Thermal Physics

Question 1

When does energy transfer between objects take place?

Answer 1

If one object does work on another object or if one object is hotter than another object.

Question 2

How is the internal energy of an object increased?

Answer 2

Energy transfer by heating or work done on the object.

Question 3

What does the first law of thermodynamics state?

Answer 3

Change in internal energy = total energy transfer due to heating and work done.

Question 4

Define internal energy:

Answer 4

The sum of the random distribution of the kinetic and potential energies of its molecules.

Question 5

Define thermal equilibrium:

Answer 5

when the temperature of 2 objects is equal so no energy transfer takes place.

Question 6

What is absolute zero?

Answer 6

0K or -273 degrees Celsius. The object has minimum internal energy.

Question 7

What does temp. rise of an object depend on when it is heated?

Answer 7

The mass, amount of energy supplied and the substance(s) the object is made of.

Question 8

What is specific heat capacity?

Answer 8

Amount of energy required to raise a unit mass by 1K. JKg(-1)K(-1)

Question 9

SHC equation?

Answer 9

Q =MC (change in temp)

Question 10

What experiment can be used to measure SHC?

Answer 10

Inversion tube experiment. Equate gpe to internal energy when it hits the bottom of the tube. Often used with lead. Can also be done by electrically heating a insulated container and equating energy in with rise in temperature recorded by thermometer. Can be done for liquid but must also account for container that the liquid is in.

Question 11

Give equation for continuous flow heating electrically

Answer 11

IV = mc (change in temp/t)

Question 12

Which process requires more energy, Vaporisation or melting?

Answer 12

Vaporisation.

Question 13

Define specific latent heat:

Answer 13

The energy needed to change the state of 1kg of a substance without a change of temperature. Q = mL The unit is J Kg (-1)

Question 14

Equation for time taken to melt a substance:

Answer 14

t = mL/P

Question 15

What are the 2 units of pressure?

Answer 15

Pa and N m^(-2)

Question 16

What angle does pressure act at?

Answer 16

90 degrees to the surface.

Question 17

What is Boyles Law?

Answer 17

pV = constant. Pressure is inversely proportional to volume.

Question 18

Why does Boyles Law not work at high pressures?

Answer 18

The particles are so close together that their volume becomes significant which doesn’t obey ideal gas laws.

Question 19

What is Charles’ Law?

Answer 19

Volume is directly proportional to temperature. V/T = constant.

Question 20

Equation involving work done, pressure and volume?

Answer 20

W = p * change in volume. This only works when pressure is constant.

Question 21

What is the pressure law?

Answer 21

p/T = constant

Question 22

What is Brownian Motion?

Answer 22

Random and unpredictable motion of particles due to collisions with other particles which causes a force into a random direction.

Question 23

How was Brownian Motion discovered?

Answer 23

Smoke particles were observed under light.

Question 24

Define the Avogadro constant:

Answer 24

The amount of molecules in 1 mole of a substance or the number of atoms in exactly 12g of Carbon 12.

Question 25

Define molar mass and its units.

Answer 25

The mass of 1 mol of a substance in kgmol^(-1)

Question 26

Give the equation relating molar mass, no. of moles and mass.

Answer 26

n = mass/molar mass.

Question 27

Give the equation relating number of molecules in a substance:

Answer 27

no. of molecules = Na*mass/molar mass

Question 28

Define ideal gas:

Answer 28

A gas that obeys Boyle’s law.

Question 29

What is combined ideal gas law?

Answer 29

PV/T =constant. This is only to be used when all 3 are changing.

Question 30

What is true about ideal gases with equal volumes and equal temperatures?

Answer 30

They have the same number of moles.

Question 31

What are the units of molar gas constant R?

Answer 31

Jmol^(-1)K^(-1)

Question 32

What is the ideal gas equation?

Answer 32

pV = nRT

Question 33

Why does pressure increase when reducing temperature?

Answer 33

There is less distance to travel between collisions so there is more force exerted per second.

Question 34

How to take root mean square?

Answer 34

square all values, take the mean and then square root the value.

Question 35

Name 5 assumptions of kinetic gas theory:

Answer 35

1) Volume of molecules is negligible to volume of the gas.
2) There are no forces of attraction.
3) They have random motion.
4) The collisions are all elastic.
5) Time of collision is negligible compared to time between collisions.

Question 36

Summarise part 1 of kinetic theory derivation:

Answer 36

Change in momentum due to impact is -mv - mv (changes direction hence negative) = -2mv
Time between successive collisions = 2l/v (2l because it travels from one wall then bounces back).
Therefore, F= -2mv/ 2l/v = -mv^(2)/l

Pressure = force/area = mv^2/l/l^2 = mv^2/l^3 = mv^2/V

Question 37

Summarise part 2 of kinetic theory derivation:

Answer 37

Sum all the pressures together from part 1 and take out factor of m/v so it becomes Nmv^2/V where N is the number of molecules. Since 1/3 particles can be assumed to move in each direction, u, v and w, p = Nm/3V(u^2 + w^2 + v^2) therefore p = Nm/3V(Crms)^2 so pV = 1/3 Nm(Crms)^2

Question 38

What is the internal energy of an ideal gas equal to?

Answer 38

The kinetic energy.

Question 39

Why are the gas laws empirical?

Answer 39

They were devised by experiments and observations rather than theory that the kinetic energy formula is derived from.