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LC Physics > Exam Qs - electricity > Flashcards

Exam Qs - electricity Flashcards

1
Q

Ohm’s Law

A

for certain conductors, current flowing through them is directly proportional to the p.d. across them at constant temperature

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2
Q

charge

A

particles that exert electrostatic forces on each other are said to be charged

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3
Q

resistivity

A

of a material is the resistance of an object of that material with unit area and unit length

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4
Q

RMS

A
  • root mean square value

- effective d.c. equivalent of the a.c.

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5
Q

parallel plate electric field diagram

A

on sheet

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6
Q

ampere

A

current that, if maintained in 2 infinitely long wires of negligible cross-section placed 1 metre apart in a vacuum, would produce between the wires a force of 2x10⁻⁷ N/m

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7
Q

MCB

A

miniature circuit breaker

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8
Q

RCD

A

residual current device

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9
Q

uses of capacitors

A
  • in camera flash

- tuning mechanism of radios

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10
Q

coulomb

A

a coulomb is equal to amount of charge accumulated in 1 sec by a current of 1 ampere

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11
Q

application of surface charge

A

electrostatic shielding

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12
Q

effective resistance of resistors in series (proof?)

A

V = V₁ + V₂

Ir = IR₁ + IR₂

R = R₁ + R₂

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13
Q

How fuse operates

A
  • in live part of circuit
  • gets hot if current exceeds certain rated value
  • melts/breaks
  • circuit is broken
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14
Q

uses of LDRs

A

security alarms

light meters

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15
Q

Why frame of electroscope is earthed

A

p.d between leaves + frame determine degree of deflection of leaves

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16
Q

how metal suit protects from high-voltage

A

blocks out external electrical fields

/no field lines inside (hollow conductor)

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17
Q

show capacitor stores energy

A
  • charge cap by connecting to battery
  • disconnect battery by flipping switch so capacitor connects to light bulb
  • bulb lights for several minutes showing it is storing energy
  • diagram on sheet
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18
Q

current, charge, time

A

I = Q/t

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19
Q

demonstrate that capacitance of parallel plate capacitor depends on distance between its plate

A
  • connect electroscope to parallel plate capacitor
  • earth one plate +place charge on other using h.t. power supply
  • slowly separate plates
  • leaves diverge indicating capacitance is lowered (voltage increasing)
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20
Q

application of surface charge on conductor

A
  • Tv signal cable
  • electrostatic shielding
  • co-axial cable
  • to protect persons/equipment, enclose them in hollow conductors
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21
Q

resistance vs temp graph

A

on sheet

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22
Q

effect of resistance as temp falls

A

r decreases

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23
Q

effect on resistance as length increases

A

r increases

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24
Q

effect on resistance as diameter increases

A

r increases

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25
Q

an RCD has a rating of 30 mA, what does this mean?

A

RCD = electrical safety device. Measures current through live + neutral wires. It will trip a switch if there is a difference of 30mA or more + shut circuit down

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26
Q

depletion layer diagram

A

on sheet

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27
Q

high why its more economical to transmit electrical energy at high voltage

A
  • Joule’s law, P ∝ I²
  • high current in a wire will lead to high energy losses
  • if volt increases, allows to transmit similar level of electrical energy at low current
  • reduces energy loss
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28
Q

p.d.

A

between 2 points in an electric field is the work done in bringing a charge of +1C from one point to the other

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29
Q

current

A

flow of (electric?) charge

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30
Q

d.c.

A

current flows in 1 direction

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31
Q

a.c.

A

current reverses direction every so often

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32
Q

resistance

A

ratio of the pd across a conductor to the current flowing through it

R = V/I

unit: ohm Ω

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33
Q

resistance in series

A

R = R₁ + R₂ …

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34
Q

resistance in parallel

A

1/R = 1/R₁ + 1/R₂ …

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35
Q

balanced wheatstone bridge formula

A

R₁/R₂ = R₃/R₄

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36
Q

joules law

A

rate at which heat is prod in conductor is directly proportional to the square of the current provided its resistance is constant

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37
Q

faraday’s law of electromagnetic induction

A

when there is a change in the magnetic flux linking a coil, an emf is induced in that coil. The strength of emf is proportional to rate of change of flux linking that coil

