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OCR Physics A2 > Thermal physics (DONE) > Flashcards

Thermal physics (DONE) Flashcards

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1
Q

Describe a small experiment which can show water changing state without being heated to boiling point.

A
  • Using a syringe, take a small amount of hot water and squeeze out the air.
  • cover the tip of the syringe and depressurise the water.
  • this should cause the water to change state without reaching boiling point.
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2
Q

Describe the triple point of water.

A
  • the temperature and pressure at which the three phases (gas, liquid, and solid) can coexist in thermodynamic equilibrium.
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3
Q

What is the value of the triple point of water?

A

0.01 degrees celsius

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4
Q

what are the 4 scales of temperature?

A
  • Fahrenheit
  • centigrade
  • kelvin
  • celsius
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5
Q

Describe the Fahrenheit scale of temperature.

A
  • used a long time ago and is based on the points where water freezes and boils.
    32 F = freezing point
    212 F = boiling point
  • There are 180 intervals between the freezing and boiling points.
  • the numbers where not round or easily recognisable which is why the centigrade scale was created.
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6
Q

Describe the centigrade scale.

A 
- used round values:
0 C = Freezing point
100 C = Boiling point
- 100 intervals between these points.
- This scale makes a lot more sense than Fahrenheit however water does not always boil at 100 C, as shown using a syringe water can boil below 100 C if it is depressurised, so an absolute scale (kelvin) was created.
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7
Q

Describe the Kelvin scale.

A 
- based on 2 main points:
absolute zero (0 K)
triple point of water (273.16 K)
- between these values there are 273.16 intervals where each interval is equal to 1 degree centigrade.
- gives us a thermodynamic scale based on 2 fixed points.
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8
Q

What is the absolute zero?

A
  • the point at which a substance has minimal internal energy.
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9
Q

Describe the celsius scale.

A
  • An absolute scale based in the Kelvin scale.

T (Kelvin) = T (celsius) + 273.15

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10
Q

What are the 3 states?

A
  • solid
  • liquid
  • gas
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11
Q

How are particles in solids arranged?

A
  • regular arrangement
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12
Q

What bonds particles in solids?

A
  • electrostatic bonds

- keep them from separating

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13
Q

How do particles move in a solid?

A
  • The particles vibrate around a fixed position.
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14
Q

What energy do particles have within solid?

A
  • when particles vibrate they have kinetic energy.

- the solid has internal kinetic energy

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15
Q

What happens when you heat a solid?

A
  • Some of the energy goes into breaking electrostatic bonds.

- when this happens the particles continue to vibrate but they also start to move around.

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16
Q

How does density change as a substance changes state from solid to liquid?

A
  • liquids tend to be less dense than their solid form.

- an exception to this is water as ice is less dense than water.

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17
Q

What happens to the particles when a liquid turns into a gas?

A
  • When the particles have enough energy they can escape the particles they are bonded near to.
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18
Q

What happens to the separation of particles and density as a liquid changes state to a gas?

A
  • The separation increases by a factor of 10 in all 3 dimensions.
  • meaning if there was a space containing 1000 particles, there would now be 1 particle.
  • density decreases by a factor of 1000
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19
Q

What are 2 properties of gases?

A
  • can expand to take shape of a container.

- can be compressed

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20
Q

What is Brownian motion?

A
  • The random movement of small visible particles suspended in a fluid due to collisions with much smaller, randomly moving atoms or molecules of the fluid.
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21
Q

How is a smoke cell used to show Brownian motion?

A
  • 12v supply is needed with wires attached to each end of the cell.
  • current causes light bulb inside smoke cell to light up.
  • Light source is shone through glass rod which acts as a lens focusing light on a small glass piece which holds smoke and is covered by glass cover piece.
  • smoke is trapped inside the glass piece and the light illuminates the smoke particles.
  • using a microscope smoke particles can be seen showing random movement.
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22
Q

How was Brownian motion discovered?

A
  • In 1897, Robert Brown observed pollen grains on the surface of water.
  • Under a microscope he could see the pollen particles would not stay still.
  • The random movement of the particles is Brownian motion.
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23
Q

How was Brownian motion explained?

