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Y13 Classes > Paper 2 Topics > Flashcards

Paper 2 Topics Flashcards

1
Q

What is 1 mole?

A

A collection of 6.02×1023 molecules

(Avogadro’s constant)

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2
Q

What is the molar mass of a substance?

A

The mass of each mole (every 6.02×1023 molecules)

Eg for He each mole has a mass of 4g

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3
Q

How do you calculate the molar mass of a compound eg NO2

A

Add up the nucleon numbers

(14+16+16=46gmol-1)

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4
Q

How do you calculate the number of molecules in a substance?

A

N = n × NA

(Number of molecules = moles × Avogadro’s constant)

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5
Q

What is the molecular mass and how is it calculated?

A

The mass of each molecule of the substance

m = M/N

(molecular mass = total mass / number of molecules)

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6
Q

How is the total mass of a substance calculated?

A

M = n × mr

(Total mass = moles × molar mass)

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7
Q

How do you convert a temperature from °C to K?

A

T(K) = T(°C) + 273

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8
Q

Define absolute zero

A

The point at which an ideal gas exerts no pressure

(0K, -273°C, molecules have no kinetic energy)

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9
Q

What is Boyle’s Law?

A

The pressure in a gas is inversely proportional to the volume it occupies

at a fixed temperature

and a fixed mass of gas

(P ∝ 1/V)

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10
Q

What does the P-V graph look like for an ideal gas?

A
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11
Q

How do you prove Boyle’s law graphically?

A

Plot a graph of P against 1/V

Should be a straight line passing through the origin

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12
Q

What is Charles’ Law?

A

Volume a gas occupies is directly proportional to the temperature of the gas

at a fixed pressure

and a fixed mass of gas

(V ∝ T)

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13
Q

How do you prove Charles’ law by graph?

A

Plot a graph of V against T

Should be a straight line passing through the origin

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14
Q

For an ideal gas, what does a graph of V against T(°C) look like?

A

Note: x-intercept represents absolute zero

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15
Q

What is the Pressure law?

A

The pressure of a gas is directly proportional to the temperature of the gas

at a fixed volume

and a fixed mass of gas

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16
Q

How do you prove the pressure law graphically?

A

Plot a graph of P against T

Should be a straight line passing through the origin

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17
Q

For an ideal gas, what does a graph of P against T(°C) look like?

A

Note: x-intercept is absolute zero

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18
Q

What is the ideal gas relationship?

A
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19
Q

When can you use the ideal gas relationship?

A

If the mass of the gas is constant

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20
Q

How do you calculate the work done compressing or expanding a gas?

A

Calculate the area under the curve

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21
Q

What is the general equation for pressure?

A

P = F / A

(Pressure = Force / Area)

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22
Q

How does a gas exert a pressure on a container?

A
  • The gas molecules collide with the container walls changing their momentum.
  • This creates a force on the molecule and the wall
  • Exerting a pressure
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23
Q

What are the 5 conditions for an ideal gas?

A
  1. Volume of the molecules must be much smaller than the volume of the gas itself
  2. The intermolecular forces are negligible
  3. The collision time of molecules with each other and the walls is much less than the time between them
  4. The collisions are elastic (no loss in KE)
  5. The molecules’ motion is random
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24
Q

How does Brownian motion explain the random motion of smoke?

A
  • Air molecules are moving randomly
  • They collide with the smoke changing momentum and exerting a force on the smoke particles
  • If at one moment there are more collisions on one side than the other
  • The smoke particle has a resultant force so accelerates in that direction
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25
Q

Explain Boyle’s Law using the molecular Kinetic Theory

A
  • When volume of container is decreased
  • More collisions per second
  • So total momentum change bigger (▲p)
  • So force exerted bigger
  • So pressure bigger (From P = F/A)
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26
Q

Explain Charles’ Law using the molecular kinetic theory

A
  • When temperature is increased
  • Volume increases to increase the distance travelled between collisions
  • Molecules have greater kinetic energy but travel further so frequency stays same
  • Change in momentum (▲p) stays constant
  • So pressure is constant (P = F/A)
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27
Q

Explain the Pressure law using the molecular kinetic theory

A
  • As temperature increases
  • The average kinetic energy of the molecules increases
  • Increasing the number of collisions per second with container walls
  • So greater change in momentum
  • Greater force and pressure exerted (P = F/A)
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28
Q

How would you use this equation to work out the density of a gas?

A
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29
Q

How do you calculate crms from a list of speeds?

A
  1. Square the speeds and add up
  2. Take a mean of the squares
  3. Square root the value
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30
Q

How is cms calculated?

A

cms = (crms)2

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31
Q

What are the units of cms?

A

[m2s-2]

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32
Q

What does the maxwell-boltzmann distribution tell us about gases?

A

Molecules have a range of kinetic energies.

So temperature of the gas is a measure of the average kinetic energy.

