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

Which scale is commonly used to measure temperature?

Answer 2

The celcius scale

Question 3

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

Answer 3

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 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

Explain what happens when one object is hotter than another

Answer 5

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 6

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

Answer 6

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

This means that any objects in thermal equillibrium must be at the same temperature

Question 7

What is the problem with the celcius scale?

Answer 7

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

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

Question 8

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

Answer 8

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

Question 9

What is the triple point of a substance?

Answer 9

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 - in other words there is no net transfer of thermal energy between the phases.

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 absolute 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

So a a temperature change of 1K is the same as a change of 1°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 advantage of using the absolute temperature scale / thermodynamic temperature scale comapared to the Celcius scale?

Answer 17

The absolute temperature scale / thermodynamic temperature scale does not depend on the property of any particular substance, nor surrounding atmospheric pressure.

Question 18

What is the kinetic model?

Answer 18

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 19

Describe the simple kinetic model for solids

Answer 19

• 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 20

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

Answer 20

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 around - a liquid.

Question 21

Describe the simple kinetic model for liquids

Answer 21

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

Question 22

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

Answer 22

• 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 23

Describe the simple kinetic model for gases

Answer 23

• In gases, the atoms are much far apart and travel at different high speeds, in different directions
• This is because they have a lot more kinetic energy than those in liquids
• There are almost no forces of attraction between them

Question 24

Compare the volume of gases to other states of matter

Answer 24

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 25

Define Internal energy

Answer 25

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 26

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

Answer 26

* A beaker of water at room temperature contains 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 27

Now describe what happens when the beaker of water is cooled | (Linked to flashcard no. 26)

Answer 27

* When the beaker of water is cooled, the water will freeze and the water molecules move more slowly as the ice gets colder. * Absolute zero is the lowest possible temperature. At this temperature, the internal energy of the substance is at a **minimum**. * The kinetic energy of all the atoms/molecules is **zero** - as they stopped moving * However, the internal energy is not zero because the substance still has **electrostatic potential energy** stored between the particles. So even at 0k, you cannot reduce the potential energy of a substance to 0.

Question 28

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

Answer 28

* 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 29

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

Answer 29

* 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. * In general, the hotter a substance, the faster the molecules that make up the substance move, and the greater the internal energy of the substance.

Question 30

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

Answer 30

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

Question 31

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

Answer 31

During the change of state/phase, the temperature of a substance remains the same, and does not change. Nor does the kinetic energy of the atoms/molecules. However, the electrostatic potential energy increases significantly.

Question 32

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

Answer 32

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 33

Explain the electrostatic potential energies of each state

Answer 33

Question 34

Define Specific Heat Capacity

Answer 34

Question 35

What is the formula for specific heat capacity?

Answer 35

E = mcΔθ Where E is the **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 36

Compare the specific heat capacitiy values of metals with water

Answer 36

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

Question 37

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

Question 38

Define Specific Latent Heat

Answer 38

* 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:

Question 39

What are the two types of **Specific Latent Heat**? | Describe them

Answer 39

- **Specific Latent Heat of Fusion** - when the substance changes from a solid to liquid phase - **Specific Latent Heat of Vaporisation** - when the substance changes from liquid to gas

Question 40

# INFO CARD:

Question 41

What is one mole?

Answer 41

One mole of any substance is defined as the amount of substance that contains as many as 6.02 x 10²³ individual atoms or molecules. ## Footnote This means, that one mole of Carbon and one mole of Oxygen will have different masses, although having the same quantity / number of atoms or molecules.

Question 42

What is the number, 6.02 x 10²³, also known as?

Answer 42

6.02 x 10²³ is also known as the **Avogadro Constant** | Also known in equations as **N_A**

Question 43

One mole of any substance contains...

Answer 43

One mole of any substance contains **6.02 x 10²³ individual atoms or molecules** | However, atoms can have different masses. ## Footnote So 1 mole of oxygen does not have the same mass as one mole of carbon

Question 44

Give the formula to find the number of atoms or molecules in a substance

Answer 44

N = *n* x N_a N = *n* x 6.02 x 10²³ - N is the **total number of atoms or molecules** in a substance - *n* is the **number of moles** of the substance

Question 45

**Kinetic theory of gases and kinetic theory of an ideal gas**

Answer 45

Question 46

What are the assumptions made in the kinetic model for an ideal gas?

Answer 46

The assumptions made in the kinetic model for an ideal gas are as follows: - The gas contains a large number of atoms or molecules moving in random directions with random speeds - The atoms or molecules of the gas occupy a negligible volume compared with the volume of the gas - The collisions of atoms or molecules with each other and the container walls are *perfectly elastic* (no kinetic energy is lost) - The time of collisions between atoms or molecules is negligible compared to the time between the molecules - Elecrostatic forces between atoms or molecules are negligible except during collisions

Question 47

Explain how the atoms or molecules in an **ideal gas** cause pressure, using the assumptions made in the kinetic model for an ideal gas.

Answer 47

- The atoms or molecules in a gas are always moving, and when they collide with the walls of a container, the container exerts a force on them, changing their momentum as they bounce off the wall - When a single atoms collides with the container wall elastically, its speed does not change, but its velocity changes from +*u*ms⁻¹ to -*u*ms⁻¹. The total change in momentum is -2*mu. - The atom bounces between the container walls, making frequent collisions. According to Newton's 2nd Law, the force acting on the atom is *F_atom = Δp / Δt, where Δp = -2mu - From Newton's 3rd law, the atom also exerts an equal but opposite force on the wall. - A large number of atoms collide randomly with the walls of the container. If the total force they exert on the wall is *F*, then the pressure they exert on the wall is given by *p* = F / A *where A is the cross-sectional area of the wall*.

Question 48

Gas law between pressure and volume

Answer 48

| You need to briefly understand it, and memorise equation

Question 49

Gas law between pressure and temperature

Answer 49

| You need to briefly understand it, and memorise equation