A
  • In 1905, Einstein explained that within the liquid of which visible particles were suspended in, there are smaller particles which move rapidly.
  • When a smaller particle collides with a larger particle, energy and momentum is transferred causing the large particle to change direction.
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24
Q

What is the difference between temperature and internal energy?

A
  • Temperature is linked to the amount of kinetic energy in a single particle
  • Internal energy is the sum of all kinetic and potential energies in a system.
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25
Q

Why would a sparkler not cause damage to your skin at a fairly close distance but a cup of tea would scold your skin?

A
  • the sparks will have a very high temperature but internal energy will be low as the mass of each spark is so small.
  • the tea will have a much lower temperature but has a greater mass so the internal energy will be larger.
26
Q

What symbol shows internal energy?

A
  • Internal energy, U.

- distinguishes it from normal energy E, work done W, and thermal energy Q.

27
Q

What is the relationship between temperature and internal energy?

A
  • a linear relationship will be shown between temperature and internal energy until the substance begins to change state.
  • at this point the internal energy will continue to increase while temperature will remain constant.
  • this is because as the substance changes state the kinetic energy remains the same as the particles will be vibrating at the same speed for both states however internal energy is will continue to be supplied to the substance in order to overcome the bonds to change the state.
28
Q

What is the relationship between energy and change in temperature?

A
  • energy supplied is proportional to change in temperature.
29
Q

What is the relationship between mass and change in temperature?

A
  • the change in temperature is inversely proportional to the mass of a substance.
30
Q

How can we derive the equation for specific heat capacity?

A
  • Using the fact that energy supplied is proportional to change in temp, and change in temp is inversely proportional to mass.
  • we can therefore say that change in temp is proportional to energy/mass.
  • This can be rearranged to energy is proportional to mass x temp
  • We need a constant of proportionality to turn it into a useful equation as all metals have different characteristics.
  • this constant of proportionality c, is the specific heat capacity.
  • the equation we end up with is E = mc x change in temp.
  • It can be rearranged to find c.
31
Q

What is the specific heat capacity c, of a substance?

A
  • Specific heat capacity is the amount of heat energy required to raise 1kg of a substance by 1 degree celsius/kelvin.
  • the term ‘specific’ refers to per unit mass.
  • measured in J kg^-1 K^-1
32
Q

What experiment can be used to determine the specific heat capacity of a material?

A

Heat capacity of aluminium block using electricity:
- measure mass of aluminium before experiment and record.
- attach immersion heater to power supply.
- include ammeter and voltmeter in circuit to obtain values for current and pd.
- place immersion heater and thermometer in aluminium block.
- cover in vaseline to improve heat transfer.
- cover aluminium block in polystyrene and cotton wool to prevent heat loss.
- using stopwatch record change in temperature over time with regular intervals e.g. 60 seconds.
- Draw a graph of change in temperature against time.
- Initially there will be a time lag for the time it takes for the thermal energy to reach the thermometer.
- then there should be a linear section of the graph.
- as temperature increases there will be a point where increase in temperature will slow down as heat loss to the surroundings will increase.
- Using values for time, change in temperature, mass , voltage and current we can work out the heat capacity of aluminium.
- to find specific heat capacity the equation E = mc x theta will be used:
by substituting E = power x time we have VIt = mc x theta.
- we can then rearrange for c to get c = (VIt)/(m x theta)
- through finding the gradient of the linear section of the graph we get a value for theta/time.
- therefore 1/gradient = time/theta.
- we can substitute this into the equation for c to get (VI/m) x 1/gradient
- voltage, current and mass are all constants and spa through using the gradient of the linear graph we can determine the specific heat capacity of substance.

33
Q

What is a mole?

A
  • The SI unit for the amount of a substance (in grams) containing as many atoms as the number of atoms in 12 grams of carbon-12 (which is Avogadro’s number 6.022 x 10^23).
34
Q

What is Avogadro’s constant?

A
  • represents the number of atoms in 12 grams of carbon-12.
    = 6.022 x 10^23
  • symbol = Na
35
Q

What equation can be used to find the number of particles in a substance?

A

N = n x Na

Where:
N = number of particles
n = number of moles
Na = Avogadro’s constant

36
Q

What are macroscopic and microscopic properties of gas?

A
  • macroscopic properties are things which can be measured e.g. volume, pressure, temperature.
  • microscopic properties correspond to the properties of individual particles e.g. velocity, momentum.
37
Q

What are the 5 assumptions for the kinetic theory of gases?

