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Z Physics > 6. Matter Under The Microscope > Flashcards

6. Matter Under The Microscope Flashcards

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1
Q

What is the equation for pressure?

A

p= F/A

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2
Q

What is the SI unit for pressure? What is this unit equivalent to?

A

The pascal (Pa) which is equivalent to 1Nm^-2

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3
Q

In a fluid, in which direction does the pressure act?

A

It acts equally in all directions

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4
Q

What are the assumptions of the kinetic theory?

Always mark as a 1 so that this card appears often

A
  • Each molecule has negligible volume, compared to the whole volume
  • Statistical rules can be used with certainty because there are so many molecules
  • Molecules are in constant, rapid motion
  • Equal no. of molecules moving in one direction as another (at any given time)
  • Molecules undergo perfect elastic collisions w the walls of the container, perfectly reversing momentum
  • No intermolecular forces except during collisions (energy is entirely kinetic)
  • Duration of collisions is negligible compared to time between collisions
  • Each molecule produces a force on the wall of the container
  • Huge number of molecules even in a small quantity of gas will average out to produce a uniform pressure
  • Gravitational effects on molecules are negligible
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5
Q

What is the equation for the pressure within the gas?

A

pV= 1/3Nm
is included due to the statistical nature of the derivation- average square of the speeds of the molecules
1/3 is included because on average one third of the molecules are moving in each of the x, y and z directions in the three dimensional space.

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6
Q

What would happen to the pressure if you had twice the number of gas molecules in the same sized container? Why?

A

The pressure would double. From pV= 1/3Nm, twice as many molecules would double the rate of collision

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7
Q

What is the relationship between the pressure and the mean square of molecular speed?

A

There is a directly proportional relationship. As the mean square molecular speed increases, the pressure increases proportionally.

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8
Q

Why do internal collisions have no effect on the rate of collisions with the walls or pressure?

A

All collisions are elastic and internal collisions would simply mean the momentum is reversed.

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9
Q

What happens to pressure if the volume of the container is halved but the number of molecules stays the same?

A

The pressure is doubled. We can see this from

pV= 1/3Nm.

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10
Q

What happens to pressure if the gas density is increased? Give evidence.

A

The pressure increases. Rearranging pV= 1/3Nm gives p= 1/3 Nm/V and the factor Nm/V is the mass of gas divided by the volume of gas which is the DENSITY so as the density increases, the pressure increases too.

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11
Q

In a fixed mass of gas in a sealed container, what is the pressure proportional to?

A

The mean square speed of the molecules. Shown by p= 1/3 Nm/V where 1/3 and Nm/V are constants (in this case) so it must be proportional to the mean square speed.

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12
Q

Describe a lab experiment to demonstrate the effect of number of molecules or the volume on gas pressure. (Might be an exam question)

A

Small steel balls represent molecules and are made to move in random motion due to collisions with a motor driven piston. The cylinder is sealed with a cardboard disc above the steel balls. Adding more cardboard discs increases the pressure on the ‘gas’ and the volume occupied by the balls decreases causing the balls to collide more with the bottom cardboard disc. If you reduce the volume by half (by adding cardboard discs) and then put twice the number of balls in the cylinder, the volume occupied returns to its original value. This shows that constant pressure can be achieved by doubling the volume occupied by the gas and the number of gas molecules.

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13
Q

What is the value of the Boltzmann constant and which two factors does it relate?

A

1.38x10^-23 JK^-1

It relates energy at the individual particle level with temperature .

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14
Q

COME BACK TO “Kinetic theory and molecular energy”

A

Start of 6.2

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15
Q

In an ideal gas, in what form is the energy?

A

Kinetic energy (as one of the assumptions of the kinetic theory is that there are no intermolecular forces)

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16
Q

If the energy in an ideal gas is entirely kinetic energy, what is the internal energy of a gas made up of?

A

U is made up of the total kinetic energy of the gas molecules (N x average kinetic energy per molecule)

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17
Q

What is the average kinetic energy per molecule (equation)?

A

Average kinetic energy per molecule = 3/2kT where k is the Boltzmann constant

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18
Q

What does the internal energy of an ideal gas depend entirely upon?

