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 motion
Pressure exists from surface collisions exerting a force
Volume decreases
Collisions occur more frequently
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 heat capacity

Answer 13

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

Question 14

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

Answer 14

Area under the graph

Question 15

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

Answer 15

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

Question 16

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 16

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

Question 17

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 17

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 18

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

Answer 18

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

Question 19

What is the equation for Force?

Answer 19

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

Question 20

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

Answer 20

Horizontal line

Question 21

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

Answer 21

Work Done

Question 22

How do you change from Celsius into Kelvin?

Answer 22

Celsius Temperature + 273

Question 23

What is specific latent heat?

Answer 23

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

Question 24

What are the three equations for an ideal gas?

Answer 24

pV = NkT pV = nRT pV = (1/3)Nmc²

Question 25

What are the equations for average kinetic energy?

Answer 25

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

Question 26

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

Answer 26

It has minimum internal energy

Question 27

Define temperature

Answer 27

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

Question 28

Define heat

Answer 28

the flow of thermal energy

Question 29

Define thermal equilibrium

Answer 29

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

Question 30

What causes pressure?

Answer 30

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

Question 31

What are the properties of an ideal gas

Answer 31

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 32

Describe Brownian motion

Answer 32

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

Question 33

What is Avogadro’s constant?

Answer 33

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

Question 34

What is the specific latent heat of fusion?

Answer 34

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

Question 35

What is the specific latent heat of vaporisation?

Answer 35

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

Question 36

Define empirical

Answer 36

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

Question 37

What is meant by the term elastic collisions?

Answer 37

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

Question 38

Define absolute scale

Answer 38

Measured in K starting at absolute zero

Question 39

Particles exert pressure on the wall of a container because…

Answer 39

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

Question 40

Why is average velocity in a container zero?

Answer 40

Velocity is a vector
random direction
velocities cancel

Question 41

Sublimation

Answer 41

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

Question 42

What is the difference between heat and temperature?

Answer 42

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.