Module 5: C14 - Thermal Physics

Question 1

What is the triple point of a substance?

Answer 1

A triple point of a substance is one specific temperature and pressure where 3 phases of matter (solid, liquid, and gas) of that substance can exist in thermal equilibrium (no net transfer of thermal energy between the phases)

Question 2

What is temperature a measure of?

Answer 2

Temperature is a measure of the hotness of an object on a chosen scale. The hotter an object is, the higher its temperature.

Question 3

What happens when one object is hotter than another (where does the net flow of thermal energy go)

Answer 3

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 hotter one. For example, when the outside air temperature is lower than your body temperature, there is a net flow of energy from you to your surroundings.

Question 4

What happens/what does it mean when 2 objects are in thermal equilibrium

Answer 4

When two objects are in thermal equilibrium, there is no net flow of thermal energy between them. This means any objects in thermal equilibrium must be at the same temperature.

Question 5

What is the Zeroth Law

Answer 5

The zeroth law states that if two objects are each in thermal equilibrium with a third, then all three are in thermal equilibrium with each other.

If both A and C are in thermal equilibrium with B, then A is in thermal equilibrium with C. This means that all three objects are at the same temperature.

Question 6

What is the relationship between Kelvin and Celsius

Answer 6

T (K) = Θ (°C) + 273

Temperatures in K are always positive, and the lowest temperature on the absolute scale is 0K

Question 7

What is the Kinetic Model

Answer 7

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

Question 8

How are atoms or molecules arranged in a solid?

Answer 8

In solids the atoms or molecules are regularly arranged packed closely together, with strong electrostatic forces of attraction between them, holding them in fixed positions, but they can vibrate and so have kinetic energy.

Question 9

How are atoms or molecules arranged in a liquid?

Answer 9

In liquids the atoms or molecules are still very close together, but they have more kinetic energy than in solids, and - unlike in solids - they can change position and flow past each other.

Question 10

How are atoms or molecules arranged in a gas?

Answer 10

In gases, the atoms or molecules have more kinetic energy than those’d in liquids, and they are much further apart. They are free to move past each other, as there are negligible electrostatic forces between them, unless they collide with each other on the container walls. They move randomly with different speeds in different directions.

Question 11

In general how does the state of matter of something affect how dense it is?

Answer 11

The spacing between the particles (atoms or molecules) in a substance in different phases affects to the density of the substance. In general, a substance is most dense in its so,I’d phase and lest dense in it’s gaseous phase.

Question 12

Why is solid water less dense than liquid water?

Answer 12

Water freezes into a regular crystalline pattern held together by strong electrostatic forces between the molecules. In this structure, the molecules are held slightly further apart than in their random arrangement in liquid water, so ice is slightly less dense.

Question 13

How does a thermometer detect a change in temperature?

Answer 13

Some physical property of a system changes as the system’s temperature changes, such as

• the volume of a liquid
• the dimensions of a solid
• the pressure of a gas at constant volume
• the volume of a gas at constant pressure
• the electric resistance of a conductor

Question 14

What does it mean 2 objects are in thermal contact?

Answer 14

Two objects are in thermal contact with each other if energy can be exchanged between them due to a temperature difference.

Thermal contact and physical contact are not necessarily the same.

Example:
Warm your hands near a fire

Question 15

What does it mean 2 objects are in thermal equilibrium?

Answer 15

A situation in which two objects would not exchange energy if they were placed in thermal contact.

Example:
Mixing hot water and cold water. Hot water cools off and cold water warms up. Eventually, a final intermediate temperature is reached, which is equal throughout the mixture. At this point, energy exchange ceases, reaching thermal equilibrium.

Question 16

What is the internal energy of substance defined as

Answer 16

The sum of the randomly distributed kinetic and potential energies of the atoms or molecules within the substance.

Question 17

Heat definition

Answer 17

Heat is the amount of energy stored in a substance and can be regarded as the sum of the random kinetic potential energies of all the molecules in that substance. This is sometimes called the Internal Energy of the substance.

Question 18

Specific Heat Capacity Definition

Answer 18

The specific heat capacity (c) of a substance is the amount of energy needed to raise the temperature of 1kg of the substance by 1K (or 1ºC). It’s unit are J kg^-1 °C^-1 or J kg^-1 K^-1

Question 19

Equation for Specific Heat Capacity

Answer 19

ΔE = mcΔθ
or
c = E/mΔθ

where
∆E is the energy change in J
m is the mass in kg
c is the specific heat capacity in J K-1 kg-1 (or J°C^-1 kg^-1 )
∆θ is the temperature change in K (or °C)

Question 20

Worked Example:

A bucket containing 11.5 litres of cold water at 10°C is
taken into a house at a warmer temperature and left
inside until it has reached thermal equilibrium with it
new surroundings. The specific heat capacity of water
is 4200 J kg^-1°C^-1.

If 504 kJ of energy is absorbed from the surroundings
to heat the water, what is the temperature of the
room?

Answer 20

1 litre = 1kg

ΔE = 504kJ = 504x10^3J
m = 11.5kg
c = 4200J kg^-1 °C^-1
ΔΘ = ?

ΔE = mcΔΘ
ΔΘ = ΔE/mc
ΔΘ = 504x10^3 / 11.5x4200
ΔΘ = 10.4°C

Water was 10°C
Now it is 20.4°C

Question 21

Worked Example:

A 60 W immersion heater takes 2.5 minutes to heat 0.5 kg of water from 21to 25°C, what is the specific heat capacity of the water. Why is it higher than the accepted value of c = 4200 J kg^-1 °C^-1?