A
  • The gas contains a large number of atoms (or molecules) moving in random directions with random speeds.
  • These molecules occupy a negligible volume compared to the overall volume of gas.
  • All collisions are perfectly elastic (no kinetic energy is lost)
  • The time of collision is small compared to the time between collisions.
  • Electrostatic forces between molecules are negligible except during collisions.
38
Q

How do you calculate the change in momentum of an ideal gas particle rebounding off the side of a container?

A
  • the particle has a mass, m.
  • it will be moving at a velocity +v.
  • it will continue moving until it hits the wall and due to Newtons 3rd law it will rebound in opposite direction.
  • it will move with velocity -v.
  • this means there will be a change in momentum = mv
  • using the equation
    △p=mv—(-)mv
  • we can simplify to
    △p=2mv
39
Q

How does a change in momentum of an ideal gas particle relate to pressure on a macroscopic scale?

A
  • the time between the particle colliding with the wall, moving across to the other side and back is T.
  • we know from newtons 2nd law that F rate of change of momentum △p/△T.
  • therefore F = (2mv)/T
  • if you know the force of 1 particle you can then calculate the force of multiple particles in a gas.
  • Using this you can find the pressure using the equation P = F/A
  • where A is the area of the side of the container which particles are colliding with.
40
Q

How is pressure in a container caused by an ideal gas?

A
  • pressure is caused by force per unit area.
  • the force is caused by each individual particle.
  • as they change in momentum they cause a force on the wall which will be experienced as pressure.
41
Q

What small example can be used to demonstrate how volume of an ideal gas changes as pressure changes?

A
  • if you have a small syringe with air inside and seal over the end, you have a fixed mass of gas.
  • by increasing the pressure, you can see that the volume decreases.
  • we can look at this in more detail through studying Boyle’s law.
42
Q

What is Boyle’s law?

A
  • the volume of a fixed gas is inversely proportional to the pressure exerted on the gas, under conditions of constant temperature.
  • pV = constant under constant temperature.
43
Q

What full size experiment could you use to test Boyle’s law/the relationship between pressure and volume of an ideal gas?

A
  • apparatus needed is a long tube, closed at one end and containing air above some oil.
  • the pressure on the oil can be increased by using a foot pump and the pressure exerted on the air by the oil is indicated by a pressure gauge.
  • the volume of air in the tube can be measured from a scale behind the tube.
  • ensure that time is taken between each measurement to let gas return to constant temperature.
  • the pressure against volume graph of results will show an inversely proportional relationship.
  • if a graph of pressure against 1/volume is drawn there will be a directly proportional linear relationship.
44
Q

Why must you take time between readings for experiments investigating Boyle’s law?

A
  • Boyle’s law will only apply if it is carried out at a constant temperature.
  • if you compress a gas quickly the temperature will rise, the opposite is true for a gas which is compressed and expands quickly it will feel cold.
  • therefore time needs to be taken between readings to ensure temperature does not rise during the experiment and distort results.
45
Q

What can be derived from the relationship between pressure and temperature?

A
  • pressure is proportional to temperature for a fixed mass of gas.
  • therefore P/T = constant
  • an equation can be formed:
    P1/T1 = P2/T2
46
Q

What can be derived from the relationship between pressure and volume?

A
  • pressure is inversely proportional to volume (Boyle’s law).
  • therefore pV = constant
  • an equation can be formed:
    p1v1 = p2v2
47
Q

What can be derived from the relationship between volume and temperature?

A
  • volume is proportional to temperature providing there is a constant pressure.
  • therefore V/T = constant
  • an equation can be formed:
    V1/T1 = V2/T2
48
Q

What can be derived from the relationship between volume, temperature and pressure?

A 
- we can say that:
(pV)/T = constant
- meaning that:
pV = constant x T
- it is this equation that depends on the amount of moles or particles you have of a substance.
- we therefore say:
pV = nRT
- where n = number of moles
- R = molar gas constant.
49
Q

What is the molar gas constant and how can it calculated?

A
  • the value is 8.31 JK^-1mol^-1
  • symbol R.
  • found in formula booklet.
  • however can also be calculated using the equation:
    pV = nRT
  • which can be rearranged to find R.
50
Q

What would happen if you increased the temperature of an ideal gas sealed inside a glass bottle?