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33
Q

For these equations how do you calculate the internal energy of the gas?

A

Multiply each by the number of molecules of the gas.

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34
Q

How do two objects brought into contact reach thermal equilibrium?

A
  • There is a net flow of thermal energy from the hotter object to the colder object
  • Until both objects are at the same temperature
  • And there is now no net flow of thermal energy
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35
Q

Define specific heat capacity

A

The energy required to increase 1kg of a substance by 1K [Jkg-1K-1]

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36
Q

When would you use this equation?

A

To calculate the mass flowing per kg of a fluid

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37
Q

Why does the temperature of a substance changing state not increase?

A

The thermal energy is used to break some of the intermolecular bonds (solid → liquid) or the rest of the intermolecular bonds (liquid → gas)

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38
Q

Define specific latent heat of fusion

A

The energy required to change the state of 1kg of a solid to a liquid at its melting point.

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39
Q

Define specific latent heat of vaporization

A

The energy required to change the state of 1kg of a liquid to a gas at its boiling point.

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40
Q

What is wrong with this?

A

Haven’t considered the change of states. Need to break it into 3 equations:

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41
Q

How are these two gravitational fields similar? How are they different?

A

Both are uniform (constant field strength)

Closer field lines represent stronger field

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42
Q

How are radial and uniform fields different?

A

Radial fields have a decreasing field strength

(Field lines increasing in separation)

Uniform fields have a constant field strength

(Field lines constant\ separation)

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43
Q

In gravitational fields when can you use the equation EP = mgh?

A

Over small distances

When radial fields are approximately uniform

And g is approximately constant

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44
Q

Why can’t SUVATs be used for radial gravitational fields?

A

SUVATs need a constant acceleration

Radial fields have a variable field strength and so a variable acceleration

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45
Q

What are equipotentials and how are they related to field lines?

A

An equipotential has the same potential along that line

(So no work is done moving along the equipotential)

They are always perpendicular to field lines

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46
Q

What is Newton’s Universal Law of Gravitation?

A

Force acting between two bodies is:

  1. Directly proportional to the product of their masses (F∝m1m2)
  2. Inversely proportional to the square of their separation (F∝1/r2)
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47
Q

Define gravitational field strength and state its units

A

The force acting per unit mass on an object in a gravitational field

[NKg-1] or [ms-2]

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48
Q

In the gravitational field strength equation what does M represent?

A

The mass of the object creating the field

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49
Q

If the Earth is exerting a force on the rocket of 5000N,

What force is the rocket exerting on the Earth?

A

5000N also. An equal and opposite force from Newton’s 3rd Law

(Which has little effect on the Earth because it has so much more mass)

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50
Q

How do you calculate the resultant gravitational field strength at a point between two bodies?

A
  1. Calculate the field strength for each body in turn (ignoring the other one)
  2. Calculate the difference between the field strengths (g is a vector)
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51
Q

How do you you calculate the field strength (or force) neutral point between two bodies in a gravitational field?

A
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52
Q

What is the definition of and the equation for absolute potential energy in a gravitational field?

A

The work done moving an object from infinity to that point in the field

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53
Q

Why is gravitational potential energy always negative?

A
  • Gravitational potential energy is 0 at infinite distance
  • And decreases inwards as you move towards object creating field
  • (So must go negative)
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54
Q

What is gravitational potential?

A

The work done per unit mass moving an object from infinity to that point in a field

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55
Q

In this equation for gravitational potential what object is represented by mass M?

A

The mass of the object creating the field

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56
Q

Which astronaut has a greater loss in gravitational potential energy?

A

Neither. Potential energy (and potential) are scalar quantities so are unaffected by the path

Both decrease by 1440MJ

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57
Q

What is the mistake here?

A

In the second stage the mass of the satellite must be used

(Not the Earth’s mass again)

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58
Q

If a gravitational fields question uses the word ‘height’ what must you do?

A

Height is the distance above the surface

So you must add on the radius of the planet/star/object

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59
Q

Why can’t two objects have a neutral point for gravitational potential? (or GPE)

A

Gravitational potential from both is negative

So they combine

To increase the magnitude of the potential

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60
Q

When can you use these proportionality equations in Gravitational fields?

A

When the mass or masses are constant

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61
Q

In gravitational fields What does a force-separation graph look like?

And what else does the graph tell you?

A

The area under the curve is the change in potential energy moving between the two separations

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62
Q

In gravitational fields What does a field strength-separation graph look like?

And what else does the graph tell you?

A

The area under the curve is the change in potential moving between the two separations

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63
Q

In gravitational fields What does a potential energy-separation graph look like?

And what else does the graph tell you?

A

The gradient of a tangent is the magnitude of the force at that point

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64
Q

In gravitational fields What does a potential-separation graph look like?

And what else does the graph tell you?

