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AQA A Level Physics > 7. Fields and their Consequences > Flashcards

7. Fields and their Consequences Flashcards

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1
Q

When can charge transfer between two objects?

A

When they slide past each other

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2
Q

When two objects slide past each other, how is charge transferred?

A

Electrons leave one surface and join the other

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3
Q

What is earthing?

A

When electrons move to or from the earth to balance charges on charged objects

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4
Q

What happens, when charge is transferred between objects, if one of the objects is an insulator?

A

The charge can build up

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5
Q

Why does charge build up on the dome of a Van de Graaf generator?

A

The dome is metal, but charge builds up as it is isolated

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6
Q

What happens when a charged polythene rod is brought near to uncharged objects?

A

It attracts the object

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7
Q

How do you get charge onto an electroscope?

A

∙ charged object e.g. polythene rod is brought near the metal plate

∙ gold leaf moves towards rod

∙ electroscope is earthed so charge is stored

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8
Q

How can electric fields be represented?

A

Field lines

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9
Q

What do field lines show?

A

The direction of the force that would be felt by a small positive charge

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10
Q

What do equally spaced field lines show?

A

A uniform field

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11
Q

What practical can be done to plot field lines?

A

Electrolytic tanks and conducting paper

∙ damp filter paper, potassium permanganate and 250V electrodes

∙ plot equipotential lines using a point probe attached to a voltmeter (field lines plotted perpendicular to equipotential line)

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12
Q

What are the rules about field lines?

A

∙ never start or stop in empty space

∙ never cross

∙ density of field lines shows strength of field

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13
Q

Where do field lines stop and start?

A

Either on a charge or at infinity

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14
Q

Why do field lines never cross?

A

If they did, a small positive charge place there would feel forces in different directions, which could be resolved into the one true direction of the field there

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15
Q

What is a neutral point?

A

A point exactly between two like charges where no field exists

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16
Q

What is the equation for electric field strength?

A

E = F / Q

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17
Q

What is field strength?

A

The force per unit (positive) charge exerted by the field

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18
Q

What are the units for electric field strength?

A

N C-1

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19
Q

Summary of Coulomb’s law?

A

The force between two point charges is

∙ directly proportional to each of the charges Q1 and Q2

∙ inversely proportional to the square of their separation

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20
Q

What is the equation for Coulomb’s law?

A

F = kQ1Q2 / r²

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21
Q

What does k depend on in F=kQ1Q2 / r²?

A

The permittivity of the substance separating the charges

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22
Q

What permittivity does every insulating material have?

A

Greater than the permittivity of free space

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23
Q

What does it mean, in terms of charges, that the permittivity of water is about 80 times the permittivity of free space?

A

Makes the forces between charges 1/80th of value - when salt is put in water the forces are reduced and crystal structure collapses

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24
Q

What equation do you get when the electric field and coulombs law equation are combined?

A

E = kQ / r²

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25
Q

What does k equal in E = kQ / r²?

A

1 / 4πℇ

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26
Q

How can the strength of the uniform field between parallel plates be made stronger?

A

∙ increase p.d. across plates

∙ moving plates closer together

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27
Q

Equation for strength of a uniform field?

A

E = V / d

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28
Q

What can electric field strength be measured in?

A

Vm-1 or NC-1

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29
Q

Why can electric field strength be measured in Vm-1 or NC-1?

A

Work is done moving an object, and work is done when a charge moves through a p.d.

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30
Q

How are electrons used in TVs, oscilloscopes and X-ray machines?

A

∙ electron gun produces electrons by thermionic emission

∙ then they are accelerated by an electric field

∙ as electrons accelerate across field, they lose potential energy but gain kinetic

(gain in Ek = loss in Ep)

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31
Q

What happens in an electron deflection tube?

A

Moving electrons pass through an electric field between two plates

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32
Q

What is the equation for the force on an electron in an electron beam, and why is this?

