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Physics A-level Paper 2 > Thermal Physics > Flashcards

Flashcards in Thermal Physics Deck (33):
1

What is the internal energy of an object?

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

2

How can internal energy be increased?

internal energy of a system is increased when energy
is transferred to it by heating or when work is done on it

3

Why do molecules have kinetic energy?

due to their random motion

4

Why do molecules have potential energy?

the chemical bonds holding the molecules together

5

Why do gases have (almost) no potential energy?

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

6

Define temperature.

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

7

What is temperature measured in in Thermal Physics?

Kelvin

8

What does it mean to be in thermal equilibrium?

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

9

What is heat?

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

10

How does heat affect internal energy?

the more heat energy, the greater the internal energy

11

How does heat travel?

hot to cold

12

How do you convert from celsius to kelvin?

+ 273

13

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

KE doesnt change

14

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

solid to liquid or liquid to gas = less potential energy
gas to liquid or liquid to solid = more potential energy

15

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

Q = mcΔθ
where c is the specific heat capacity

16

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

Q=ml
where l is the specific latent heat

17

What is meant by specific heat capacity?

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

18

What is specific latent heat of fusion?

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

19

What is specific latent heat of vaporisation?

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

20

What must always be mentioned when referring to temperature?

Kinetic energy of molecules

21

What must always be mentioned when referring to bonds?

intermolecular forces of attraction

22

What is Boyle's gas law?

P ∝ 1/V :
pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature

23

What is Charles' gas law?

V ∝ T :
volume is directly proportional to temperature for a fixed mass of gas at a constant pressure

24

What is Avogadro's law?

P ∝ T :
pressure is directly proportional to temperature for a fixed mass of gas with constant volume

25

What is the Ideal Gas Equation?

PV = nRT
or PV = NkT

26

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

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

27

Derive the kinetic theory equation.

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)

28

How do you calculate the kinetic energy of gas molecule?

Ek = 3/2 x k x T

29

How do you calculate work done?

work = PΔV

30

How do you convert between moles and molecules?

N = n x avogadros constant

31

What is brownian motion?

the random motion of gas molecules caused by collisions between molecules

32

What is brownian motion evidence for?

the existence of atoms

33

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

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