A

The gradient of a tangent is the field strength at that point

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65
Q

What is the equation for gravitational field strength within a planet?

(r ≤ R)

A

This part of the graph is linear as g ∝ r

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66
Q

How do you derive the equation for gravitational field strength inside a planet?

A
  • Use general equation for density (M/V)
  • With V as the volume of a sphere (4/3πr3)
  • Sub into general equation for field strength
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67
Q

How do you derive Kepler’s 3rd Law? (r3 ∝ T2)

A
  1. Equate centripetal force to force due to gravity
  2. Substitute in angular speed formula (from circular motion)
  3. Rearrange
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68
Q

How do you derive the formula for the velocity of a satellite orbiting a planet or star?

A
  1. Equate centripetal force to force due to gravity
  2. Rearrange
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69
Q

Which planet has the greatest orbital velocity and why?

A

Mercury

It is closest to Sun so smallest r

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70
Q

How do you derive the formula for the escape velocity of a planet or star?

A
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71
Q

In these 3 equations what does the mass refer to?

A

The mass of the object creating the field

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72
Q

Why does a satellite not need to be above the escape velocity to reach low Earth orbit?

A
  1. Escape velocity only applies to objects without engines (that can’t increase their KE)
  2. Satellite isn’t escaping the field (so doesn’t need as much KE)
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73
Q

How do you calculate the Kinetic Energy of a satellite orbiting a planet?

A

Substitute orbital velocity into equation for kinetic energy

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74
Q

How do you calculate the total energy of an orbiting satellite?

A

Add the kinetic and potential energy together…

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75
Q

What is the difference between a geosynchronous and geostationary orbit?

A

Both have orbital periods of 24 hours (the same as the Earth)

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76
Q

How are geostationary and polar satellites different?

A

Geostationary satellites orbit above the same point of the equator and have an orbital period of 24 hours

Polar satellites orbit over the North and South pole with an orbital period of much less (around 2 hours)

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77
Q

What are polar satellites used for?

A
  • Communication for high latitude regions (close to the poles)
  • Espionage (spying)
  • Meteorology (weather)
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78
Q

What are Geostationary satellites used for?

A
  • Satellite television
  • Mobile Phone Communications
  • GPS
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79
Q

What 4 things do magnetic fields affect?

A
  1. Charges moving in the field
  2. Conductors with a current passing through
  3. Other magnets
  4. Magnetic materials
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80
Q

What do the field lines for a bar magnet look like?

A

Field lines always act North → South

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81
Q

What do the field lines look like between two opposite poles?

A

Field is uniform between the poles

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82
Q

What do the field lines look like between two like poles?

A
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83
Q

How are field lines represented ‘going into the page’?

A
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84
Q

How are field lines represented ‘coming out of the page’?

A
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85
Q

This conductor in a magnetic field has a current passing through

But doesn’t experience a force

Why?

A

Because it is parallel to the field lines

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86
Q

When do you use Fleming’s Left hand Rule?

A
  1. Looking at DC motors
  2. Looking at charges moving in a magnetic field
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87
Q

How do you calculate the force on a conductor placed at an angle in a magnetic field?

A

First use trigonometry to calculate the perpendicular component of its length

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88
Q

Why does the reading on the balance increase when a current runs through the conductor?

A

The magnetic field pushes up on the conductor

So the conductor pushes the magnets down (Newton’s 3rd Law)

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89
Q

How can you increase the mechanical energy produced by the DC motor?

A

Increase the torque by:

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90
Q

What do the commutator rings do in the DC motor?

A

Switch connections of the bars every 180°

So direct current is produced

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91
Q

What happens if the commutators are removed from the DC motor?

A

Force on each bar won’t change

So coil will reach equilibrium in vertical position

And won’t continue spinning

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92
Q

Why does an electron move in a circular path in a magnetic field?

A

Force from magnetic field perpendicular to velocity of electron

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93
Q

How do you apply Flemming’s left hand rule to a negative charge moving in a field?

A

Current acts opposite to the velocity

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94
Q

How do you apply Flemming’s left hand rule to a positive charge moving in a field?

A

Current acts in the same direction as the velocity

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95
Q

When should you use each equation?

A

F=BIL on a conductor in a magnetic field (with current)

F=BQv on a charge in a magnetic field (moving)

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96
Q

How do you calculate the radius of the orbit of a charge moving in a magnetic field?

A

Equate the magnetic and centripetal forces

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97
Q

How do you explain the different curvatures of radiation () passing through a magnetic field?

A

Greater the specific charge → Smaller r (Bigger deflection)

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98
Q

How do you calculate the speed of a charged particle accelerated through an electric field?

A
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99
Q

How much work does a magnetic field do on a moving charge?

A

0J because the force and velocity vectors are perpendicular

So the charge does not increase its kinetic energy

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100
Q

In a mass spectrometer how does the velocity selector work?