A

∙ F = e V / d

(as E=V/d and force on an electron charge is F=Ee

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33
Q

What is the shape of the path when electron beams are deflected?

A

Curved, as the force (F=eV/d) is constant

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34
Q

What is the difference between electric and gravitational fields, in terms of what ‘feels’ the force?

A

∙ electric - charge q

∙ gravitational - mass m

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35
Q

What is the difference between electric and gravitational fields, in terms of their definition?

A

∙ electric - force per unit charge

∙ gravitational - force per unit mass

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36
Q

What is the difference between electric and gravitational fields, in terms of their constant of proportionality?

A

Both inversely proportional to r²

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37
Q

What is the difference between electric and gravitational fields, in terms of their force equation?

A

∙ electric - F=Eq

∙ gravitational - F=mg

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38
Q

What is the difference between electric and gravitational fields, in terms of their direction of force?

A

∙ electric - like charges repel, unlike charges attract

∙ gravitational - all masses attract

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39
Q

What is the difference between electric and gravitational fields, in terms of their relative strength?

A

∙ electric - strong at close range (responsible for chemical bonding)

∙ gravitational - weak except for massive bodies (responsible for motion of planets)

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40
Q

Do all points in an electric field have an absolute electric potential?

A

Yes

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41
Q

What is absolute electric potential?

A

The electric potential energy that a unit positive charge would have at that point

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42
Q

What is absolute electric potential calculated using?

A

V = 1/4πℇ x Q/r

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43
Q

What does r equal in V = 1/4πℇ x Q/r?

A

Distance from the charge Q

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44
Q

What does the sign of electric potential depend on?

A

The sign of Q (V is +ve when Q is +ve and the force is repulsive)

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45
Q

When will V (electric potential) be zero?

A

When r is

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46
Q

For a V-r graph, what does the gradient of a tangent to the graph give?

A

The field strength at that point

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47
Q

When will two points in an electric field have an electric potential difference between them?

A

If they have a different absolute electric potential

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48
Q

What is electric potential difference?

A

The energy required to remove a unit charge between two points in an electric field that have a different electric potential

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49
Q

How can electric potential difference be found?

A

Using the area under the graph of E against r

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50
Q

Where are field lines in relation to equipotential lines?

A

Perpendicular

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51
Q

What happens in terms of energy when you travel along an equipotential line?

A

No work is done so no energy is transferred

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52
Q

For a charged sphere, where may charge considered to be?

A

At the centre

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53
Q

When is electric potential zero?

A

At infinity

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54
Q

What equation links V to E?

A

E = ΔV/Δr

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55
Q

What is G?

A

The universal gravitational constant

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56
Q

What is the unit of G?

A

N m2 kg-2

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57
Q

What does the formula F = Gm1m2/r2 apply to?

A

Point masses, but spherical masses can be treated as point masses with all their mass concentrated in the centre

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58
Q

Is the force in F = Gm1m2/r2 attractive or repulsive?

A

Always attractive

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59
Q

What is a field?

A

A region of space around an object where other bodies feel a force due to it

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60
Q

Is your pull on the earth the same as the earth’s pull on you?

A

Yes, but your gravitational field is far weaker

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61
Q

What is field strength?

A

A point in a body’s field as the gravitational force exerted on an object placed at that point, per kg of the object’s mass

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62
Q

If two objects of different masses are placed at the same point in a field will they experience the same field strength and gravitational force?

A

Will experience same field strength, but different gravitational forces

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63
Q

When calculating gravitational field strength, what is the mass that matters?

A

The mass below you only

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64
Q

Where does g = GM/r2 come from?

A

When objects are a distance from earth e.g. other planets

  • F = Gm1m2/r2
  • small mass m, and planet mass M - F = GMm/r2
  • but F=mg so mg = GMm/r2
  • therefore g = GM/r2
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65
Q

What is Kepler’s third law?

A

The time of one orbit T, and the distance from the planet to the sun r, are related by T² ∝ r³

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66
Q

How is T² ∝ r³ proven?

