thermal physics

Question 1

definition of specific heat capacity

Answer 1

energy required to raise the temperature of 1kg of a substance by 1 Kelvin

Question 2

units of specific heat capacity

Answer 2

J kg-1 K-1

Question 3

experiment to calculate specific heat capacity

Answer 3

Continuous flow calorimeter:
1)Set up the experiment with a heating element heating the water and let the water flow at a steady rate until the water out is at a constant temperature.
2)Record the flow rate of the water and duration of the experiment to find the mass of the water as well as the temperature difference (∆θ) between the water in and water out.
3)Record the current and p.d to find the energy transferred to the water (E=IVt+H where H is the heat lost to the surroundings.
4)Repeat the experiment only changing the p.d of the power supply and flow rate (mass) so ∆θ remains constant. You should now have an equation for each experiment.
5)Rearrange equations to get
c=Q₂-Q₁ / (m₂-m₁)∆θ as the H’s cancel.
6) Use Q=IVt to find Q₁ and Q₂ to find the specific heat capacity.

Question 4

specific latent heat

Answer 4

the energy required to change the state of 1 kg of a substance without raising its temperature

Question 5

what happens during a change of state

Answer 5

• there is no change in temperature
• kinetic energies of particles are constant
• the potential energies of particles are changing

Question 6

definition of specific latent heat of vapourisation

Answer 6

the energy required to change the state of 1 kg of a substance from liquid to gas without changing its temperature

Question 7

definition of specific latent heat of fusion

Answer 7

energy required to change the state of 1 kg of substance from solid to liquid without changing its temperature

Question 8

Units of specific latent heat

Answer 8

Jkg-1

Question 9

definition of absolute zero

Answer 9

temperature at which there is zero kinetic energy per particle

Question 10

absolute zero in degrees celsius

Answer 10

-273 degrees

Question 11

converting between Kelvin and Celsius temperature scales

Answer 11

Kelvin to Celsius: subtract 273
Celsius to Kelvin: add 273

Question 12

pressure volume graph

Answer 12

• pressure is proportional to 1/volume so an increased temperature or mass of gas causes the curve to move right and up.
• use pV=nRT to determine changes to graphs

Question 13

pressure-temperature graph

Answer 13

• temperature is in Kelvin
• pressure is proportional to T
• decreasing the volume or increasing the mass of gas caused a steeper gradient
• use pV=nRT to determine changes to graphs

Question 14

volume-temperature graph

Answer 14

• temperature is in Kelvin
• V is proportional to T
• decreasing the pressure or increasing the mass of gas caused a steeper gradient
• use pV=nRT to determine changes to graphs

Question 15

why might energy put into a material not equal the specific heat capacity calculated

Answer 15

energy lost to surroundings means specific heat capacity may not be constant over the temperature range used

Question 16

define terms N, m and crms in pV = ⅓Nm(crms2).

Answer 16

N - number of particles(molecules)
m - mass of individual particles (molecules)
crms - square root of mean square speed

Question 17

average kinetic energy per particle of a gas at temperature

Answer 17

=3/2kT = 1/2m(Crms²)
k - boltzman constant - 1.38x10^-23

Question 18

Total kinetic energy of gas at temperature T

Answer 18

= N x 1.5kT = N x 0.5mCrms². Where N is the number of particles/molecules in gas
k - boltzman constant - 1.38x10^-23

Question 19

assumptions of ideal gases

Answer 19

• All molecules/atoms are identical
  -Molecules/atoms are in random motion = range of speeds, no preferred direction.
• Gas contains a large number of molecules
• The volume of gas molecules is negligible compared to the volume occupied by the gas
• No forces act between molecules except during collisions - Collisions are elastic
• Collisions are of negligible duration compared to the time between collisions

Question 20

what is meant by random motion in context of particles

Answer 20

Particles have no preferred direction, With a range of speeds

Question 21

what is meant by all collisions being elastic

Answer 21

a collision in which momentum and kinetic energy are conserved

Question 22

Why is the average velocity of particles in a container zero?

Answer 22

• Velocity is a vector
• Random velocity means no preferred direction of movement so velocities cancel

Question 23

by considering motion of particles, explain how they exert pressure on the wall of a container

Answer 23

• molecules/particles have momentum
• momentum changes at wall due to collisions lead to a force being exerted on the wall as F=∆mv / t

Question 24

in terms of kinetic theory, why does pressure decrease when gas is removed from a container

Answer 24

• less gas so fewer molecules
• so rate of change of momentum is less
• pressure is proportional to the number of molecules per unit volume so pressure decreases

Question 25

In terms of kinetic theory, what effect does reducing temperature of a gas have on the pressure

Answer 25

• Pressure decreases
• Since molecular collisions with wall are less frequent and rate of ∆momentum is less

Question 26

how does increasing temperature affect the motion of gas particles

Answer 26

increase in temperature causes an increase in the mean kinetic energy so mean square speed increases

Question 27

quantities that increase when temperature of a constant mass of gas at a constant volume is increased

Answer 27

• pressure
• average KE per particle
• internal energy

Question 28

Why are mean square speeds of two different gas molecules at the same temperature not the same

Answer 28

• The mean kinetic energy is the same for both gases but the masses of the atoms is different.
• Mean square speed is proportional to 1/mass
• Lighter mass atom/molecules must have a higher mean square speed in order to have the same average kinetic energy.