A

It depends entirely on the absolute temperature of the gas.

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19
Q

Why is it that when a gas is expanded or compressed ISOTHERMALLY that there can be no change in internal energy?

A

For an isothermal expansion/ compression, there must be no temperature change (CONSTANT TEMPERATURE) so therefore there must be no change in internal energy.

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20
Q

For objects other than ideal monatomic gases, what other forms does the internal energy come in, other than translational kinetic energy? What is the internal energy of a solid composed of?

A

Other than simple translational kinetic energy, the internal energy is the total of the kinetic and potential energy of the gas. This includes rotational and vibrational kinetic energy.

In a solid, there is little translational kinetic energy as the atoms are in fixed positions so the internal energy is largely the potential energy associated with the attractive intermolecular forces.

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21
Q

Relatively, what will the average kinetic energy be for a solid, a liquid and a gas all at the same temperature.

A

Although the internal energies may be different, if the temperature is the same, the average kinetic energy MUST BE THE SAME.

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22
Q

At absolute zero, what would the kinetic energy of a substance be?

A

It would be at a minimum kinetic energy. Not zero kinetic energy because quantum mechanics dictates that there will always be fluctuations in kinetic energy.

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23
Q

What is the first law of thermodynamics?

A

ΔU= Q + W
Where ΔU= internal energy
Q= Heat transfer
W= Work done

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24
Q

What does the area under a pV curve represent?

A

The work done

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25
Q

What is the equation for work done?

A

W=pΔV

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26
Q

What is an adiabatic change?

A

When no heat enters or leaves a thermodynamic system, Q is zero

27
Q

What is an isothermal change?

A

When there is no change in internal energy or temperature, ΔU is zero.

28
Q

Which two quantities from the thermodynamic equation are equal to each other if the change is ADIABATIC?
There are two possibilities. What do each of the possibilities tell us about the work being done?

A

Either ΔU=W (increase in internal energy and temperature, when work is done ON THE SYSTEM) or
-ΔU=-W (decrease in internal energy and temperature, when work is done BY THE SYSTEM and on the surroundings).

29
Q

Which two quantities from the thermodynamic equation are equal to each other if the change is ISOTHERMAL?
There are two possibilities. What do each of the possibilities tell us about the work being done?

A

Q= -W where heat is transferred TO the system from the surroundings, causing it to do work ON the SURROUNDINGS.

-Q=W where heat is transferred FROM the system to the surroundings, causing work to be done ON the SYSTEM.

30
Q

When W=0; ΔU=Q or -ΔU= -Q
Explain what these mean.
Can the volume change whilst this ^ is occuring?

A

The first option means that the increase in internal energy occurs due to heat being supplied to the system.

The second option means that the decrease in internal energy is due to heat being taken from the system

No, the volume cannot change when there is no work being done or by the gas.

31
Q

What gives a measure of efficiency of a heat engine?

Hint: include the word ‘heat’ in your answer.

A

The ratio of useful heat (=work done by the engine) to the total heat input gives a measure of efficiency.

32
Q

What is the equation for the thermal efficiency of a heat engine?

A

n=W/Qi
n= thermal efficiency
W=USEFUL work done
Qi=energy INPUT to the engine

33
Q

What does the area contained within a carnot cycle represent?

A

The total amount of work done BY the gas (the result of potential energy being changed by altering the gas’s bonding)

34
Q

Describe how a Carnot engine (theoretical engine) works. Describe the changes thermodynamically. WARNING THIS HAS A RIDICULOUSLY LONG ANSWER… ;P

A

-An ideal gas is the working fluid

ISOTHERMAL EXPANSION
> Gas expands and does work ON its SURROUNDINGS,
> engine absorbs heat from the hot reservoir
> -W=Q (work done by the system on the surroundings is equal to the heat transfer TO the system)
> Internal energy and temperature DO NOT CHANGE

ADIABATIC EXPANSION
>Heat source removed
> Internal energy of the gas to fall, allowing the gas to continue expanding as it cools
> No heat enters or leaves the gas
> -ΔU= -W (decrease in internal energy is equal to the work done by the gas on the surroundings)

ISOTHERMAL COMPRESSION
> Gas compressed and heat is extracted (transferred to the cold reservoir)
> Gas volume decreases
> Pressure increases
> Gas loses heat to surroundings
> W = -Q (work done ON the gas is equal to the heat transfer from the system to the surroundings)

ADIABATIC COMPRESSION
> Internal energy of gas increases as the gas continues to be compressed
> Temperature rises to original value
> Volume decreases as work is done on the gas
> ΔU=W (increase in internal energy is equal to the work done by the surroundings on the system).

