Year 2 Chapter 4: Thermal Physics

Question 1

What is internal energy, U?

Answer 1

The sum of the randomly distributed kinetic and potential energies of the particles in a body

Question 2

How can internal energy, U, be increased?

Answer 2

Transferring energy to the system by doing work on it or increasing its temperature

Question 3

What is the first law of thermodynamics?

Answer 3

ΔU = ΔQ - ΔW
ΔU: increase in internal energy of system
ΔQ: thermal energy added to system
ΔW: work done by the system

Question 4

How do kinetic and potential energy change during a state change?

Answer 4

Potential energy of the system changes however kinetic energy is constant, hence no change in temp during state change

Question 5

How do kinetic and potential energy change when water is being heated to 100C and boiled?

Answer 5

Energy transferred to kinetic energy, increasing temp. Once reached 100C, energy is transferred to potential energy as bonds between water molecules are broken but NOT kinetic energy, so no increase in temp

Question 6

What is the equation for thermal energy transferred?

Answer 6

Q= mcΔθ
Q: thermal energy/ J
m: mass/ kg
c: specific heat capacity/ J kg^-1 C^-1

Question 7

What is specific heat capacity?

Answer 7

The amount of energy required to increase the temperature of 1kg of a substance by 1 Kelvin

Question 8

What is specific latent heat?

Answer 8

The amount of energy required to change the state of 1kg of a substance without a change of temperature

Question 9

What is the equation for specific latent heat?

Answer 9

Q= ml
Q: thermal energy/ J
m: mass/ kg
l: specific latent heat/ Jkg^-1

Question 10

What are the types of specific latent heat?

Answer 10

Specific latent heat of fusion (solid to liquid)
Specific latent heat of vaporisation (liquid to gas)

Question 11

What are the ideal gas laws?

Answer 11

Laws describing the experimental relationship between pressure, volume and temperature for a fixed mass of gas

Question 12

What is Boyle’s Law?

Answer 12

pV= k (constant)
When temperature is constant, pressure and volume are inversely proportional

Question 13

What is Charles’ Law?

Answer 13

V/T= k (constant)
When pressure is constant, volume is directly proportional to absolute temperature

Question 14

What is the Pressure Law?

Answer 14

p/T= k (constant)
When volume is constant, pressure is directly proportional to absolute temperature

Question 15

What is the absolute scale?

Answer 15

The Kelvin scale that starts at absolute zero.
K = C + 273
The Kelvin scale is used in all thermodynamic calculations, 1K change = 1C change

Question 16

What is absolute zero?

Answer 16

The lowest possible temperature of a system, where no heat remains and the particles in the system have no kinetic energy, volume and pressure are zero

Question 17

What is the ideal gas equation?

Answer 17

pV = nRT
p: pressure
V: volume
n: moles
R: molar gas constant
T: temp in Kelvin

Question 18

What is the ideal gas equation for a number of molecules?

Answer 18

pV = NkT
p: pressure
V: volume
N: number of molecules, n x avogadros
k: Boltzmann constant
T: temp in Kelvin

Question 19

What is molar mass?

Answer 19

The mass of one mole of a substance, for atoms or molecules is the mass number in g

Question 20

How do you find the work done on a gas when given a graph of pressure against volume?

Answer 20

Find the area under the curve

Question 21

What is Brownian motion and what did it give evidence for?

Answer 21

The random motion of particles which gave evidence for the existence of atoms and molecules

Question 22

How does the simple molecular model explain Boyle’s Law?

Answer 22

If you increase the volume of a fixed mass of gas, its molecules will move further apart so collisions will be less frequent therefore pressure decreases

Question 23

How does the simple molecular model explain Charles’ Law?

Answer 23

When the temperature of a gas is increased, its molecules gain kinetic energy meaning
they will move more quickly and because pressure is kept constant (therefore frequency of
collisions is constant) the molecules move further apart and volume is increased

Question 24

How does the simple molecular model explain the Pressure Law?

Answer 24

When the temperature of a gas is increased, its molecules gain kinetic energy meaning
they will move more quickly. As volume is constant the frequency of collisions between
molecules and their container increases and they collide at higher speeds therefore
pressure is increased

Question 25

What is the difference between the ideal gas laws and kinetic theory model?

Answer 25

The gas laws are empirical in nature, they come from observation and experimental evidence whereas the kinetic theory model only comes from theory

Question 26

What are some assumptions for the kinetic theory model?

Answer 26

Motion of molecules is random
Collisions are perfectly elastic
Negligible duration of collisions compared to time between collision
Molecules follow Newton’s Laws of motion

Question 27

What is the PMT derivation for the kinetic theory model?

Answer 27

(Box with sides l, particle mass m, velocity u)
Δp = mu - (-mu) = 2mu
time to collide with same wall again= 2l/ u
F= Δp/ Δt, 2mu/ (2l/u) = mu^2/ l
P = F/A, mu^2/ l / l^2 = mu^2 / V
Total P = m((u1)^2 + (u2)^2 … ) / V
P = Nm(u^2) / V (mean square speed)
crms^2 = 3ums^2
pV = 1/3 Nm crms^2

Question 28

What are assumptions for an ideal gas?

Answer 28

Only internal energy is kinetic energy, no potential energy
No interactions other than perfectly elastic collisions