6.2 Thermal Physics Flashcards
Molar mass of Nitrogen(N2) IS 28gmol^-1
a) calculate number of molecules in 1.2 grams of nitrogen gas
b) calculate number of atoms in 1.2g of nitrogen
a) moles = mass / molar mass
12 / 28 = 0.043
molecules = moles x 6.02 x 10^23
= 0.043 x 6.02 x 10^23 = 2.58 x 10^22
b) 2.58 x 10^22 x 2 = 5.16 x 10^22 atoms
What temperature is a room typically
20 degrees
293 Kelvin
What is atmospheric pressure at sea level
101 kPa
What is the lowest temperature theoretically possible
0 kelvin = - 273 degrees
What is Boyles law
Pressure is inversely proportional to volume
- as fixed mass of gas is kept at a constant temperature
Using boyles law, sample has 100kPa at room temperature and volume of 0.48 m^3. State new pressure when temperature remains same and volume goes down to 0.12 m^3
P1V1 = P2V2
400 Kpa
What is Charles law
Volume of gas is directly proportional to temperature
- as fixed mass of gas is kept at a constant pressure
A sample of gas is kept at atmospheric pressure as it is heated from 293 kelvin to 550 kelvin. If its original volume was 0.067 m^3, what is its volume once heated
V1 / T1 = V2 / T2
V2 = 0.13 m^3
What is the pressure law (Gay - Lussac’s law)
Pressure of gas is directly proportional to temperature
- a fixed mass of gas was kept at a constant volume
What is the equation that combines the three gas laws.
(P1V1) / T1 = (P2V2) / T2
as (P1V1) / T1 = constant
Define pressure and equation for it
pressure is the normal force per unit surface area which the force acts
P = f / a
what is the equation for work done linking pressure and volume
W = P x change in volume
What is the molecular kinetic theory
- when a ball hits the wall in a container, its velocity flips direction, its momentum has also flipped, so wall has caused a change of momentum
- gas molecules exert an equal and opposite force on wall
- divide force by area to calculate pressure
What are the 5 conditions for an ideal gas
- volume of each molecule is negligible compared to total volume of the gas
- intermlcular forces between molecules are negligible
- time between collisions with wall is much greater than duration of colissions
- collision are considered elastic
- large number of molecules whos motion is random
Using Boyles law, explain what happens when volume of a container is decreased
- fixed temperature so molecule change of each molecule is the same
- more collisions per second so total momentum change bigger
- larger force exerted on walls
- larger pressure exerted
Using charles law, explain what happens when temperature of a gas is increased
- kinetic energy of molecules increases
- so change of momentum of each collision is greater
- to keep total force and pressure constant, volume increases
- increasing time between collisions
Using pressure law, explain what happens when temperature of gas is increased
- kinetic energy of molecules increases
- so change of momentun of each collission is greater
- volume is constant so total force increases
- increasing pressure exerted
Which one is root mean square speed and which one is mean square speed.
Crms = root mean square speed
(Crms) ^2 = mean square speed
using maxwell-boltzmans graph, what happens if a gas gets hotter.
Moves to the right and is lower and wider
Define thermal equilibrium
Two objects are in thermal equilibrium if there is no net energy transfer between them when they are placed in contact.
Define specific heat capacity
Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 K.
Why is the area constant in a Boltzmann graph
area corresponds to number of molecules in the gas, as only temperature changes but not mass of the gas
How do you work out total kinetic energy of gas
total number of molecules x average KE of gas molecules
What happens when two things come into contact
thermal energy transferred from hotter object to colder object until both objects are at the same temperature.
- molecules in hotter object have more kinetic energy as they collide with objects in colder object they transfer kinetic energy. It continues until both objects have same kinetic energy and reach thermal equilibrium
How do you increase temperature gained by a metal
- when power switches on, electrical energy flows along wire, converted to thermal energy in plate, transferring thermal energy to metal.
- to increase even more either increase power of hot plate or increase electrical energy converted into thermal or decrease mass of metal used.
Is a materiaI better at storing thermal energy if it has higher specific heat capacity.
YES, it absorbs more heat to increase its temperature by 1 degree
Describe an experiment to measure specific heat capacity of liquid inside calorimetry.
