Thermal Physics

Question 1

State the 9 assumptions used in the kinetic theory of gases.

Answer 1

Particles move in random directions. Except for during collisions, particles move in straight lines.
The separation between molecules is much greater than the size of the molecules.
The molecules do not exert a force on the walls of the container during the collision.
All collisions are elastic.
All the molecules obey the laws of motion.
If a gas sample is left for a sufficient time, it eventually comes to a steady state.
The density of molecules and the distribution of molecules are independent of position, distance, and time.
Any forces that act during collisions last for much less time than the time between collisions.

Question 2

What is internal energy?

Answer 2

The sum of the randomly distributed kinetic and potential energies of all the particles in a body.

Question 3

What two things can internal energy be changed by?

Answer 3

Changing temperature or doing work.

Question 4

When a substance changes state, what changes and what stays the same?

Answer 4

Internal energy changes but kinetic energy (and temperature) stays the same.

Question 5

What is specific heat capacity?

Answer 5

The amount of energy needed to raise the temperature of 1kg of the substance by 1K.

Question 6

What is specific heat capacity measured using?

Answer 6

A continuous-flow calorimeter.

Question 7

What is specific latent heat of fusion or vaporisation?

Answer 7

The quantity of thermal energy required to change the state of 1kg of a substance.

Question 8

What is 0 degrees Celsius in Kelvin?

Answer 8

273K

Question 9

What is Boyle’s law?

Answer 9

pV = constant. At a constant temperature the pressure p and volume V of a gas are inversely proportional.

Question 10

What is Charles’s law?

Answer 10

V/T = constant. At constant pressure, the volume V of a gas is directly proportional to its absolute temperature T.

Question 11

What is the pressure law?

Answer 11

p/T = constant. At constant volume m, the pressure p of a gas is directly proportional to its absolute temperature T.

Question 12

State an experiment to investigate Boyle’s law. (The one we didn’t do).

Answer 12

Use a pump and a pump connected to a tube of oil and air.

Question 13

State an experiment to investigate Charles’s law.

Answer 13

Using a thermometer, capillary tube and ruler in a container of water where the capillary tube contains a drop of sulphuric acid trapping an air column beneath it.

Question 14

What is molecular mass?

Answer 14

The mass of one molecule (the sum of the masses of all the atoms making up the molecule).

Question 15

What is molar mass? What is it’s unit?

Answer 15

The mass of one mole of a substance. Measured in grams, it is equal to the relative molecular mass of that substance. e.g. Helium has a relative mass of 4 so its molar mass is 4g and the molar mass of an oxygen molecule, which consists of 2 oxygen atoms, is 16g + 16g = 32g.

Question 16

What is the equation for number of moles?

Answer 16

n = N/N_a where n is number of moles, N is the number of molecules and N_a is Avogadro’s constant.

Question 17

What is the ideal gas equation?

Answer 17

pV/T = constant

Question 18

What is the equation for work done in changing the volume of a gas at constant pressure?

Answer 18

W = pΔV

Question 19

What is the potential energy of an ideal gas?

Answer 19

0J as there are no forces between them except when they are colliding.

Question 20

When do real gases behave like ideal gases?

Answer 20

When the pressure isn’t too big and the temperature is reasonably high compared with their boiling point.

Question 21

What is Brownian motion?

Answer 21

When particles in a fluid move in a zigzag, random way.

Question 22

What causes Brownian motion?

Answer 22

Collisions with fast, randomly-moving particles in the fluid.

Question 23

State an experiment to investigate Boyle’s law. (The one we did).

Answer 23

Hanging masses from a syringe filled with air and adding more masses and measuring how the volume inside the syringe changes.

Question 24

State the steps in the method of the Boyle’s law experiment with the syringe.

Answer 24

Measure the diameter of the syringe using a vernier caliper, draw in 4ml of air and record this, fit rubber tubing over the nozzle and clamp it with the pinch clip as close to the nozzle as possible, put the syringe in a clamp and hang a 100g mass from it with a 2kg counterweight places on the bottom of the clamp, gently move the plunger up and down to ensure it is not sticking and release it, record the new volume and add more mass then repeat until there are measurements up to 1000g, repeat the experiment twice more an find the mean of the volumes for each mass.

Question 25

What must be done when calculating pressure inside the syringe in the Boyle’s law practical?

Answer 25

Find the pressure using F/A then subtract standard atmospheric pressure, 101kPa.

Question 26

State two safety precautions in the Boyle’s law syringe experiment.

Answer 26

Using a counterweight to prevent the clamp from falling over and placing a sand pit on the floor to catch the masses and prevent them bouncing if they fall.

Question 27

State two improvements which could be made to the Boyle’s law syringe experiment.

Answer 27

Make the clamp taller so that it doesn’t distort the syringe barrel or make it more difficult for the plunger to move freely.
Lubricate the syringe to prevent the plunger from sticking.

Question 28

State the steps in the method of the Charles’s law experiment.

Answer 28

Fill a capillary tube with air at the bottom, a drop of concentrated sulfuric acid in the middle and air at the top. Tie the capillary tube to a 30cm ruler using elastic bands and place it into a beaker along with a thermometer. Add hot water from the kettle to the beaker and then stir the water with the thermometer. Read and record the temperature and length of the air sample. Record readings every 5 degrees down until room temperature.

Question 29

State two safety precautions when performing the Charles’s law experiment.

Answer 29

Wear safety goggles to avoid sulfuric acid getting in the eyes as this can cause damage to them, be careful not to spill any boiling water as that can cause burns.

Question 30

What precaution must be taken in the Charles’s law experiment with the capillary tube to ensure the results will be accurate?

Answer 30

The tube must be perfectly clean with no traces of other chemicals to prevent the thread of sulfuric acid from splitting.