A
  • planet remains in orbit due to centripetal force (F=mv2/r)
  • F=Gm1m2/r2
  • GMm/r2 = mv2/r
  • but v = 2πr/T
  • so GM/r2 = 4π2r/T2
  • T2 = (4π2/GM) r3
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67
Q

What are geostationary satellites used for?

A

Communications

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68
Q

How many geostationary satellites would be able to cover the entire earth?

A

3 placed into orbit 120 degrees apart above the equator

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69
Q

Why are the energy and costs required for launching a satellite into a geostationary orbit high?

A

Because they have to be launched so high

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70
Q

Equation for GPE in a uniform field?

A

Change in GPE = mgh

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71
Q

What is the value of GPE at infinite distance?

A

0

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72
Q

What is GPE proportional to, in terms of r?

A

1/r

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73
Q

What is the equation for GPE in a radial field?

A

GPE = –GMm/r

note this is not change in GPE

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74
Q

When GPE = –GMm/r, what happens to GPE as you move closer to M?

A

GPE becomes more negative as at infinite distance GPE=0

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75
Q

Why did someone come up with gravitational potential?

A

To get an expression involving energy which is independent of the mass placed in a field

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76
Q

What are the two ways gravitational potential can be thought about?

A
  1. GP at a point in a field is the energy per unit mass (V=Ep/m so V=gh)
  2. GP is work done per unit mass in moving a small object from infinity to that point (ΔV=ΔE/m)
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77
Q

Is gravitational potential a scalar or vector?

A

Scalar

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78
Q

Is gravitational field strength vector or scalar?

A

Vector

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79
Q

Is the force between two masses vector or scalar?

A

Vector

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80
Q

Is gravitational potential energy vector or scalar?

A

Scalar

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81
Q

What is the unit for gravitational potential?

A

J kg-1

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82
Q

What do equipotentials do?

A

Join points of equal potential

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83
Q

When V = -GM/r is used to calculate V at earth’s surface, V = -63 MJ kg-1. What does this mean?

A

63mJ of work needs to be done in order to move 1kg from the earth’s surface to infinity

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84
Q

What type of graph is the graph of V against r?

A

A 1/r curve, not an inverse-square

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85
Q

What is the derivation for the escape velocity equation?

A
  • Ep = -GMm/r and Ek=1/2mv²
  • energy is conserved so Ep+Ek=0
  • -GMm/r + 1/2mv² = 0
  • so v = √2GM/r
  • or √2gr as gr=GM/r
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86
Q

What is a null point?

A

A point between two masses where the resultant g force is zero

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87
Q

What are low earth orbits?

A
  • satellite systems used in telecommunications

* orbit between 400 and 1,000 miles above the earth’s surface

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88
Q

What is a synchronous orbit?

A

An orbit where the satellite has a period equal to that of the body being orbited

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89
Q

What is the radius of a geostationary orbit?

A

approx 42000km

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90
Q

What do capacitors do?

A

Store electric charge

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91
Q

Where are capacitors used?

A

In almost all electric circuits

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92
Q

What does a capacitor consist of?

A

Two parallel metal plates separated by an insulator called a dielectric

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93
Q

What is a dielectric?

A

The insulator in a capacitor that separates the two metal plates

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94
Q

When will a capacitor have a greater capacitance?

A

When it can store more charge

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95
Q

What is the capacitance of a capacitor?

A

The charge stored per unit of potential difference across it

96
Q

What is the equation for capacitance?

A

C = Q/V

97
Q

What is the unit of capacitance?

A

the farad, F

=1CV⁻¹

98
Q

Are farads large or small units?

A

Very large

99
Q

How are farads usually marked?

A

In pico or microfarads

1 pF = 10⁻¹²

100
Q

What may a capacitor have marked on it?

A

A working voltage which must not be exceeded

101
Q

Roughly, how much electric charge do capacitors store?