A

Unless an ion’s velocity = E/B, it will travel in a parabola and miss the gap

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101
Q

In a mass spectrometer how does the mass separation work?

A

The ions have the same velocity (from the velocity selector)

So deflect by specific charge

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102
Q

In a particle accelerator why are both magnetic and electric fields needed?

A
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103
Q

In the cyclotron what is the purpose of the alternating current and magnetic field?

A

Alternating current → Electric Field between ‘Dees’ → Increases kinetic energy

Magneti Field → Moves particle in circular path in ‘Dees’ → Containing particle

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104
Q

In the cyclotron why is the frequency of the alternating current constant?

A

As the charge speeds up → Travels further in each Dee → So takes same time

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105
Q

How do you calculate the AC frequency of the cyclotron?

A

Note: f is independent of v

So the frequency is constant

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106
Q

How do charges interact in these situations?

A
  1. Like charges repel
  2. Opposite charges attract
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107
Q

Which direction will these charges move?

A

Electric field lines shows direction of Force on +ve charges

(-ve charges are opposite)

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108
Q

How are radial and uniform electric fields different?

A
  1. Radial fields have a varying field strength (weaker when further apart)
  2. Uniform fields have a constant field strength
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109
Q

For electric fields, how are are equipotentials related to the field lines?

A

Equipotentials always perpendicular to field lines

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110
Q

How can you change this situation to increase the force on the charge?

A
  1. Increase field strength
  2. Increase magnitude of charge
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111
Q

What field lines are produced by…

a) +ve charge
b) -ve charge

A

Field lines always act…

  • Away from +ve
  • Towards -ve
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112
Q

How do the field lines look for these two interacting oppositely charged particles?

A
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113
Q

How do the field lines look for these two interacting like charged particles?

A

NOTE: Field lines never cross

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114
Q

What is the electric field strength at the following points?

A

Field strength is constant between parallel plates (capacitor)

E1 = E2 = E3

115
Q

What is the electric potential at the following points?

A

Electric potential linearly increases between parallel plates (capacitor)

116
Q

Define Coulomb’s Law

A
117
Q

Define electric field strength

A

Force per unit charge acting on a small positive charge

118
Q

What force would act on a 5C charge placed at 6NC-1?

A

F=Eq → 5x6 = 30N

119
Q

What is wrong here?

A

Electric field strength ≠ acceleration

120
Q

How do you calculate electric field strength outside a conducting sphere?

A

Treat it as a point charge

121
Q

How do you calculate electric field strength inside a conducting sphere?

A

E=0 everywhere!!!

122
Q

What is the graph of electric field strength for a conducting sphere?

A
123
Q

How is electric field strength calculated here?

A

For parallel plates calculate E first if possible

124
Q

How do you work out the resultant field strength between charges?

A
  1. Work out field strength from each
  2. Label vectors
  3. Add or subtract field strengths
125
Q

Can you use SUVATs here?

A

Yes!

Field strength constant → Acceleration constant

126
Q

Why is potential energy here +ve?

A

Ep = 0 at ∞

Increases as charge moves closer

127
Q

Why is potential energy here -ve?

A

Ep = 0 at ∞

Decreases as charge moves closer

128
Q

Why is potential energy here +ve?

A

Ep = 0 at ∞

Increases as charge moves closer

129
Q

Why is potential energy here -ve?

A

Ep = 0 at ∞

Decreases as charge moves closer

130
Q

Define electric potential

A

Work done per coulomb moving positive charge from infinity to that point

131
Q

Why does the moving charge’s potential energy increase?

A

Equipotentials show the change in potential of a +ve charge

If +ve charge → decreases

If -ve charge → increases

132
Q

How are these charges different?

A

Q is the charge creating the field

q is the charge moving in the field

133
Q

What is wrong here?

A

Electric potential is scalar

But they are opposite → must be subtracted

134
Q

How do you calculate the neutral electric field strength (or force) point between charges?

A
135
Q

How do you calculate the neutral electric potential point between charges?

A
136
Q

What is the graph of electric force for a conducting sphere?

A

Same as field strength graph

137
Q

What is the graph of electric potential for a positively charged conducting sphere?

A
138
Q

What is the graph of electric potential for a negatively charged conducting sphere?

A
139
Q

What two situations produce a uniform electric field?

A
  1. Radial field over a short distance
  2. Field between 2 parallel plates
140
Q

Define Capacitance

A

Charge stored per unit Volt [F]

141
Q

What do the gradient and area under this graph represent?

A

Gradient → Capacitance

Area → Work done (Energy Stored)

142
Q

What is wrong with this?

A

C = capacitance → not the charge!!!

143
Q

When building a capacitor how do you maximize the capacitance?

A
  1. Increase the area of the plates
  2. Decrease the plate separation
  3. Place dielectric between plates
144
Q

What does it mean if the relative permittivity of a dielectric (εr) is 5.0?

