Section 8 - Thermal physics

Question 1

What is the internal energy?

Answer 1

In a body it is the sum of all the randomly distributed kinetic and potential energies of all the particles in a body

Question 2

How can you change the internal energy?

Answer 2

Increased - by heating it or doing work to transfer energy to the system
Decreased - by cooling it or by doing work to remove energy from the system

Question 3

What is specific heat capacity?

Answer 3

The amount of energy required to raise the temperature of 1kg of a substance by 1K without a change in state

Question 4

What is specific latent heat?

Answer 4

The quantity of thermal energy required to change the state of 1kg of a substance, without changing the temperature

Question 5

What is Boyle’s law?

Answer 5

pV=constant
* At a constant temperature and with a fixed mass, the pressure and volume of a gas are inversely proportional
* A (theoretical) gas that obeys Boyle’s law at all temperatures is called an ideal gas

Question 6

What is Charles’s Law?

Answer 6

v/t = constant
* At a constant pressure and with a fixed mass, the volume of a gas is directly proportional to its absolute temperature

Question 7

What is The Pressure Law?

Answer 7

p/T = constant
* At a constant volume and with a fixed mass, the pressure of a gas is directly proportional to its absolute temperature

Question 8

What is the experiment to investigate Boyle’s Law?

Answer 8

1. The oil traps a pocket of air in a sealed tube with fixed dimensions
2. A tire pump is used to increase the pressure on the oil and the Bourdon gauge records the pressure
3. As the pressure increases, the oil level will rise, and the air will compress - the volume occupied by air in the tube will reduce
4. Measure the volume of air when the system is at atmospheric pressure, then gradually increase the pressure, keeping the temperature constant
5. Write down the pressure and the volume of air as it changes
6. Multiplyng these together should result in the same value
7. If you plot a graph of p against 1/V you should get a straight line

Question 9

What is the experiment to investigate Charles’s Law?

Answer 9

1. A capillary tube is sealed at the bottom and contains a drop of conc H2SO4 acid halfway up the tube which traps a column of air between the bottom of the tube and the acid drop
2. The beaker is filled with near-boiling water, and the length of the trapped column of air increases - as the water cools, the length of the air column decreases
3. Regularly record the temperature of the water and the air column length as the water cools
4. Repeat with fresh near-boiling water teice more,m letting the tube adjust to the new temperature between each repeat - recorf the length at the same temperatures each time and take an average
5. If you plot your average results on a graph of length against temperature and draw a line of best fit, you will get a straight line - shows the length of the air column is proportional to temperature
6. The colume of the column of air is equal to the volume of a cylinder, whch is proportional to its length

Question 10

What is molecular mass?

Answer 10

Sum of all the masses of all the atoms that make up a molecule

Question 11

How do you calculate the number of molecules?

Answer 11

N=nNA
* Where n is the number of moles, N the number of molecules and NA avogadro’s constant

Question 12

When does the ideal gas equation work well?

Answer 12

For gases at low pressures and fairly high temperatures

Question 13

What must be done for a gas to expand or contract at constant pressure?

Answer 13

Work must be done
* This usually involves the transfer of heat energy
* The work energy in changing the volume of a gas at constant pressure is given by work done = pΔV

Question 14

Draw the derivation for the pressure on one wall of a box in the x direction

Question 15

Draw the derivation for the general equation of the pressure of a gas

Question 16

What is the root mean square speed or Crms

Answer 16

• c̄² is the means square speed which is the average of the square speeds of all the particles with units m²s-²
• so the sqare root of it gives you the average speed with units ms^-1

Question 17

What are the assumptions for the kinetic theory?

Answer 17

R - Random motion of particles
A - Attraction between particles = none
V - Volume of the molecules is negligible
E - Elastic collisions
D - Duration of collisions is negligible

Question 18

What is an ideal gas?

Answer 18

Obeys Boyle’s law with perfect precision

Question 19

What is a perfect gas?

Answer 19

A real gas under conditions that Boyle’s law is a valid enough description of its behaviour

Question 20

What are empirical laws?

Answer 20

Based on observations and evidence which means they can predict what will happen but they don’t explain why - gas laws and ideal gas equation

Question 21

What are theories?

Answer 21

A theory is based on assumptions and derivations from knowledge and theories we already had - kinetic theory

Question 22

How has our understanding of gases developed?

Answer 22

1. Democritus had ideas 2000 years ago
2. Robert Boyle discovered the relationship between pressure and volume at a constant temperature
3. Jacques Charles discovered that the volume of a gas is proportional to temperature at a constant pressure
4. The pressure law was discovered by Guillaume Amontons who noticed that at a constant volume, temperature is proportional to pressure, it was then re-discovered by Joseph Louis Gay-Lussac
5. Daniel Bernoulli explained Boyle’s law by assuming that gases were made up of tiny particels - the beginning of kinetic theory
6. Robert Brown discovered Brrownian motion which helped support kinetic theory

Question 23

What is Brownian motion?

Answer 23

1. Botanist Robert Brown noticed that pollen grains in water moved with a zigzag random motion
2. This supports the kinetic particle theory as it states that the random motion is a result of collisions with fast, randomly-moving particles in the fluid

Question 24

When you need to find the temperature of a gas and you’re given the Crms, which set of equations do you use?

Answer 24

Kinetic theory equations

Question 25

How do you convert from kgmol^-1 to kg?

Answer 25

Divide by avogadro's constant

Question 26

By the kinetic theory, why does the pressure decrease when gas particles leave?

Answer 26

1. Fewer atoms means fewer collisions per second 2. Therefore the total momentum change per second drops 3. So pressure falls

Question 27

When calculating the moles of gas that has escaped what should you do?

Answer 27

Initial pressure - remaining pressure and then pV=nRT

Question 28

If temperature is constant, what does that mean for the velocity, even if volume or another factor is changed?

Answer 28

1. Ek=3/2kt 2. Therefore Ek is constant 3. So velocity/Crms is constant

Question 29

When the temperature of a gas is increased, but there's no change in state, how does the motion of the atoms change?

Answer 29

Mean kinetic energy increases therefore mean speed increases

Question 30

How do you get Crms?

Answer 30

1. Square all the speeds 2. Divide by the amount of speeds 3. Square root after

Question 31

How do you find the work done by a gas when expanding due to a change in state?

Answer 31

W=pΔV

Question 32

How does the kinetic theory interpret absolute zero temperature?

Answer 32

Temperature at which the random motion stops

Question 33

How do the gas laws interpret absolute zero temperature?

Answer 33

The temperature at which a volume or pressure agaisnt temperature graph extrapolates to 0

Question 34

In terms of the kinetic theory, why is there a pressure exerted by a gas?

Answer 34

1. The particles collide with the walls of the piston and change momentum 2. Force = rate of change of momentum exerted on the piston by the gas 3. Pressure = force/area

Question 35

When given a graph of temp in celsius and pressure, how do you find the gradient?

Answer 35

Change in y divided by 273 + temp on x axis

Question 36

Why when the thing the particles are in is moving faster, is the rise in termperature greater?

Answer 36

1. The container/whatever is doing work on the water 2. Work is converted to thermal energy

Question 37

What is work done on a pressure volume graph?

Answer 37

Area under the graph