Thermal Physics

Question 1

Define Heat

Answer 1

The energy transferred between two objects because of the temperature difference between them.

Question 2

Two objects are in thermal equilibrium if

Answer 2

No net flow of heat in thermal contact

At the same temp

Question 3

Define absolute zero

Answer 3

Zero point on thermodynamic temperature scale in kelvin

Question 4

Thermometric properties are

Answer 4

The properties of a thermometer that allows it to measure temperature

Question 5

Triple point of water

Answer 5

273.16K/0.01C

Question 6

Define ideal gas

Answer 6

An ideal gas is a gas which obeys the equation pV = nRT, where
p is the pressure of the gas,
V is the volume of the gas,
T is the thermodynamic temperature of the gas,
and N is the amount of gas in moles.

Question 7

Pressure law

Answer 7

Pressure is proportionate to volume

Question 8

Charles law

Answer 8

Volume is proportionate to temperature

Question 9

Boyle’s law

Answer 9

Pressure is inversely proportional to temperature

Question 10

6 assumptions of the kinetic theory of gases

Answer 10

1. Any gas is made up of a large no. of particles
2. Constant and random motion
3. Perfectly elastic collisions between particles and wall
4. Vol is negligible, any 2 particles are far apart, low chance of collision
5. Force between particles is negligible except during time of collision
6. Duration of collision is negligible

Question 11

Define Heat capacity

Answer 11

Quantity of heat required to raise the body temp by 1 degree

Question 12

Define specific heat capacity

Answer 12

Quantity of heat required to raise the temp of a unit mass of the substance by 1 kelvin

Question 13

Formula for specific heat capacity

Answer 13

Q = mc(change in temp)

Question 14

Specific latent heat of fusion

Answer 14

Quantity of heat required to convert a unit mass of solid to liquid without any change in temperature

Question 15

Specific latent heat of vaporization

Answer 15

Quantity of heat required to convert a unit mass of liquid to gas without any change in temperature

Question 16

Define one kelvin

Answer 16

1/273.16 of the thermodynamics temp of the triple point of water

Question 17

Internal energy of a system is

Answer 17

The sum of the random distribution of microscopic kinetic energy and microscopic potential energy of all the atoms on molecules

Question 18

The first law of thermodynamics states that

Answer 18

The INCREASE in internal energy of a system is equal to the sum of the thermal energy added to it and the work done to the system.

Change in U = Q + WD

Question 19

Isothermal

Answer 19

Temp is constant. ∆U = 0, Q = -W

Question 20

Isochloric/Isovolumetric

Answer 20

Volume is constant. ∆U = Q, W = 0

Question 21

Isobaric

Answer 21

Pressure is constant. W = -p∆V, ∆U = area under graph

Question 22

Adiabatic

Answer 22

No heat gain or loss throughout. Q = 0, ∆U = W

Question 23

Cyclic

Answer 23

No net change of thermal energy. ∆U = 0

Question 24

What affects a change in microscopic kinetic energy?

Answer 24

Temperature

Question 25

What affects a change in microscopic potential energy?

Answer 25

State of body

Question 26

By reference to the energy of molecules, explain why the internal energy of an ideal gas is proportional to its thermodynamic temperature.

Answer 26

internal energy definition

For an ideal gas, the microscopic PE of the molecules is negligible because there are no intermolecular forces,

hence internal energy of an ideal gas is solely due to the microscopic KE of the gas molecules.

Since microscopic KE is proportional to thermodynamic temperature, internal energy of an ideal gas would be proportional to its thermodynamic temperature.

Question 27

Why is the specific heat capacity larger when heating takes place at constant pressure rather than at constant volume?

Answer 27

To raise the temperature of a unit mass of neon gas by 1K, the increase in internal energy Δ *U would be the same.

By the 1st law of thermodynamics ΔU = Q + W , for a constant pressure process, work is done by the gas as it expands during heating, so thermal energy required will be the sum of the increase in internal energy and work done by the gas (Q = Δ*U + W)

For a constant volume process, no work is done so thermal energy required is only to increase internal energy (Q = Δ*U)

Since for constant pressure process, more thermal energy is required for the same rise in temperature ΔT , by Q = mcΔT, c would be larger.

Question 28

Derive kinetic theory equation

Answer 28

Δp = -mcx - mcx = -2mcx

f = cx/2L

Δp/Δt = cx/2L(-2mcx) = -mcx²/L

Force exerted by wall on particle = -mcx²/L

Force exerted by particle on wall = mcx²/L

(mcx²/L)/L² = mcx²/L³ = mN<cx>²/L³ = mN/L³ (⅓c) = ⅓(Nm/V)<cx>² _</cx></cx>

PV = ⅓(Nm)<cx>² _</cx>

Question 29

Derive KE of an ideal gas particle

Answer 29

⅓(Nm)<c>² = NkT
(Nm)<c>² = 3NkT
½m<c>² = 3/2 NkT = U_</c></c></c>

Question 30

Define the internal energy of an ideal gas.

Answer 30

The sum of the random microscopic kinetic energy of all the gas molecules only, since there is no microscopic potential energy for an ideal gas because there are no intermolecular forces of attraction.