Answer 21

ΔΘ = 4°C
m = 0.5kg
t = 150s
P = 60W

E = mcΔΘ
P = E/t

60x150 = E
E = 9000J

c = E/mΔΘ
c = 9000/0.5 x 4
c = 4500J kg^-1 °C^-1

It is higher than the accepted value of c = 4200 Jkg^-1°C^-1, because heat is being lost from the object.

Question 22

Equation showing relationship between Energy gained by cooler object and Energy lost by hotter object

Answer 22

𝐸𝑛𝑒𝑟𝑔𝑦 𝑔𝑎𝑖𝑛𝑒𝑑 𝑏𝑦 𝑐𝑜𝑜𝑙𝑒𝑟 𝑜𝑏𝑗𝑒𝑐𝑡 = 𝐸𝑛𝑒𝑟𝑔𝑦 𝑙𝑜𝑠𝑡 𝑏𝑦 ℎ𝑜𝑡𝑡𝑒𝑟 𝑜𝑏𝑗𝑒𝑐𝑡

or

𝑚1𝑐1∆𝜃1= 𝑚2𝑐2∆𝜃2

Question 23

Worked Example:

If 500ml of water is in thermal equilibrium with a copper object at 25 degrees Celsius and the specific heat capacity of water is 4200 J kg^-1 °C^-1 ,the water was initially at 20 °C, the copper at 100 °C and the copper has a mass of 0.155 kg, what is the specific heat capacity of copper? (Hint: What is the density of water?)

Answer 23

Water:
m = 0.5kg
c = 4200 J kg^-1 °C^-1
ΔΘ = 5°C

Copper:
m = 0.155kg
c = ?
ΔΘ = 75°C

𝑚1𝑐1∆𝜃1= 𝑚2𝑐2∆𝜃2

c2 = 𝑚1𝑐1∆𝜃1/𝑚2∆𝜃2
c2 = 0.5x4200x5/0.155x75
c2 = 903 J kg^-1 C^-1

Question 24

Specific Latent Heat Definition

Answer 24

The specific latent heat (L) of fusion or vaporisation is the quantity of thermal energy required to change the phase of 1kg of a substance.

• Fusion (liquid → solid)
• Vaporisation (liquid → gas)
• …or the other way (Melting/Condensing)

Question 25

Equation for Energy in Specific Latent Heat

Answer 25

E = mL

(E in Jkg^-1)

Question 26

Worked Example:
The specific latent heat of fusion (melting) of ice is 330,000 J kg-1
.
What is the energy needed to melt 0.65 kg of ice?

Answer 26

E = mL

0.65 x 330000 = 210,000J

Question 27

Worked Example:

The power of the immersion heater in the diagram is 60 W. In 5 minutes, the top pan balance reading falls from 282g to 274g. What is the specific latent heat of vaporisation of water?

Answer 27

P = 60W
t = 300s
Δm = 8g = 0.008kg

P = E/t
E = Pt
E = 60x300
E = 18000 J

E = mL
L = E/m
L = 18,000/0.008
L = 2.25x10^6 Jkg^-1

Question 28

What equation for change in energy should be used when a question includes heating and then changing the phase of a substance

Answer 28

ΔE = mcΔΘ + mL

Question 29

Worked Example:

A container of water at 18°C containing 1500g of water is heated until it boils dry. How much energy is required to make this happen?

(Lv = 2.25x10^6 Jkg^-1
c = 4200 J kg^-1 °C^-1)

Answer 29

m = 1.5kg
Lv = 2.25x10^6 J kg^-1
Θ = 18°C
c = 4200 J kg^-1 °C

ΔE = mcΔΘ + mL
ΔE = 1.5 x 4200 x 82 + 1.5 x 2.25x10^6
ΔE = 3.8916x10^6 J

ΔE = 3.89MJ

Question 30

What happens in Specific Latent Heat of Fusion?

Answer 30

When the substance changes from solid to liquid phase, we refer to the specific latent heat of fusion, Lf.

Question 31

What happens in Specific Latent Heat of Vaporisation?

Answer 31

When the substance changes from solid to liquid phase, we refer to the specific latent heat of vaporisation, Lv.

Question 32

How can you find the energy transferred from a heater to a substance (specific heat capacity)

Answer 32

E = IVt

c = ItV/mΔΘ

Question 33

How can you determine the specific latent heat of fusion using current and voltage

Answer 33

Lf = IVt / m

Question 34

What happens when a substance reaches its melting or boiling point

Answer 34

When a substance reaches its melting or boiling point, while it is changing phase the energy transferred to the substance does not increase its temperature. Instead, the electrostatic potential energy of the substance increases as the electrical forces between the atoms or molecules change. Only once the phase change is complete does the kinetic energy of the atoms or molecules increase further, and so the temperature rises again.

Question 35

What is the electrostatic potential energy of atoms in a gas

Answer 35

The electrostatic potential energy is zero because there negligible electrical forces between atoms or molecules

Question 36

What is the electrostatic potential energy of atoms in a liquid

Answer 36

The electrostatic forces between atoms or molecules give the electrostatic potential energy a negative value.

The negative simple means that energy must be supplied to break the atomic or molecular bonds.

Question 37

What is the electrostatic potential energy of atoms in a solid

Answer 37

The electrostatic forces between atoms or molecules are very large, so the electrostatic potential energy has a large negative value.

The negative simple means that energy must be supplied to break the atomic or molecular bonds.