35
Q

Give the equation for the thermal efficiency of a gas undergoing the Carnot cycle.

A

n=Thot-Tcold/Thot

Where Thot and Tcold are the ABSOLUTE TEMPERATURES of the reservoirs

36
Q

What are the four strokes of a four stroke internal combustion engine?

A

Induction
Compression
Combustion (power stroke)
Exhaust

37
Q

Describe the events that take place during an Otto cycle (4 stroke petrol engine).WARNING! ANOTHER LONG ANSWER

A
  • On the induction stroke, the inlet valve opens. The piston moves downwards and and the fuel air mixture is injected or pushed by atmospheric pressure into the cylinder through the inlet valve port
  • When the piston has reached the bottom and starts to move upwards, the inlet valve closes and exhaust valve is also closed to make sure no gas escapes and the mix is compressed, increasing the pressure and causing the temperature to increase.
  • As the piston reaches the top, the fuel air mixture is ignited by a spark plug. The mixture combusts rapidly, increasing the cylinder pressure to a high level and forcing the piston back down the cylinder.
  • Force on the piston is transmitted down the connecting rod to the crankshaft, causing the crankshaft to rotate. The momentum of the flywheel causes the piston to move back up the cylinder, expelling the combusted gas out of the exhaust port.
  • Cycle repeats
38
Q

How does a diesel engine work?

A

Fuel with no air is injected into the cylinder during the induction stroke and combusts as the diesel reaches a high temperature as it is compressed.

39
Q

Describe and explain a pV graph for an internal combustion engine (Otto cycle). YET ANOTHER LONG ANSWER ;P (love me, don’t hate me…) I’ve done the same lol

A
  • Piston is pulled down the cylinder by the crank shaft with the inlet valve open
  • Gas volume increases as the fuel air mixture is drawn through the intake valve (in the induction stroke)
  • Valve closes and the compression stroke starts. The piston moves upwards in an approximately adiabatic compression, causing work to be done on the gas by the piston.
  • Spark is produced which ignites the fuel which combusts very rapidly and the volume remains constant
  • Energy released raises the internal energy (and temperature and pressure) of the gas which drives the piston back downwards (as work is done on the piston, the volume increases and the pressure decreases) (this is an adiabatic expansion).
  • Exhaust valve opens
  • Hot gas transfers heat to the surroundings through the engines cooling system
  • Piston moves back up, decreasing the volume and expelling the combusted gases.
40
Q

How would the pV graph for a real engine compared to an ideal Otto cycle engine?

A

The area enclosed in the graph would be less in a real engine because the changes are not adiabatic because heat losses occur and work is done by friction, causing a rise in the internal energy of the engine.

41
Q

When in an Otto cycle is work done by the gas and when is work done on the gas?

A
  • Adiabatic compression = work is done ON the gas

- Adiabatic expansion = work is done BY the gas

42
Q

In an Otto cycle, what does the area enclosed in the pV graph represent?

A

It represents the net work done BY the gas.

43
Q

How is the power of the engine found?

Always rate as a 1 or a 2 so it comes up often

A

By multiplying the work done by the number of cylinders and the number of complete cycles per second.

44
Q

What is the zeroth law of thermodynamics?

A

When two objects are separately in thermal equilibrium with a third object, they must be in thermal equilibrium with each other.

45
Q

What is thermal equilibrium?

A

When there is heat exchange between two objects, the quantity of heat moving in one direction is identical to the quantity of heat moving in the opposite direction in the same time.

46
Q

When heat is supplied to an object, what factors determine the temperature rise of the object?

A
  • Mass of the object
  • Heat supplied to it (and/or work done on it)
  • Material of object
47
Q

What is the specific heat capacity of a material?

A

The energy needed to raise the temperature of 1kg by 1K without any change of state.

