Thermal Physics

Question 1

What does it mean when 2 objects are in thermal equilibrium ?

Answer 1

there is no NET flow of thermal energy between them and
they have the same temperature

this does not imply they must have the same internal energy as internal energy depends also on the number of molecules in the 2 bodies, which is unknown

Question 2

Absolute zero means ?

Answer 2

Temperature at which all substances have a minimum internal energy {NOT: zero internal energy}

K = degree C + 273.15

0K, -273.15 degree C

Question 3

Thermodynamic (Kelvin) scale / absolute scale of temperature is ?

Answer 3

is the theoretical scale that is independent of the properties of any particular substance

Question 4

Specific latent heat of fusion and vaporisation ?

Answer 4

energy per unit mass required to change a substance from solid to liquid without a change in temperature
(CHANGE IN STATE)
from liquid to gas

Question 5

Specific latent heat capacity

Answer 5

energy per unit mass required to raise the temperature of the substance by one unit temperature WITHOUT A CHANGE OF STATE

Question 6

microscopic view of energy

what is U

Answer 6

internal energy ( random motion and attractive forces ) = kinetic energy due to random movement of molecules + potential energy among molecules due to intermolecular forces of attraction between them

Question 7

The mole

Answer 7

amount of substance that contains the same number of particles as the number of atoms in 12g of carbon-12.

Question 8

properties of ideal gas

Answer 8

refer to notes

Question 9

“Internal energy is determined by the state of the system”. Explain what this means.

Answer 9

Internal energy is determined by the values of N & T of the current state, ie it is independent of the path taken to reach its current state

Question 10

A The internal energy of a system can be increased without transfer of energy by heating.
B The internal energy of a system depends only on its temperature.
C When the internal energy of a system is increased, its temperature always rises.
D When 2 systems have the same internal energy, they must be at the same temperature.

Answer 10

option A

Question 11

First Law of thermodynamics

Answer 11

The INCREASE in internal energy of a system is equal to the sum of the heat supplied to the system and the work done on the system.

Question 12

Specific latent heat of vaporisation > specific latent heat of fusion for a given substance because:

Answer 12

During vaporisation, there is a greater increase in volume than in fusion;
 thus more work is done against atmospheric pressure during vaporisation.
 The greater increase in volume also means the increase in the (molecular) potential energy and a large amount of energy is also required to break the intermolecular bonds in liquid to separate the molecules far apart in vapour state., & hence, internal energy, during vaporisation > that during melting.
 Hence by 1st Law of Thermodynamics, heat supplied during vaporisation > that during melting;
hence lv > lf {since Q = ml = Change in U - (-W)

Question 13

by applying the first law of thermodynamics how is it possible for a gas to be supplied wth heat and its temperature not to rise

Answer 13

The fact that temperature remains constant shows that there is no increase in internal energy, since internal energy is directly proportional to thermodynamic temperature.

By 1st Law of Thermodynamics,
ΔU = Q + Won = 0 J
Since Q = - work done on the gas = +work done by the gas

The heat supplied to the gas is all used to do work against the atmosphere when the gas expands.

Question 14

what is an ideal gas

Answer 14

An ideal gas is a gas that obeys the equation of state (PV = nRT) at all values of volume, pressure and temperature.