A

Small amounts, providing power for a short amount of time

102
Q

Why can charged capacitors be dangerous?

A

They can discharge all of their charge in a fraction of a second

103
Q

Where can capacitors be used?

A
  • camera flash
  • back up power supply
  • to smooth dc power supplies
104
Q

What happens in terms of electrons when a capacitor is charged?

A
  • electrons are pushed onto one plate and off the other
  • the power supply does work on the electrons so the Ep increases
  • this Ep is stored in the electric field between the plates
105
Q

In a V-Q graph, what is the area underneath the graph?

A

Energy stored

106
Q

Explain how a capacitor ‘gets’ charge.

A
  • during charging electrons flow from -ve terminal of power supply to one plate of capacitor and from other plate to +ve terminal
  • switch closed, charging starts, rate of flow of charge is large (i.e. big current) and this decreases with time and plates become more charged so resist further charging
107
Q

What happens, when charging a capacitor, to the rate of flow of charge?

A
  • at first rate of flow of charge/current is large

* this decreases with time as the plates become more charged so resisting any further charging

108
Q

What effect would adding a resistor in the circuit when charging a capacitor have?

A

Only affects time taken for capacitor to become fully charged and not the eventual p.d. across it

109
Q

What does the gradient of the tangent to the curve at a point on a Q-t graph give?

A

The current at that time

110
Q

How to find current at a particular time using a Q-t graph?

A

Gradient of tangent at a certain point

111
Q

On a current-time graph, what does the area underneath the curve give?

A

The charge stored

112
Q

What is T½ on a Q-t graph?

A

The time for the charging current to half

113
Q

What actually happens in a discharge circuit?

A
  • as soon as switch is closed, ‘large’ current flows and p.d. across capacitor drops
  • as charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls
  • rate of decrease of p.d. also falls
114
Q

When is a capacitor fully discharged?

A

When the charge on the plates is zero and the current and p.d. are also zero

115
Q

How does a resistor affect the discharge of a capacitor?

A

The value of the resistor doesn’t affect the final p.d., only the time it takes to reach this value

116
Q

How does a larger resistor affect the time taken for a capacitor to discharge?

A

The bigger the resistor, the longer time taken to discharge

117
Q

What can a discharge curve apply to?

A

V, Q or I against time

118
Q

What is the area under the discharge curve of a I-t graph equal to?

A

The charge that has flowed

119
Q

What is the gradient of the tangent on a Q-t graph equal to?

A

The current at that point in time

120
Q

What is half life equal to?

A

0.69RC

121
Q

What is the time constant?

A

The time taken for the p.d. across the capacitor and the charge stored on the capacitor to drop to 1/e of their original values

122
Q

What is the gradient of the graphs of loge(current) etc. graphs?

A

All have same gradient; -1/RC

123
Q

What equation shows the factors affecting the capactiance of a parallel plate capacitor?

A

C = Aε0εr/d

124
Q

What does C equal in C = Aε0εr/d?

A

Capacitance (F)

125
Q

What does A equal in C = Aε0εr/d?

A

Cross sectional area of overlap of the plates (m²)

126
Q

What does ε0 equal in C = Aε0εr/d?

A

Permittivity of free space (Fm⁻¹)

127
Q

What does εr equal in C = Aε0εr/d?

A

Relative permittivity of the dielectric

128
Q

What does d equal in C = Aε0εr/d?

A

Distance between the plates (m)

129
Q

What is relative permittivity also known as in terms of capacitors?

A

The dielectric constant of the material

130
Q

What is εr calculated from?

A

εm / ε0

where εm is the permittivity of the material used as the dielectric

131
Q

What is the unit of εr?

A

No unit

132
Q

What is a dielectric material (dielectric for short)?

A

An electrical insulator that can be polarised by an applied electric field

133
Q

What happens when a dielectric is placed in an electric field?

A

Electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarisation

134
Q

When does dielectric polarisation occur?