A

The capacitor stores 5x more charge with the dielectric between the plates!

145
Q

How does adding a dielectric increase the capacitance of a capacitor?

A
  1. Dielectric contains polarised molecules
  2. They align with the field between the plates
  3. Bigger negative charge attracts more electrons onto negative plate
  4. Repels more electrons away from positive plate
  5. V same but Q has increased
146
Q

What happens if the dielectric is removed?

(Capacitor still connected to battery)

A
  1. Polarised molecules removed
  2. Some electrons leave negative plate
  3. Attracts more electrons to positive plate
  4. Q has decreased but V same
  5. C decreases (C=Q/V)
147
Q

What happens if the dielectric is removed?

(When the Capacitor is disconnected from battery)

A
  1. Polarised molecules removed
  2. But charge is trapped on plates
  3. Same Q but with lower C
  4. V increases (V=Q/C)
148
Q

How does this capacitor charge?

(When switch 1 is closed)

A
  1. Electrons flow from the negative terminal of the battery
  2. To the connected parallel plate (right plate)
  3. Electrons are repelled from the opposite plate (left)
  4. And attracted to the positive terminal of the battery
  5. Charge across Parallel plates
149
Q

How does this capacitor discharge?

(When switch 2 is closed)

A
  1. Electrons flow from the negative plate (right)
  2. Through the resistor
  3. To the other plate (left)
  4. Decreasing charge difference across plates
150
Q

Define time constant

A

Time constant is how long it takes for a capacitor to…

  1. Charge to 63% of max charge (0.63Q0)
  2. Discharge 63% of Q0 (down to 0.37Q0)
151
Q

What factors affect the time constant of a circuit?

A
  1. The resistance of the components in the circuit (Capacitor R=0)
  2. Capacitance of the capacitor
152
Q

Complete this discharging curve for a capacitor

A
153
Q

Complete this discharging curve for a capacitor

A
154
Q

Complete this discharging curve for a capacitor

A
155
Q

Complete this charging curve for a capacitor

A
156
Q

Complete this charging curve for a capacitor

A
157
Q

Complete this charging curve for a capacitor

A
158
Q

How do you read off the time constant from this graph?

A

Read off time when charge (or current or voltage) has decreased to 37% initial

159
Q

How do you read off the time constant from this graph?

A

Read off time when charge (or current or voltage) has increased to 63% final

160
Q

Explain why the I-t graph is exponential when a capacitor discharges

A
  1. Potential difference across capacitor drives large current through resistor
  2. Charge across plates decreases
  3. Potential difference across the plates decreases
  4. Current gets smaller and smaller
161
Q

Explain why the I-t graph is exponential when a capacitor charges

A
  1. Battery drives current round circuit
  2. Charge build up on capacitor plates
  3. Potential difference builds up across plates
  4. Difference in PD between battery and capacitor gets less
  5. So smaller push on electrons
  6. Smaller current
162
Q

What is wrong here?

A

80% is the decrease in charge (∆Q)

So it discharges to 20% of initial Q=0.2Q0

163
Q

How do you make a capacitor charge/discharge at a constant rate?

A

Use a variable resistor

Decreasing resistance

To keep charging/discharging current constant

164
Q

How do the graphs change if a capacitor is charging at a constant rate?

A

Current → Constant

Voltage and Charge → Linear

165
Q

How do the graphs change if a capacitor is discharging at a constant rate?

A

Current → Constant

Voltage and Charge → Linear

166
Q

How do you show Q=0.37Q0 after 1 time constant?

A

Set t=RC

167
Q

What is wrong here?

A

Capacitor is discharging at a constant rate

So current is constant

168
Q

How does the potential difference of the resistor change as the capacitor charges?

A

NOTE: VR+VC=V0

169
Q

How does the potential difference of the resistor change as the capacitor discharges?

A

NOTE: VR+VC=V0

170
Q

For a discharging capacitor, what does the gradient of the Q-t graph give?

A

Current at that instant

171
Q

For a charging capacitor, what does the gradient of the Q-t graph give?

A

Current at that instant

172
Q

For a discharging capacitor, what does the area of the I-t graph give?

A

Charge lost in that region

173
Q

For a charging capacitor, what does the area of the I-t graph give?

A

Charge gained in that region

174
Q

How is magnetic flux calculated?

A

Magnetic flux density x Perpendicular area

175
Q

How do you calculate the effective flux through this coil?

A

Must take component of field perpendicular to surface

(Or component parallel to normal of surface)

176
Q

How does the number of turns in a coil affect its flux?

A

Flux doesn’t change

But flux linkage increases

177
Q

How do you find the flux linkage through this coil?

A

Must take component of field perpendicular to coil

(Or component parallel to normal of coil)

178
Q

How do you induce an emf in this coil?