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38
Q

effective resistance in parallel

A

proof

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39
Q

wire containing the fuse

A

live wire

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40
Q

colour of live wire

A

brown

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41
Q

show point discharge

A
  • attach van de graaff generator to electroscope
  • charge VDGG, presence of charge causes leaves to diverge
  • bring sharply pointed obj close to generator
  • leaves collapse, charge removed from system
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42
Q

demonstrate electric field pattern

A

semolina experiment, diagram needed

  • add semolina to shallow dish of oil
  • attach electrodes
  • semolina particles become slightly charged at each end + line up along lines of force, following shape of electric field
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43
Q

capacitors - storing energy

A

store energy for short periods of time + smaller quantities of energy

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44
Q

battery - storing energy

A

store energy for a long time

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45
Q

capacitance

A

capacitance of a conductor is the ratio of the charge on the conductor to its potential

C = Q/V

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46
Q

coulomb’s law

A

states that force of attraction or repulsion between two points is directly proportional to product of the charges + is inversely proportional to square of distance between them

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47
Q

electric field

A

any region of space where a static electric charge experiences a force other than force of gravity

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48
Q

electric field strength

A

electric field strength at a point in an electric field is the force per unit charge at that point

formula + notation

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49
Q

volt

A

the p.d. between two points is 1 volt if 1 joule of work is done when 1 coulomb is brought from one point to the other

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50
Q

ohmmeter advantage

A

easily portable

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51
Q

ohmmeter disadvantage

A

less accurate

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52
Q

how we known wheatstone bridge is balanced

A

-no current flows through central galvanometer

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53
Q

when coil is used in a circuit (in ac circuit)

A

create a changing magnetic field (due to ac), creating a back emf.
known as self-induction
will reduce effective voltage + therefore, current

54
Q

when diode used in a circuit

A

current will only flow in one direction

55
Q

if diameter of wire is increased by a factor of 3, what will be its effect on resistance?

A

R ∝ 1/²R² , so R is decreased by a factor of 9

56
Q

joule’s law

A

rate at which heat is produced in a conductor is directly proportional to the square of the current provided its resistance is constant

57
Q

How can a galvanometer be converted into a voltmeter?

A

a very high resistance known as “series resistance” is connected in series with the galvanometer.
/
A galvanometer is converted to a voltmeter by connecting it in series with a relatively high resistance called multiplier.

58
Q

why gravitational force can be ignored for sub-atomic particles

A
  • masses are very small
  • force is proportional to mass
  • gravitational force is the weakest
59
Q

distinguish between resistance + resistivity

A

R = V/I + notation

p = RA/I + notation

60
Q

voltage-time graph for ac and dc supply

A

know from notes

61
Q

why does a transformer not work with direct current?

A
  • current not changing

- magnetic flux/magnetic field not changing

62
Q

why its inefficient to use low voltage when transmitting electricity

A
  • large current

- more heat loss

63
Q

why it’s necessary to use rms voltage when comparing ac and dc

A

so as to make the power output equivalent between ac and dc

64
Q

factors that affect heat produced in a current-carrying conductor

A
  • resistance
  • current (squared)
  • time
65
Q

How to charge an electroscope by induction

A
  • hold + charged rod near cap of uncharged GLE
  • earth cap (touch with finger)
  • remove finger and then remove + rod
66
Q

describe an experiment to investigate the principle by which an operator in a metal-foil suit is protected

A

-distribution of charge on an object experiment

67
Q

circumstances that point discharge will occur

A
  • large electric field strength
  • high charge density at a point
  • potential at a point
68
Q

principle on which definition of the ampere is based

A

current-carrying conductor experiences a force in a magnetic field

69
Q

all the charge resides on the surface of a van de graff generator’s dome. Why?

A

like charges repel

charges are a max distance apart on surface of dome

70
Q

demonstrate that total charge resides on outside of a conductor

A

-distribution of charge on an object experiment with metal can/pot

  • surface of a conducting can is connected with a wire to the Van de Graaff generator.
  • Van de Graaff generator is turned on and a lot of charge is deposited on the conducting can.
  • proof plane used to sample charge on outside surface of can + the charge sample placed on gold foil electroscope.
  • leaves of the electroscope deflect to show there is charge on the outside of the can.
  • the charge on inside surface is sampled.
  • proof plane is carefully inserted through a hole in the top of the can where it touches the inside surface and is removed.
  • charge sample is deposited on the foil electroscope.
  • leaves do not deflect + shows that no charge is on inside surface. t.
71
Q

why does the current produced in a coil vary

A

induced emf / flux varies

72
Q

why is coulomb’s law an example of the inverse square law?