A

When a dielectric is placed in an electric field and the electric charges shift from the average equilibrium positions

135
Q

What is the effect of dielectric polarisation?

A
  • +ve charges are displaced toward field and -ve shift in opposite direction
  • creates internal electric field that reduces overall field within dielectric so reducing p.d. across capacitor
136
Q

How is an internal electric field created in a capacitor?

A

The dielectric is placed in an electric field, dielectric polarisation occurs and creates an internal electric field

137
Q

What is the effect of an internal electric field in a dielectric?

A

It reduces the overall field within the dielectric itself, reducing the p.d. across the capacitor

138
Q

How can the p.d. return to its original value after dielectric polarisation?

A

Requires the addition of more charge onto the plates

139
Q

What effect does dielectric polarisation have on the capacitor overall?

A

The capacitor can store more charge for the same p.d. and increase its capacitance

140
Q

When will a wire in a magnetic field experience a force?

A

When it carries a current

141
Q

How can the direction of the force on a current carrying wire be worked out?

A

Using Fleming’s left hand rule

142
Q

What does each finger mean in Fleming’s left hand rule?

A

First finger = field

Second finger = Current

Thumb = Thrust/Force

143
Q

What equation is used to work out the magnitude of force on a current-carrying wire?

A

F = BIL

144
Q

What does F mean in F=BIL?

A

Force (N)

145
Q

What does B mean in F=BIL?

A

Flux Density (T)

146
Q

What does L mean in F=BIL?

A

Length of conductor in field (m)

147
Q

What does I mean in F=BIL?

A

Current (A)

148
Q

How is the tesla defined using words?

A

One newton per amp per metre

149
Q

How is the tesla defined using the equation?

A

1T = 1 N/Am

150
Q

How can the strength of a magnetic field be measured?

A

By the force per unit current per unit length acting on a current carrying conductor placed perpendicular to the lines of a uniform field

151
Q

How can magnetic flux density B be measured?

A

By the force per unit current per unit length acting on a current carrying conductor placed perpendicular to the lines of a uniform field

152
Q

What does the right hand thumb rule determine?

A

The direction of the magnetic field in a direction of current

153
Q

What directions are field and current relative to each other in F=BIL?

A

Perpendicular

154
Q

How is a wire with the current coming towards you drawn?

A

A circle with a dot in

155
Q

How is a wire with the current travelling away from you drawn?

A

A circle with a cross in

156
Q

What happens to a charged particle when it moves through a magnetic field?

A

It experiences a force

157
Q

When a charged particle moves through a magnetic field and experiences a force, what is this force proportional to?

A
  • B - magnetic flux density
  • Q - charge on the particle
  • v - velocity of the particle
158
Q

When a charge is moving in a magnetic field, how can the force be calculated?

A

F = BQv

159
Q

What condition must be true when F=BQv is used to calculate the force on a charge?

A

The charge must be moving at 90° to the field

160
Q

In the equation F=BQv, what is the direction of the force given by?

A

Fleming’s left hand rule

161
Q

When a charged particle moves at right angles a magnetic field, what is the constant force perpendicular to?

A

Both the velocity and the field

162
Q

Why does the constant force on a particle in a magnetic field change the particle’s direction of motion and has no effect on speed?

A

The constant force is perpendicular to both the velocity and field

163
Q

Does the force from a magnetic field on a charged particle affect the particle’s speed?

A

No

164
Q

Does the force from a magnetic field on a charged particle affect the particle’s direction of motion?

A

Yes

165
Q

What is the result when a constant force acts on a charged particle in a magnetic field?

A

The particle travels in a circular path

166
Q

What does it mean for a charged particle in a magnetic field, that the constant force is perpendicular to the velocity and field?

A

The particle travels in a circular path

167
Q

How is the equation for the radius of the circular path which a charged particle follows derived?

A
  • BQv = mv²/r

* r = mv/BQ

168
Q

What is the equation for the radius of the circular path which a charged particle follows?