- measure mass of calorimeter empty, then full of liquid, subtract full from empty to calculate mass of liquid
- steady current ran through liquid at a steady potential difference
- use stopwatch and thermometer to measure temperature at suitable intervals
- Plot temperature against time to get straight line through origin
- gradient = P / mc
re arrange to calculate c
State changes of state that happens at 0 degrees
ice melts to water
water freezes to become ice
state changes of state that happen at 100 degrees
- water vaporises to become vapour
- water vapour condenses to become steam
Define specific latent heat of fusion
Specific latent heat of fusion is the energy (required) to change 1 kg of material from the solid state to the liquid state
Define latent heat of vaporisation
energy required per kg to change liquid to gas at its boiling point
equation for flow rate
mass / time
A solar panel transfers energy at a rate of 1.2 kW to liquid passing through it. The liquid has a specific heat capacity of 4.0 kJ kg−1 K−1 .When the liquid flows through the solar panel, its temperature increases by 3.0 K. What is its flow rate
flow rate = mass / time
power = Q / t
where Q = mass x c (specific heat capacity) delta theta
re arrange for m / t to get m / t = p / c x change in temperature
1200 / 4000 x 3 = 0.10
A gas occupies a volume V. Its particles have a root mean square speed (crms) of u.
The gas is compressed at constant temperature to a volume 0.5V.
What is the root mean square speed of the gas particles after compression?
1/2 m (Crms)^2 = 3/2 kT
(Crms)^2 is proportional to temperature
hence it remains as u
Which statement is true about an experiment where Brownian motion is demonstrated using
smoke particles in air?
A
The experiment makes it possible to see the motion of air
molecules.
B
The motion is caused by the collisions of smoke particles
with each other.
C
The motion is caused by collisions between air molecules
and smoke particles.
D
The motion occurs because air is a mixture of gases and the
molecules have different masses.
C
A liquid flows continuously through a chamber that contains an electric heater. When the steady
state is reached, the liquid leaving the chamber is at a higher temperature than the liquid entering
the chamber. The difference in temperature is Δt.
Which of the following will increase Δt with no other change?
A Increasing the volume flow rate of the liquid
B Changing the liquid to one with a lower specific heat capacity
C Using a heating element with a higher resistance
D Changing the liquid to one that has a higher density
B
as r increases temperature decreases
wont heat up as quickly
What is the total internal energy of 2.4 mol of an ideal gas which has a temperature of 15 °C?
3 / 2 kT x number of molecules
1.5 x (1.38 x 10^-23) x (15 + 273) x (2.4 x 6.02 x 10^23)
= 8.6 x 10^3
when calculating molar mass, what unit is it originally in
grams
Atoms of a gas rapidly rotate in a drum. State why drum must be in a vacuum.
If not in a vacuum gas atoms will collide with air atoms, changing their direction or speed distribution
molar mass of a gas is 0.209 kg mol^-1. Calculate the mass of one atom
0.209 / 6.022 x 10^23 = 3.47 x 10^-25
oven temperature is kept constant during the experiment but the pressure in the oven decreases as atoms leave through the exit hole.
Explain, using the kinetic theory, why the pressure decreases.
Pressure is due to collisions with atoms against the wall as there are fewer atom collisions per second.
therefore total momentum change per second falls so pressure drops
Describe motion of atoms as the temperature increases
- mean kinetic energy increases / mean speed increases
Units for specific heat capacity
J Kg^-1 K^-1
What equation links work done and volume
work done = pressure x change in volume
State what is meant by the internal energy of a gas.
Internal energy is the total energy stored within a system due to the random motion of its particles. It includes the sum of the kinetic energy of the particles and the potential energy arising from interactions between them.
Absolute zero of temperature can be interpreted in terms of the ideal gas laws or the kinetic energy of particles in an ideal gas.
Describe these two interpretations of absolute zero of temperature.
- it is the temperature at which the pressure or volume goes to zero
- with kinetic energy, it is when random motion of particles stop / or kinetic energy = zero
A mixture of argon atoms and helium atoms is in a cylinder enclosed with a piston. The mixture is at a temperature of 310 K.
Calculate the root mean square speed (crms) of the argon atoms in the mixture.
molar mass of argon = 4.0 × 10–2 kg mol–1
mass = molar mass / number of molecules
(4 x 10^-2) / 6.02 x 10^23 = 6.64 x 10^-26
8use equation from formula sheet to get 440 ms^-1
Compare the mean kinetic energy of the argon atoms and the helium atoms in the mixture.
In equilibrium at the same temperature both gases have the same mean or average kinetic energy
Explain, in terms of the kinetic theory model, why a pressure is exerted by the gas on the piston.
- Particles collide with piston walls and change momentum
- piston provides force as force is equal to rate of change of momentum
- pressure is equal to force / area so the particle produces a force on the pistol exerting a pressure
The mixture of gases in the cylinder stays the same.
Explain, using the kinetic theory model, two changes that can be made independently to reduce the pressure exerted by the gas.
- pressure is inversely proportional to volume so volume could be increased
- leads to increase in time between collisions and results in less frequent collisions, it increases area and reduces pressure
- pressure is proportional to temperature so if temperature is decreased then time between collisions increases as rate of change of momentum decreases with the wall.
The pump can circulate the water at different speeds.
When working at higher speeds the rise in temperature is greater.