Question 29

Root mean square speed

Answer 29

The square root of the mean square speed of the particles in the gas ∴ the average speed.

Question 30

derivation of ideal gas equation from kinetic theory

Answer 30

Particle, of mass m, travelling with velocity v in a box of length a, width b and height c. Travelling back and forwards along length a and hitting end of area bc.

• Change in momentum of a molecule hitting the end wall ∆p = - 2mv
• from Newton’s 2nd Law : F = ∆p/∆t - Time between collisions of a molecule hitting the end wall ∆t = 2a / v
• Force, F, on the molecule hitting the end wall F= -mv² / a
• from Newton’s 3rd Law : force on molecule is equal and opposite to force on box
• Force, F, on wall from one molecule hitting the end wall; F = mv² / a
• Pressure = Force / Area
• Area of end wall = bc
• Pressure, p, from one molecule hitting end wall; p = mv² / abc
• Volume V = abc;
• Pressure, p, from one molecule hitting end wall; p = mv² / V
• Pressure, p, from N molecules hitting end wall; p = N mv² / V
• Particles have a range of speeds. Average of speeds squared = (crms)²
• Pressure, p, from N molecules with a range of speeds hitting end wall: p = Nm(crms)² / V
• On average in a box, 1/3 of molecules will move in each of the 3 directions.
• Ideal gas equation pV = 1/3 Nm(crms)²

Question 31

ideal gas equation

Answer 31

pV=nRT for n moles
pV=NkT for N molecules
By combining the 3 gas law equations
(pV/T=constant)

Question 32

molecular mass

Answer 32

sum of the masses of all the atoms that make up a molecule = mass of one molecule = A

Question 33

molar mass

Answer 33

the mass of one mole of a gas. One mole of gas contains Avogadro’s number of particles so N=nNa. Volume of one mole of gas = 24dm³

Question 34

average molecular kinetic energy derivation

Answer 34

pV=nRT=1/3 Nm(Crms)²
∴0.5m(Crms)² = 1.5nRT/N (multiplying by 1.5)
0.5m(Crms)²=1.5kT (Nk=nR)
0.5m(Crms)²=1.5RT/Na (k=R/Na)

Question 35

internal energy

Answer 35

the sum of kinetic and potential energies. for ideal gases it is the sum of the kinetic energy as they have no potential energy due to no IM forces

Question 36

kinetic energy of particles

Answer 36

randomly distributed kinetic energy = number of particles x average kinetic energy per particle

Question 37

potential energy

Answer 37

intermolecular forces increased by: - transfer of thermal energy (heat)
- doing work on a substance

Question 38

thermal equilibrium

Answer 38

thermal energy will continue to be transferred until objects are in thermal equilibrium

Question 39

1st law of thermodynamics

Answer 39

change in internal energy = total energy transferred due to work and heat ∆u = ∆Q + ∆W

Question 40

temperature definition

Answer 40

average energy per particle

Question 41

boyle’s law pV = constant

Answer 41

• the volume of a fixed mass of gas at constant temperature is inversely proportional to the pressure of the gas
• if the volume of a gas is reduced, particles will be closer together so collisions between themselves and the container will be more frequent
• therefore the pressure increases

Question 42

charles’ law V/T = constant

Answer 42

• at constant pressure, the volume of a gas is directly proportional to its absolute temperature
• when a gas is heated, its particles gain kinetic energy so constant pressure means the gas particles move quicker and further apart increasing the volume

Question 43

the pressure law p/T = constant

Answer 43

• at constant volume, the pressure of a gas is directly proportional to its absolute temperature
• as a gas is heated, its particles gain kinetic energy thus if the volume of the container remains the same, gas particles will collide more frequently with themselves and the walls of the container, increasing the pressure of the gas

Question 44

experiment for Boyle’s law

Answer 44

1. put oil in a tube of fixed dimensions which traps a pocket of air
2. use a tyre pump to increase the pressure on the oil
3. as pressure increases, more oil is pushed up the tube so the air compresses (reduced volume)
4. measure the volume of air at room temperature and increase the pressure recording volume and pressure at regular intervals
5. a graph of p against 1/v should be a straight line as pV = constant

Question 45

experiment for Charles’ law

Answer 45

1. Seal a capillary tube at the bottom and place a drop of concentrated sulfuric acid halfway up the tube trapping a column of air
2. fill a beaker with it inside with near boiling water and the length of the trapped air column should increase. Water cooling will cause it to decrease
3. record the temperature and air column length in regular intervals as the water cools and repeat twice more at the same temperature intervals and average your results
4. plot a graph of length of column against temperature which will give a straight line showing Charles’ law

Question 46

gas laws vs kinetic theory

Answer 46

the gas laws are empirical - they are based on observations and evidence of how gases respond to change in their environment and can be proven
kinetic theory is theoretical - it is based on assumptions and derivations from knowledge and theories we already had

Question 47

brownian motiion

Answer 47

• the random motion of particles suspended in a fluid resulting from their collisions with each other and other atoms
• this can be seen when smoke moves in a zigzag, random motion due to its collisions with smaller lighter air particles travelling at high speeds
• this is evidence that the air is made up of tiny atoms moving quickly hence gained acceptance for the concept of the atom