Thermal Physics

Question 1

What is the internal energy of an object?

Answer 1

the total kinetic energy of the molecules and potential energy of the bonds added together

Question 2

How can internal energy be increased?

Answer 2

internal energy of a system is increased when energy

is transferred to it by heating or when work is done on it

Question 3

Why do molecules have kinetic energy?

Answer 3

due to their random motion

Question 4

Why do molecules have potential energy?

Answer 4

the chemical bonds holding the molecules together

Question 5

Why do gases have (almost) no potential energy?

Answer 5

their molecules are so far apart that there are no intermolecular interactions

Question 6

Define temperature.

Answer 6

the measure of the average kinetic energy of all particles within the substance.

Question 7

What is temperature measured in in Thermal Physics?

Answer 7

Kelvin

Question 8

What does it mean to be in thermal equilibrium?

Answer 8

two objects are the same temp so no heat energy transfer between them

Question 9

What is heat?

Answer 9

the type of energy which flows when there is a difference in temperature.

Question 10

How does heat affect internal energy?

Answer 10

the more heat energy, the greater the internal energy

Question 11

How does heat travel?

Answer 11

hot to cold

Question 12

How do you convert from celsius to kelvin?

Answer 12

+ 273

Question 13

How does the amount of kinetic energy change when an object changes state?

Answer 13

KE doesnt change

Question 14

How does the amount of potential energy between the molecules change when an object changes state?

Answer 14

solid to liquid or liquid to gas = less potential energy

gas to liquid or liquid to solid = more potential energy

Question 15

How do you calculate the energy change when an objects temperature change?

Answer 15

Q = mcΔθ

where c is the specific heat capacity

Question 16

How do you calculate the energy change when an object changes state?

Answer 16

Q=ml

where l is the specific latent heat

Question 17

What is meant by specific heat capacity?

Answer 17

heat energy required to raise the temperature of the unit mass of a given substance by a given amount. every substance has its own unique specific heat capacity

Question 18

What is specific latent heat of fusion?

Answer 18

heat needed to change a mass of 1 kg the substance from a solid at its melting point into liquid at the same temperature.

Question 19

What is specific latent heat of vaporisation?

Answer 19

heat needed to change a mass of 1 kg the substance from a liquid at its boiling point into a gas at the same temperature

Question 20

What must always be mentioned when referring to temperature?

Answer 20

Kinetic energy of molecules

Question 21

What must always be mentioned when referring to bonds?

Answer 21

intermolecular forces of attraction

Question 22

What is Boyle’s gas law?

Answer 22

P ∝ 1/V :

pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature

Question 23

What is Charles’ gas law?

Answer 23

V ∝ T :

volume is directly proportional to temperature for a fixed mass of gas at a constant pressure

Question 24

What is Avogadro’s law?

Answer 24

P ∝ T :

pressure is directly proportional to temperature for a fixed mass of gas with constant volume

Question 25

What is the Ideal Gas Equation?

Answer 25

PV = nRT

or PV = NkT

Question 26

What assumptions must be made when deriving the kinetic theory equation?

Answer 26

any gas contains a large enough sample of particles for statistics to be used
motion is rapid and random
collisions are perfectly elastic (no loss of energy)
there are no attractive forces between particles
repulsive forces only act during collisions
volume of particles is negligible

Question 27

Derive the kinetic theory equation.

Answer 27

1. calculate the change in momentum:
ΔP = -2mu
2. Calculate force on particle:
i. Time between collisions: 
t = s/u
t = 2l/u
ii. Calculate number of collisions per second (freq):
f = 1/t
f  =u/2l
iii. calculate the rate of change of momentum
ΔP/ t = ΔP x f
ΔP/ t = -m(u^2) /  l  
iv. use Newton II:
F = ΔP / Δt
F on particle from wall = -m(u^2) / l
v. use Newton III:
F on particle = - F on wall
F on wall from particle = m(u^2) / l
3. Consider N particles in box:
F on wall = F1 + F2 + ... + Fn
F on wall = m(u^2) / l + m(u^2) / l + ... + m(u^2) / l
F on wall = m/l ( u1^2 + u2^2 + ... + un^2)
4. Use mean of squares
F on wall = mN(mean u^2) / l
5. convert force to gas pressure:
P = F/A
P = mN(mean u^2) / lA
P = mN(mean u^2) / V
PV = mN(mean u^2)

Question 28

How do you calculate the kinetic energy of gas molecule?

Answer 28

Ek = 3/2 x k x T

Question 29

How do you calculate work done?

Answer 29

work = PΔV

Question 30

How do you convert between moles and molecules?

Answer 30

N = n x avogadros constant

Question 31

What is brownian motion?

Answer 31

the random motion of gas molecules caused by collisions between molecules

Question 32

What is brownian motion evidence for?

Answer 32

the existence of atoms

Question 33

What is the difference in validity of the gas laws and the kinetic theory model?

Answer 33

gas laws are empirical by nature whereas kinetic theory is simply a theory