A

r = mv/BQ

169
Q

How is the equation for the time period for an electron in a magnetic field to make one rotation derived?

A
  • time = distance/speed
  • time = length of circular path/speed of electron or T=2πr/v
  • since r = mv/BQ, T = 2πmv/vBQ
  • T = 2πm/BQ
170
Q

Equation for time period of an electron in a magnetic field to make one rotation?

A

T = 2πm/BQ

171
Q

In a magnetic field of constant flux density, does the time period of an electron depend on its speed?

A

No

a faster moving electron moves in a circle of larger radius, but takes the same time to make one revolution

172
Q

Where might the circular path of charged particles in a magnetic field be applied?

A

Cyclotron

173
Q

What is magnetic flux given by?

A

Φ = BA

174
Q

What does Φ mean in Φ=BA?

A

Magnetic flux (Wb)

175
Q

What does A mean in Φ=BA?

A

Cross sectional area (m2)

176
Q

What does B mean in Φ=BA?

A

Flux density (T)

177
Q

What is magnetic flux measured in?

A

weber (Wb)

178
Q

What is 1 weber defined as?

A

1 Wb = 1T m2

179
Q

What happens to the equation Φ = BA if the plane of the area is not perpendicular to the field?

A

Φ = BAcosΘ

180
Q

What is flux linkage?

A

The magnetic flux linking the coil

181
Q

What is flux linkage given by?

A

Flux linkage = NΦ

182
Q

What does Faraday’s law state?

A

The magnitide of the induced emf is equal to the rate of change of flux linkage

183
Q

What does N mean in flux linkage = NΦ?

A

The number of turns in a coil i.e. the number of turns cutting the flux

184
Q

What is the equation relating to Faraday’s law (words)?

A

Induced emf = change in flux linkage/time taken

185
Q

What is the equation relating to Faraday’s law (symbols)?

A

E = ΔNΦ / Δt

186
Q

What does Faraday’s law essentially tell us?

A

The size of the induced emf

187
Q

How can we find the direction of the induced emf described in Faraday’s law?

A

Using Lenz’s law

188
Q

What is Lenz’s law

A

The direction of the induced emf is such that it will try to oppose the change in flux that is producing it

189
Q

Equation that relates to Lenz’s law?

A

E = - ΔNΦ / Δt

190
Q

What does the minus sign in Lenz’s law equation show?

A

That the emf is always induced in a direction so as to oppose the change in flux

191
Q

What does a generator do?

A

Converts kinetic energy to electical energy

192
Q

What do the slip rings in a generator do?

A

Rotate with the coil and press against stationary carbon brushes

193
Q

In a generator, which side of the coil makes contact with which brush?

A

Each side of the coil always makes contact with the same brush

194
Q

In a generator, what happens to flux linkage as the coil rotates at a steady rate?

A

The flux linkage constantly changes

195
Q

In a generator, how much a.c. does one revolution of the coil give?

A

One revolution of the coil gives one cycle of a.c.

196
Q

Overall, what increases the peak emf of a generator?

A

By increasing the rate of change of flux linkage of the coil as it spins

197
Q

How can the rate of change of flux linkage of the coil in a generator be increased, in order to increase peak emf?

A
  • using a coil with more turns
  • using a coil with a larger cross-sectional area
  • increasing the strength of the magnetic field
  • increasing the frequency of rotation of the coil
198
Q

What will the frequency of rotation of the coil affect in a generator?

A

The rate of change of flux linkage of the coil, as well as the frequency of the a.c. signal

199
Q

In a generator, what is the relationship between the frequency of rotation of the coil and the frequency of the ac signal?

A

They are directly proportional

200
Q

What are the equations for flux linkage when looking at generators?

A
  • flux linkage = BANcosΘ
  • flux linkage = BANcosωt
  • flux linkage = BANωsinωt
201
Q

In the equation flux linkage = BANcosΘ, what does Θ depend on? What equation does this result in?