Explain why.
assume that no heat is transferred to the surroundings
when the pump is working at speed, the pump is doing work on the water.
work and heat can both raise the temperature of a body, as work can be converted into thermal
pump increases the randomness of the movement of the molecules
mean speed / mean kinetic energy is proportional to absolute temperature
State two assumptions made about the motion of the molecules in a gas in the derivation of the kinetic theory of gases equation.
- molecules continually move about in a random motion
- collisions of molecules with each other and the walls are elastic
- time in contact with wall is small compared to time between collisions
- the molecules move in straight lines between collisions
Use the kinetic theory of gases to explain why the pressure inside a football increases when the temperature of the air inside it rises. Assume that the volume of the ball remains constant.
- mean kinetic energy and mean speed or air molecules increases
- more collisions with the inside of the football per second
- leading to higher rate of change of momentum which is proportional to the force, due to p = f/a as force increases, pressure increases
Define the Avogadro constant.
number of particles in one mole of a substance
Calculate the mean kinetic energy of krypton atoms in a sample of gas at a temperature of 22 °C.
Use of 3/2 kT
convert degrees to kelvin
= 6.1 x 10^-21
Outline what is meant by an ideal gas.
molecules have negligible volume
collisions are elastic
no interaction between molecules except collisions
gas obeys ideal gas law / obeys boyles law etc
at any given temperature / pressure
Describe one way in which the motion of the molecules of air inside the bicycle tyre is similar and one way in which it is different at the two temperatures.
similar :
- rapid random motion
- range of speeds
different :
- mean kinetic energy
- root mean square speed
- frequency of collisions
The concept of an absolute zero of temperature may be explained by reference to the behaviour of a gas.
Discuss one experiment that can be performed using a gas which would enable you to explain absolute zero and determine its value.
It is not necessary to give full details of the apparatus. Your answer should:
* include the quantities that are kept constant
* identify the measurements to be taken
* explain how the results may be used to find absolute zero
* justify why the value obtained is absolute zero.
The quality of your written communication will be assessed in your answer.
- the mass of the gas must be kept constant and the volume or the pressure must be kept constant
- Record pressure or volume over a range of temperatures. Experiment must involve changing the temperature with pressure or volume being the dependent variable
- Gas must be heated uniformly by a temperature bath to control the temperature
- Show a graph of pressure / volume against temperature that is linear. Linear relationship shows Pressure law or charles law as line of best fit implies a linear graph
- Use the results in a graph of pressure against temperature (or volume against temperature) which can be extrapolated to lower temperatures which has zero pressure (or volume) at absolute zero, which is at 0 K or −273 °C
- Absolute zero is obtained using any gas (provided it is ideal or not at high pressures or close to liquification)
- absolute zero is the temperature at which volume / pressure / mean KE is zero or particles are not moving.
Scientists continue to try to produce a viable fusion reactor to generate energy on
Earth using reactors like the Joint European Torus (JET). The method requires a
plasma that has to be raised to a suitable temperature for fusion to take place.
State two nuclei that are most likely to be used to form the plasma of a fusion reactor.
DEUTERON - HYDROGEN - 2
Triton / tritium / hydrogen−3
State one method which can be used to raise the temperature of the plasma to a suitable temperature.
Electrical heating
using radio waves
Lead has a specific heat capacity of 130 J kg−1 K−1
Explain what is meant by this statement.
It takes 130 Joules to raise temperature of mass of 1kg of lead by 1 kelvin without changing its state
Define the specific latent heat of vaporisation of water.
the energy required to change the state of a unit mass of water to steam / gas
when at its boiling point temperature / 100°C / without a change in temperature
What does the graph look like highlighting the the relationship between the internal energy of an ideal gas (y-axis) and the absolute temperature of the gas (x-axis)
straight line through the origin as they are directly proportional
Sketch a graph of Charles Law, draw two lines of one with lower pressure and one with higher pressure
Sketch a graph of Charles Law, draw two lines of one with lower pressure and one with higher pressure
b) explain how s one of the energy transferred to the sample changes arrangement of the atoms during the interval tA to tB
c) state what happens to the potential energy of the atoms and to the kinetic energy of r her atoms during the time interval tA to tB
e) the sample has a mass of 0.25kg. Determine the specific hear capacity of M when in the liquid state. State an appropriate SI unit for your answer
b) for a helium - 3 nucleus and a helium - 4 nucleus to fuse they need to be separated by no more than 3.5 x 10^-1. Calculate minimum total kinetic energy for them to reach separation of 3.5 x 10^-15 m
Part b - requires help
a) heat supplied = heat gained by cola
0.25 x 840 x (30 - T) = 0.2 x 4190 x (T - 3)
T = 8.41 degrees
Specific hear capacity = 4200