A

Moving the magnet in and out of the coil
Causes a change in flux linkage
(Faraday’s Law)

179
Q

If the magnet is stationary why is there no emf induced?

A

Emf only induced if the flux linkage is changing

(Faraday’s Law)

180
Q

Which way does the current act in this coil and why?

A

Current acts to create a magnetic field opposing the increase in flux linkage
(Tries to keep magnet from entering)

Due to Lenz’s law

181
Q

Which way does the current act in this coil and why?

A

Current acts to create a magnetic field opposing the decrease in flux linkage
(Tries to keep magnet from leaving)

Due to Lenz’ law

182
Q

Why does moving the magnet faster induce a larger emf?

A

The rate of change of flux linkage is greater

(Faraday’s law)

183
Q

What is Faraday’s Law of electromagnetic induction?

A

The magnitude of the emf is proportional to the rate of change of flux linkage

184
Q

What is Lenz’ law?

A

The direction of an induced emf tends to produce a current which opposes the
change causing it

185
Q

Why can’t the current act in this direction?

A

The magnetic field produced attracts the magnet
Increasing its kinetic energy
Energy has been created out of nothing!!!

(Against Lenz’ law)

186
Q

What is the right hand grip rule?

A

A current in a coil produces a magnetic field as shown

187
Q

What is wrong here?

A

Flux linkage becomes negative when coil flips!!!

188
Q

How is the induced emf related to the graph of flux linkage?

A

(From Faraday’s law)

189
Q

What is the corresponding graph of induced emf?

A

emf = -ve gradient

190
Q

If the coil moves across the field at a constant speed what do the flux linkage and emf graphs looks like?

A

emf = -ve gradient

191
Q

How do you calculate the emf induced between the ends of this conducting bar moving across a magnetic field?

A
192
Q

When do you use Fleming’s Right Hand Rule

A

For generators!!!

193
Q

For the AC generator when is the max emf induced?

A

When change in flux linkage is max

(When flux linkage = 0)

194
Q

For the AC generator when is 0 emf induced?

A

When change in flux linkage is 0

(When flux linkage = max)

195
Q

What is the corresponding emf graph for the AC generator?

A
196
Q

How is the max induced emf calculated for the AC generator?

A
197
Q

How do you increase the max induced emf of the AC generator?

A

Increase any of the terms in the equation:

198
Q

How is the AC generator different to the DC motor?

A

Generator: Kinetic energy -> Electrical energy

(Motor: Electrical energy -> Kinetic energy)

Generator: Slip rings keep each side of coil connected to same side of circuit

(Motor: Commutator ring switch polarity every half cycle)

199
Q

What are eddy currents?

A

Currents produced in a conductor by magnetic fields

(Lenz’ law in conductors)

200
Q

Why does the magnet take longest to fall through the full copper pipe?

A
  1. Magnet in freefall
  2. Plastic not a conductor so no eddy currents (still in freefall)
  3. Copper pipe incomplete so can’t create eddy currents (still in freefall)
  4. Eddy current reduce acceleration
201
Q

How are eddy currents created in this copper pipe?

A
  1. Flux linkage decreasing above -> current creates attracting field upwards
  2. Flux linkage increasing below -> current creates repelling field upwards
202
Q

How does eddy current braking work?

A
  1. Part of disk leaving field -> current creates attraction to electromagnet
  2. Part of disk entering field -> current creates repulsion to electromagnet
203
Q

When can eddy current braking not be used?

A

To hold a car stationary on a slope

(no change in flux linkage)

204
Q

How does the oscilloscope trace look for an AC current?

(When the time base is switched on)

A
205
Q

How does the oscilloscope trace look for an AC current?

(When the time base is switched off)

A
206
Q

How does the oscilloscope trace look for an DC current?

(When the time base is switched on)

A
207
Q

How does the oscilloscope trace look for an DC current?

(When the time base is switched off)

A
208
Q

For AC supply what is Vrms and how is it calculated?

A

The equivalent DC voltage that would supply the same average power

209
Q

For AC supply what is Irms and how is it calculated?

A

The equivalent DC current that would supply the same average power

210
Q

Label V0 and Vp→p on this AC oscilloscope trace

A
211
Q

What is wrong with this calculation?

A

When using electricity formulas must use rms values for voltage (and current)

212
Q

How does a step up transformer work?

A
  1. AC current flows through primary coil
  2. Magnetic field flows through secondary coil
  3. Changing flux linkage in secondary coil larger
  4. Greater emf induced (so bigger voltage)
213
Q

How does a step down transformer work?

A
  1. AC current flows through primary coil
  2. Magnetic field flows through secondary coil
  3. Changing flux linkage in secondary coil smaller
  4. Smaller emf induced (so smaller voltage)
214
Q

Why does a transformer only work with AC supply?

A

If supply is DC
Flux linkage in secondary coil doesn’t change
So emf isn’t induced (Faraday’s law)

215
Q

How do you calculate the voltage (rms) in the secondary coil?

