answer 25

• A beaker of water at room temperature would contain a huge number of water molecules travelling at hundreds of meters per second.
• The internal energy of the water is the sum of all the individual kinetic energies of the water molecules in the beaker, and the sum of all the potential energies due to the electrostatic intermolecular forces between the molecules.

Question 26

What happens to the internal energy of a substance at absolute zero?

Answer 26

• At absolute energy, the lowest temperature possible, the internal energy of a substance is at a minimum.
• This is because the kinteric energy of all the atoms/molecules is zero - as they have stopped moving.
• However, the internal energy is not zero because the substance still has electrostatic potential energy stored between the particles.
• This means that even at 0K, you cannot reduce the potential energy of the substance to 0.

Question 27

What happens to the internal energy of a body when you increase its temperature?

Answer 27

• When you increase the temperature of a body, it will increase its internal energy.
• This is because as the temperature increases, the average kinetic energy of the atoms or molecules inside the body increases, and the faster the molecules move.

Question 28

What are the two things that increase the internal energy of a body?

Answer 28

• The increase in temperature (as it is an increase in kinetic energy)
• The changing of phase - from solid to liquid, or liquid to gas (as it is an increase in electrostatic potential energy)

Question 29

Describe what happens to the temperature of a substance during the change of state/phase

Answer 29

During the change of state/phase, the temperature of a substance remains the same, as the kinetic energy remains the same

Question 30

Explain what happens to the internal energy & temperature of a substance when the substance changes state?

Answer 30

When a substance changes phase, for example from a solid to a liquid, the temperature does not change, nor does the kinetic energy.

However the internal energy of the substance increases

This is because the electrostatic potential energy of a substance increases significantly as the electrical forces between the atoms or molecules change

Question 31

Explain the electrostatic potential energies of each state

Answer 31

Question 32

Define Specific Heat Capacity

Answer 32

Question 33

What is the formula for specific heat capacity?

Answer 33

E = mcΔθ

Where E is energy supplied to the substance in Joules
m is the mass in kg of the substance
Δθ is the change in temperature measured in K or °C, since both give the same numerical value for change

Question 34

Compare the specific heat capacitiy values of metals with water

Answer 34

Metals tend to have low specific heat capacity values, whereas water has an exceptionally high specific heat capacity

Question 35

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

Question 36

Define Specific Latent Heat

Answer 36

• The specific latent heat of a substance, L, is a property of a substance.
• It is defined as the energy required to change the phase per unit mass of a substance, while at constant temperature.

Therefore:

where E is the energy supplied to change the phase of mass M of the substance