A

The angular speed ω of the coil

giving flux linkage = BANcosωt

202
Q

What does ω stand for?

A

Angular speed in rad s-1

203
Q

How does induced emf in a generator vary? Why is this?

A

Sinusoidally, as max. change happens when Θ=90°

204
Q

What type of current do cells and batteries supply?

A

Direct

205
Q

What is a direct current?

A

Current flowing in one direction only

206
Q

What type of current does mains supply?

A

Alternating

207
Q

What is the peak value of ac current or pd?

A

The maximum in either direction

208
Q

How can peak value of ac be measured?

A

From the wave as the amplitude

209
Q

How is peak current denoted?

A

I₀

210
Q

How is peak voltage denoted?

A

V₀

211
Q

What is the peak to peak value of current or pd?

A

The range of values - the distance from the peak above the zero line to the peak below the zero line

212
Q

What is the time period of an ac current?

A

The time taken for one complete cycle/wave

213
Q

What is the root mean squared?

A

A value of current that produces the same heating effect in a resistor as the equivalent dc

214
Q

Why, for an ac current, is it impossible to assign a value to pd and current for a fixed value of time? What is done instead?

A

Current and pd is constantly changing so average would be zero

  • root mean squared produces same heating effect in a resistor as the equivalent dc
215
Q

What is an oscilloscope used for?

A

To show the sizes of voltages and currents in both dc and ac circuits

216
Q

What does a dc trace on an oscilloscope look like? Why?

A

A straight line, as the current is constant, so the voltage is constant

217
Q

What does an ac trace on an oscilloscope look like? Why?

A

A wave, as the current is constantly changing from maximum flow in one direction to maximum flow in the other direction, so voltage does the same

218
Q

What controls do we use on an oscilloscope?

A
  • volts/div dial

* time base dial

219
Q

What does the volts/div dial on an oscilloscope allow you to do?

A

Change how much each vertical square is worth

220
Q

What does the time base dial on an oscilloscope allow you to do?

A

Change how much each horizontal square is worth

221
Q

How can you measure the voltage of a dc supply using an oscilloscope?

A

Counting number of vertical squares from origin to line and multiply by volts/div

222
Q

How can the time for one period be measured using an oscilloscope?

A

Counting how many horizontal squares one wavelength is and multiply by time base

223
Q

What does a transformer do?

A

Changes the value of ac voltage

224
Q

What do transformers consist of?

A

Two coils wound around a soft iron core

225
Q

How does a transformer work?

A
  • ac current flows in the primary coil
  • producing a changing magnetic flux in the soft iron core
  • meaning the flux linkage of the secondary coil is constantly changing
  • and so an ac voltage is induced across it
226
Q

What does a step-up transformer do? Why is this?

A

Increases ac voltage, as secondary coil has more turns than primary coil

227
Q

What does a step-down transformer do? Why is this?

A

Decreases ac voltage, as secondary coil has fewer turns than primary coil

228
Q

In a transformer, what is the ratio of voltages equal to?

A

The ratio of turns

229
Q

What equation shows the ratio of voltages in transformers?

A

Vs / Vp = Ns/ Np

230
Q

What happens in terms of energy losses in an ideal transformer?

A

No energy is lost

231
Q

What is the equation for power in an ideal transformer, and under what condition?

A

VpIp = VsIs

provided no energy is lost

232
Q

What are eddy currents?

A

Looping currents induced by the changing magnetic flux in the core

233
Q

What do eddy currents do?

A

Create a magnetic field that acts against the field that induced them

234
Q

How do eddy currents dissipate energy?

A

By generating heat

235
Q

How can the energy loss from eddy currents be reduced?

A

By laminating the core

236
Q

How can the efficiency of a transformer be calculated?

A

Ratio of power out to power in:

E = IsVs / IpVp

237
Q

What is voltage stepped up to through the National Grid?

A

400 000 V

AQA A Level Physics (10 decks)

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