A

This equation always works (no matter what efficiency)

216
Q

How do you calculate the efficiency of a transformer?

A

(The voltages and currents must be rms values)

217
Q

What are the main causes of thermal loss in a transformer?

A
  1. Large Eddy currents in magnet (P=I2R)
  2. Large currents in primary and secondary coils
  3. Hysteresis losses (magnet’s resistance to change in flux linkage)
  4. Flux losses (not all flux passing through secondary coil)
218
Q

How are the main causes of thermal losses in a transformer reduced?

  1. Large Eddy currents in magnet
  2. Large currents in primary and secondary coils
  3. Hysteresis losses
  4. Flux losses
A
  1. Laminate the core
  2. Use wire with low resistance
  3. Use soft iron core for magnet
  4. Wind primary coil over secondary coil
219
Q

Why are step up transformers used to transport electricity over long distances?

A

Smaller currents = smaller energy losses to thermal

(P=I2R)

220
Q

What happened in the Rutherford scattering experiment?

A
  1. Most alpha particles passed through gold leaf undeflected
  2. Some were slightly deflected
  3. A tiny proportion (1 in 8000) reflected
221
Q

What did Rutherford’s experiment reveal about the atom?

A
  1. Atom is mostly empty space
  2. Centre of atom is small, dense and positively charged (nucleus)
222
Q

Compare the ionising power of the three main types of radiation

A
  • *Alpha most ionising**
  • *Beta medium ionising**
  • *Gamma least ionising**
223
Q

Compare the penetrative power of the three main types of radiation

A
  • *Gamma most penetrating**
  • *Beta medium penetrating**
  • *Alpha least penetrating**
224
Q

Compare the range of the three main types of radiation

A 
Alpha = 3-7cm
Beta = 0.2-3m
Gamma = Very long distance
225
Q

What are the main sources of background radiation?

A
226
Q

How does a Geiger-Muller tube detect radiation?

A
  1. Radiation ionises gas in tube
  2. Negative ions attracted to metal rod
  3. Positive ions attracted to casing
  4. Small current generated in circuit
227
Q

How is radiation used to control the thickness of metal?

Why can’t alpha radiation be used?

A

If detector count too low -> metal too thick -> Rollers move closer together

If detector count too high -> metal too thin -> Rollers move apart

Alpha won’t be detected

228
Q

What materials can shield the three main types of radiation?

A
  • *Alpha = Paper**
  • *Beta = Aluminum**
  • *Gamma = Lead or several meters of concrete**
229
Q

How is radiation used in a smoke detector?

A

Alpha radiation ionizes air between detector
Ionized air causes current to flow
Smoke blocks ionization of air
Current stops
Alarm sounds

230
Q

If the detector is moved 3x further away what will happen to the count rate?

A

9x smaller

Gamma radiation follows inverse square law

231
Q

How should you safely use radiation?

A
  1. Minimise exposure time
  2. Maximize distance from source
  3. Store in shielded containers
  4. Don’t consume food or drink near source
232
Q

How is radiation used for medical imaging?

A

Medical tracer with short half life injected

Tumors absorb radionuclides and emit gamma

Gamma detected outside body

233
Q

How is radiation used to destroy tumors?

A

Gamma radiation focused on tumor

High energy breaks apart tumor

Low levels through other tissue

234
Q

Sketch the graph of nuclear stability

A
235
Q

On this graph of nuclear stability highlight regions of the decays…

  1. α
  2. β-
  3. β+
  4. Proton emission
  5. Neutron emission
A
236
Q

What makes a nucleus unstable? (and radioactively decay)

A
  1. An incorrect balance of protons and neutrons (off line of stability)
  2. Too many nucleons
  3. Nucleus in excited state
237
Q

What is electron capture and what is it’s equation?

A

Proton captures inner shell electron and becomes neutron

238
Q

What two forms of radiation are released after electron capture?

A

X-ray → electron de-excites to fill inner shell

γ → Nucleus reorders and de-excites

239
Q

How does distance of closest approach work?

A

KE at distance → PE closest

Use to get rough size of nucleus

240
Q

What two graphs could you plot to prove this relationship?

A
241
Q

What does r0 represent?

A

Average radius of each nucleon

242
Q

How do you calculate the average density of a nucleus?

A
243
Q

Why is the average nucleus density so large? (∼2.3x1017kgm-3)

A

Atom is mostly empty space

244
Q

How was electron scattering used to determine nuclear diameter?

A

Graph plotted

First minima used to calculate diameter

(Don’t need to know equation)

245
Q

How is electron scattering better than alpha recoil to determine nuclear radius?

