Thermal physics

Question 1

Define internal energy

Answer 1

Sum of the individual potential and kinetic energies of all molecules within the body

Question 2

State the first law of thermodynamics

Answer 2

The change of internal energy of the object is equal to the total energy transfer due to work done and heating

Question 3

Define specific heat capacity

Answer 3

The energy required to change the temperature of 1kg of the substance by 1K without changing state

Question 4

Describe the experiment to measure the specific heat capacity of a metal

Answer 4

Metal block of known mass in an insulated container. Electrical heater inserted into a hole in the metal, thermometer into another.
Measure the temperature change over a given time

Question 5

Which measurements are used to measure specific heat capacity of a metal, and how?

Answer 5

Heater's current (I)
Heater's pd (V)
Time of heating (t)
Temperature change (ΔT)
mcΔT=IVt (rearrange for c)

Question 6

State Boyle’s law

Answer 6

When a fixed mass of gas is reduced in volume by a compressing piston, the pressure inside the piston increases

Question 7

Explain why Boyle’s law is true

Answer 7

• Gas particles all in random motion
• Pressure occurs from particles collides with the surface of a wall exerting a force
• As volume decreases
• Collisions occur more frequently
• Therefore pressure increases

Question 8

State Charles’ law

Answer 8

For a fixed mass of an ideal gas at constant pressure, its volume is directly proportional to its absolute temperature

Question 9

State the pressure law

Answer 9

For a fixed mass of an ideal gas at constant volume, its pressure is directly proportional to its absolute temperature

Question 10

Define Avogadro constant

Answer 10

The number of atoms in exactly 12g of carbon-12

Question 11

Define molar mass

Answer 11

The mass of 1 mol of the substance

Question 12

What are the five assumptions that are made for the kinetic theory equation?

Answer 12

Molecules are point molecules
No attractive forces between molecules
Molecules move in continual random motion
All collisions are elastic
Time of collisions is negligible

Question 13

Define molar heat capacity

Answer 13

The energy required to change 1 mol of gas by 1K

Question 14

Define heat capacity

Answer 14

The energy required by an entire object to change temperature by 1K

Question 15

How is work done on a gas calculated from a pressure against volume graph?

Answer 15

Area under the graph

Question 16

Explain what is meant by specific latent heat of fusion (2)

Answer 16

The energy required to change one kg of an object from liquid to solid without changing temperature

Question 17

Water is pumped througha heater in a hot tub, why is the temperature rise greater when the pump works at a higher speed? (2)

Answer 17

The pump is doing work on the water. Work can raise the temperature of the body

Question 18

Use the kinetic theory of gases to explain why the pressure inside a football increases when the temperature of the air inside it rises. Assuming volume remains constant (3)

Answer 18

Molecules have a greater rms velocity. Number of collisions between molecules and inside surface of ball increases per second. pressure = F/A and F = rate of change of momentum

Question 19

What is the equation for Work Done in a Piston with a set mass of gas in?

Answer 19

A: W = p ΔV W = Work Done p = Pressure ΔV = Change in Volume

Question 20

What is the Second Law of Thermodynamics?

Answer 20

It is not possible to convert heat continuously into work, without at the same time, transferring some heat from a warm body into a cold body. : the total entropy of a system can never decrease over time (temperature of an object always wants to be in equilibrium with surroundings).

Question 21

What is the equation for Force?

Answer 21

F = p x A F = Force p = Pressure A = Cross-Sectional Area

Question 22

What is the equation for internal energy for constant pressure processes?

Answer 22

ΔU = ΔQ – pΔV ΔU = Change in Internal Energy ΔQ = Change in amount of heat supplied to the system p = Pressure ΔV = Change in Volume

Question 23

What does the p-V graph look like for constant pressure (isobaric process)?

Answer 23

Horizontal line

Question 24

What does the area below a p-V graph show?

Answer 24

Work Done

Question 25

What are the rules for a cycle process?

Answer 25

1) For a complete cycle, ΔU = 0 (No change in internal energy)
2) the net work done = area enclosed by the cycle

Question 26

How do you change from Celsius into Kelvin?

Answer 26

Celsius Temperature + 273

Question 27

What is specific latent heat?

Answer 27

The amount of thermal energy required to change the state of 1 kg of a substance at constant temperature

Question 28

What are the three equations for an ideal gas?

Answer 28

pV = NkT pV = nRT pV = (1/3)Nmc²

Question 29

What are the equations for average kinetic energy?

Answer 29

KE = (1/2)m c² = (3/2)kT = (3/2)(RT/Nₐ)

Question 30

What is the main property of a gas at absolute zero?

Answer 30

It has minimum internal energy

Question 31

Define temperature

Answer 31

a measure of the average kinetic energy of the particles in a system

Question 32

Define heat

Answer 32

the flow of thermal energy

Question 33

Define thermal equilibrium

Answer 33

when two objects are at the same temperature so there is no flow of heat.

Question 34

What causes pressure?

Answer 34

The collisions between gas particles and the inside walls of the container

Question 35

What are the properties of an ideal gas

Answer 35

1. Molecules move with random motion, 2. Molecules obey Newton’s laws, 2. Molecules have same average kinetic energy, 3. There are no attractive forces between molecules, 4. Collisions between molecules are elastic, 5. Molecules are point masses (have zero volume), 6. Time taken between collisions is much greater than during collisions.

Question 36

Describe Brownian motion

Answer 36

particles moving randomly due to collisions with many other small, fast particles.

Question 37

State the third law of thermodynamics

Answer 37

the entropy of an ideal substance will be zero at absolute zero (0K).

Question 38

What is Avogadro’s constant?

Answer 38

The number of atoms in exactly 12g of carbon-12 (6.02x1023)

Question 39

What is the specific latent heat of fusion?

Answer 39

The energy needed to change the state a substance from solid to liquid without a change of temperature.

Question 40

What is the specific latent heat of vaporisation?

Answer 40

The energy needed to change the state a substance from liquid to gas without a change of temperature.

Question 41

Define empirical

Answer 41

Based on observation or experience rather than theory or pure fact.

Question 42

What is meant by the term elastic collisions?

Answer 42

Both KE and momentum are conserved and total E is constant before and after the collision

Question 43

Define absolute scale

Answer 43

Measured in K starting at absolute zero

Question 44

Particles exert pressure on the wall of a container because…

Answer 44

Molecules have momentum
Momentum change at wall
Momentum change at wall leads to force

Question 45

Why is average velocity in a container zero?

Answer 45

Velocity is a vector
random direction
velocities cancel

Question 46

Sublimation

Answer 46

a chemical process where a solid becomes a gas without going through a liquid stage

Question 47

What is the difference between heat and temperature?

Answer 47

Heat is the flow of thermal energy as it flows from a high temperature to a low whereas temperature is a measure of the kinetic energies of the particles in a selected substance.