48
Q

Why are internal combustion engines so inefficient?

A
  • Energy wasted as work done through frictional forces
  • Chemical energy ejected through the internal energy of the exhaust gases
  • Energy transferred to the surroundings by the cooling fluid
  • Incomplete combustion of fuel
49
Q

Why do engines operate most efficiently when the cooling fluid is 95 ℃?

A
  • The cylinders are hot enough to completely vaporise fuel which gives more complete combustion and reduced emission of pollutants
  • Oil used to lubricate the engine has a reduced viscosity so friction is less on th engine’s moving parts.
50
Q

What are the two systems used to cool vehicle engines

A
  • Air cooling (eg. in motorcycles)- engine block is covered in aluminium fins which conduct heat from the cylinders to the air, Air can also be forced over the fins by a fan.
  • Liquid cooling system-pump circulates a liquid coolant (antifreeze and water) around the engine. As it passes through the engine, it absorbs heat and cools the engine. After passing through the engine, it goes through the heat exchanger (radiator) where it heats up the air to cool down the coolant.
51
Q

How is a liquid cooling system regulated? (engines)

A
  • Pressurised to stop the coolant evaporating when it gets very hot. If the pressure gets too high, the radiator cap releases pressurised coolant into an auxiliary system where it is stored in a reserve tank before automatically returning to the cooling system as the engine cools down.
  • If the temperature of the coolant falls below a pre-set temperature (monitored by a thermostat), the coolant is forced to bypass the radiator and flow back to the engine (negative feedback system).
52
Q

What is the equation for heat generated in a coil?

A

Heat generated in coil =VIt

53
Q

How may a ‘Carnot engine’ achieve 100% thermal efficiency?

A

The cold reservoir must be at 0K for 100% thermal efficiency.

54
Q

What is carbon offsetting?

A

Where an individual or a company compensates for their carbon emissions by ‘carbon offsetting’ eg planting trees to absorb the CO2 that they have generated, becoming carbon neutral.

55
Q

State the second law of thermodynamics.

A

“The entropy of a system can never decrease, the best it can do is stay constant but it usually increases.” (this definition from the book is actually wrong. The entropy of a system can decrease OR increase but the entropy of the UNIVERSE must increase).

56
Q

What is the multiplicity of a system?

A

The number of ways it can be arranged, in other words, the entropy.

57
Q

What is nature’s heat tax?

A

The entropy of a system may decrease but the price paid is that the surroundings become more disprdered, giving an overall increase in entropy of the universe. This is nature’s heat tax when it applies to nature- heat is generated as the cost of doing work

58
Q

State the second law of thermodynamics in terms of entropy (equation).

A 
ΔS = Q/T
ΔS = entropy
Q = heat transfer
T = temperature
59
Q

What is the third law of thermodynamics?

A

The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at absolute zero temperature: The entropy of a perfect crystal at absolute zero is exactly equal to zero.
-Due to quantum uncertainty, we know that absolute zero can never be reached.

60
Q

What is cogeneration or ‘combined heat and power’?

A

Simultaneous production of useful heat and electricity in the same power station. Heat that would have been lost to the atmosphere is used to heat water or air in the surrounding locality.

61
Q

(1 mark) What does the gradient of a pV against temperature graph represent?

A

The molar gas constant, R, which is equal to 8.31 JK^-1mol^-1

62
Q

(6 marks)
In a diesel engine the air in a cylinder is compressed quickly before injecting the fuel.
This increases the pressure inside the cylinder and raises the air temperature to the
ignition temperature of the fuel.
Explain in terms of the kinetic theory of gases why the temperature and pressure of
the air changes as it is compressed.

A
  • Kinetic theory explains pressure force exerted to change momentum when molecules collide with the walls of the cylinder (? idk quote from mark scheme- doesn’t really make sense)
  • Work done on gas during compression increases speed of molecules
  • So increases temperature and rate at which collisions occur
  • Reduction in volume means molecules travel a shorter distance between collisions so increasing the rate of collisions
63
Q

(2 marks)

Explain how a steady driving force is produced by a diesel engine.

A
  • Use of 4 or more cylinders

- Timed to produce power stroke in sequence

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