A

Alpha Recoil

  • Closest approach so only an estimate
  • Recoil of nucleus not considered
  • Effect of strong force not known

Electron Scattering:

  • Not affected by strong force (leptons)
  • Electron λdb tunable
246
Q

Define the decay constant λ

A

The probability that an unstable isotope decays in one second

247
Q

Define the activity, A, of a radioactive sample

A

The total number of unstable isotopes that decay after one second

248
Q

What does the activity, A, of a radioactive sample depend on?

A
  1. The decay constant λ
  2. The number if unstable isotopes N
249
Q

Define the half life, T½, of a radioactive sample

A

Time taken for either…

  1. Activity of sample to halve
  2. Number of unstable isotopes remaining to halve
250
Q

How do you derive the half life T½ equation?

A

Set N as 0.5N0

251
Q

How do you prove this graph is exponential?

A

Find multiple T½ and compare

252
Q

What’s wrong with this calculation?

A

Activity and time must be the same units

253
Q
  1. Derive the equation of this graph
  2. What are the gradient and y-intercept?
A
254
Q

What is the gradient of this graph?

A

r0 → average radius of nucleon

255
Q

What do the gradient and y-intercept of this graph represent?

A

where r0 → average radius of nucleon

256
Q

When should you treat the neutron and proton as having slightly different masses?

A

Dealing with fusion or fission

(Calculating mass defects, binding energies or mass difference)

257
Q

Define mass defect (∆m)

A

Mass lost when nucleons (protons and neutrons) come together to form nucleus

258
Q

Define binding energy

A

Energy released when nucleons (protons and neutrons) come together to form nucleus

259
Q

How is binding energy related to mass defect (∆m)?

A
260
Q

Define 1 atomic mass unit

A

Mass of 1/12 of a carbon-12 atom

(1.661x10-27kg)

261
Q

What’s the next step here?

A

Multiply by 931.5MeV

262
Q

What’s the next step here?

A

Use E=mc2

263
Q

Sketch the binding energy per nucleon graph

A
264
Q

Label the fusion and fission regions

Why are they there

A

Energy only released if binding energy per nucleon increases

265
Q

Define metastable state in radioactive decay

A

Long-lived excited state of nucleus

Eventually it de-excites emitting γ

266
Q

How many possible decays are there?

A

2β- and 3γ

267
Q

What is nuclear fission?

A

Heavy nucleus splits into two lighter nuclei releasing energy and neutrons

268
Q

What are the 2 main isotopes used as fuel in nuclear reactors?

A

U-235, U-238

269
Q

How do you calculate the energy released in this fission reaction?

A

Calculate the mass difference (∆mdiff)

270
Q

How do you calculate if a reaction is possible?

A

If mass difference is positive reaction is possible

271
Q

What are the main components of the nuclear reactor?

A
  1. Fuel rods - U-235
  2. Control rods - Boron
  3. Moderator - water or graphite
  4. Coolant - CO2 or water
272
Q

What does the moderator do in a nuclear reactor?

A

Reduce neutrons’ speeds to thermal speeds

(More likely to be absorbed by U-235)

273
Q

What do the control rods do in a nuclear reactor?

A

Absorb some neutrons

Stop chain reaction occurring

274
Q

What are the main safety features of the nuclear reactor?

A
  1. Reaction happens inside thick walled concrete vessel
  2. Control rods fully inserted if meltdown starts
  3. Reactor flooded with water to remove thermal energy if meltdown starts
275
Q

In nuclear reactors how are spent fuel rods disposed?

A
  1. Removed remotely from reactor
  2. Stored in cooling ponds for up to 1 year
  3. Vitrified by mixing with molten glass
  4. Sealed in barrels
  5. Stored in mountains or deep underground
276
Q

What is nuclear fusion?

A

Two lighter nuclei are combined to form one heavier nuclei and release energy

277
Q

Why is nuclear fusion so difficult to achieve?

A

Requires incredibly high temperatures and pressures

  1. To ionise the isotopes
  2. To bring isotopes close enough to overcome electromagnetic repulsion
278
Q

In fusion how close do the ionised isotopes need to get?

A

Close enough for the strong force to be larger than electromagnetic

(<1fm)

279
Q

What are the two main fusion isotopes in stars (in their main sequence)

A

Deuterium → H-2

Tritium → H-3

280
Q

What does the coolant do in a nuclear reactor?

A

Transfer heat from fuel rods to the water that spins the turbines

281
Q

How is 1 Tesla defined?

A

The magnetic field that applies a force of 1N to a 1 metre conductor with a 1A current flowing perpendicular to the field (B=F/IL)

282
Q

If this unstable isotope of caesium decays by α emission where does it end up on the graph?

A

N → neutron number

Z → Proton Number

283
Q

If this unstable isotope of caesium decays by β- emission where does it end up on the graph?

A

N → neutron number

Z → Proton Number

284
Q

If this unstable isotope of caesium decays by β+ emission where does it end up on the graph?

A

N → neutron number

Z → Proton Number

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