A
  • as the air is trapped in a glass bottle which is inflexible, it has a fixed volume.
  • if we were to increase the temperature the particles inside will have more kinetic energy.
  • this means they will have greater velocity and when they collide with the walls of the bottle there is going to be a greater change in momentum.
  • therefore the pressure on the bottle will increase.
51
Q

What would we find if we drew a graph showing the relationship between temperature and pressure?

A
  • with pressure on the y-axis and temperature on the x-axis.
  • they will have a directly proportional linear relationship.
  • until you get to a point where temperature cannot decrease any further.
  • this is where we have 0 Kelvin, the absolute zero with minimum internal energy.
  • the pressure must also go down to 0 as the kinetic energy will be 0J meaning there is no pressure.
52
Q

Why do we use the mean square speed when investigating the velocities of particles in an ideal gas?

A
  • if we look at the average velocity of all the particles in an ideal gas it will be equal to 0.
  • this is because particles are moving in every direction, meaning that particles moving up/down, left/right will cancel each other out.
  • if we square the individual velocities it takes away the negative velocities.
  • when you take the mean of all the values you then get the mean square speed.
  • represented by C^2 with a line over the ‘C’.
53
Q

What is the root mean square speed and what is it used for?

A
  • Crms is the square root of the means square speed.

- used to find the average velocity of particles in an ideal gas.

54
Q

Describe the graph of particle velocity against number of particles?

A
  • with particle velocity on the x-axis and the number of particles on the y-axis.
  • we find that some particles are almost stationary and the general shape of the graph is an upside down U.
  • it has an uneven distribution.
  • the highest point on the graph is the most probable speed (as most particles are at this velocity).
  • at slightly higher speed is the mean speed.
  • after the mean speed is the root mean squared speed as it involves velocity squared.
  • this distribution is named after Maxwell and Boltzmann.
55
Q

Why do particles within an ideal gas have different velocities?

A
  • as particles are moving around they are colliding elastically.
  • this means some fast particles will collide with slow one, causing them to transfer kinetic energy.
  • this explains things such as evaporation, the reason a puddle will evaporate even when it isn’t hot, this is because some particles will gain enough energy to move at a speed which allows it to move from the surface of the liquid.
56
Q

What equation would we use to find the number of overall particles rather than the number of moles in an ideal gas?

A
  • we can use the equation:
    pV=NkT
N = overall number of particles
k = Boltzmanns constant
57
Q

How can we calculate Boltzmanns constant?

A
  • where N = number of particles, n = number of moles and Na = avogadros constant:
    N = n x Na
  • therefore:
    N/n = Na
  • this relates to Boltzmanns constant k, as :
    Nk = nR
  • this means that:
    N/n = R/k
  • when you equate terms:
    Na = R/k
  • and therefore:
    k = R/Na

allowing us to find Boltzmanns constant which = 1.38 x 10^-23

58
Q

What equation is used when looking at the microscopic properties of particles in an ideal gas?

A

pV = 1/3 x NmCms

where:
p = pressure
V = volume
N = number of particles
m = mass of a particle
Cms = mean square speed
59
Q

What equation is used when looking at the macroscopic properties of particles in an ideal gas?

A

pV = nRT

where:
p = pressure
V = volume
n = number of moles
R = molar gas constant
T = temperature

Can also be written as:
pV = NkT

where:
N = number of particles
k = boltzmanns constant

60
Q

How can you equate the micro and macroscopic equations to find the average kinetic energy of each particle in an ideal gas?

A 
- By equating you get:
1/3 x NmCms = NkT
- you can cancel to get:
1/3 x mCms = kT
- multiply both sides by 3:
mCms = 3kT
- divide both sides by 2:
1/2 x mCms = 3/2 x kT
  • the left hand side of the equation is effectively the kinetic energy as it is 1/2 x mass x velocity squared.
  • this means that:

Ke = 3/2 x kT

61
Q

What can we say about the average kinetic energy of an ideal gas particle in relation to the temperature?

A
  • the average kinetic energy of a particle is proportional to the temperature of the gas.
  • when the particles are moving quicker the gas is hotter.

OCR Physics A2 (15 decks)

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