A

force inversely proportional to distance squared

F ∝ 1/d²

73
Q

why resistance of a bulb is different when not connected to the mains

A
  • cold filament/coil has lower resistance
  • hot filament/coil has higher resistance

(Diff coil temps result in diff resistance values)

74
Q

difference between grav force and electrostatic force between two electrons

A
  • gravitational force is much smaller than electrostatic force
  • gravitational force is attractive, electrostatic force is repulsive
75
Q

how charge is distributed over pear shaped conductor (drawing it)

A
  • concentration of charge at pointed end

- charge indicated throughout the conductor

76
Q

how to convert ac output voltage to dc voltage

A

diode/rectifier

77
Q

function of a diode in a circuit

A

rectifier / convers ac to do

78
Q

electricity companies transmit electricity over large distances at high voltage. Explain why high voltage is used

A

-for a given power transmission high voltage uses low current minimising power (heat) loss

79
Q

2010 q 12(b)

A

know how to do

80
Q

functions of the resistor

A
  • limits the current

- acts as a load resistor

81
Q

how output voltage displayed

A

-cathode ray oscilloscope

82
Q

output voltage is lower than input voltage. explain why

A

voltage drops across (eg. led)

83
Q

effects of an electric current

A

heating
magnetic
chemical

84
Q

2010 q 8 circuit qs

A

know how to do

85
Q

when will an RCD disconnect a circuit?

A

when magnitude of the current flowing in differs from that flowing out

86
Q

2017 q11: Why did Telstar not allow transatlantic signals to be transmitted constantly?

A
  • period of Telstar diff to period of Earth
  • not in geostationary orbit
  • not always above same place
87
Q

what particles are used for transatlantic communication in: telegraph cables

A

electrons

88
Q

what particles are used for transatlantic communication in: satellite signals

A

photons

89
Q

what particles are used for transatlantic communication in: optical fibres

A

photons

90
Q

Why it takes longer for a magnet to fall through a copper pipe than a glass pipe

A
  • falling magnet creates changing magnetic flux/field
  • emf induced
  • current flows in copper (only)
  • generating magnetic fields which oppose motion of falling magnet
91
Q

state principle on which a moving-coil galvanometer is based

A

a current carrying conductor experiences a force in a magnetic field

92
Q

how galvanometer may be converted to function as an ammeter

A

small resistance connected in parallel

93
Q

how galvanometer may be converted to function as a voltmeter

A

large resistance connected in series

94
Q

another device based on same principle as moving-coil galvanometer

A

dc motor/moving-coil loudspeaker

95
Q

functions of parts of induction coil

A

know

96
Q

application of induction coil

A
  • create spark in engine of car

- electric fence

97
Q

who invented the induction coil?

A

Dr. Nicholas Callan

98
Q

function of induction coil

A

changes low voltage dc to high voltage dc

99
Q

In an induction coil, a primary coil with a few turns of thick wire + a secondary coil with many turns of thin wire are wrapped on same soft-iron core. Why is there a large no. of turns in the secondary coil?

A

emf induced proportional to number of turns

100
Q

why primary coil has a thick wire

A

thick wire has low resistance + large current flows.

greater efficiency, reduced heating/energy losses

101
Q

why are both coils wrapped on the same soft-iron core?

A

greater flux linkage/efficiency / less energy losses

102
Q

describe how loudspeaker operates

A

Diagram showing: cone, magnet, coil

coil in magnetic field 3 changing current (in coil) 3 force on coil / coil vibrate

103
Q
  1. a bar magnet is attached to a string + allowed to swing. Copper sheet is placed underneath. Why amp of swings decrease rapidly
  2. main energy conversion as magnet slows down
A
  1. -induced emf in copper
    - current flows in copper sheet, generating a magnetic field, opposing motion of magnet
  2. kinetic energy -> electrical energy
104
Q

energy conversion that takes place in an electric motor

A

electrical energy to kinetic energy

105
Q

function of commutator in an electric motor

A

so coil rotates in one direction / reverses current every half-cycle

106
Q

function of carbon brushes in an electric motor

A

to link power supply to coil / enable current to eneter coil as it rotates

107
Q

function of magnet in an electric motor

A

provide a magnetic field

108
Q

why does motor turn when current flows through coil

A

current-carrying coil experiences a force in a magnetic field

109
Q

advantage of using induction motor over a dc motor

A
  • no brushes to replace
  • not affected by minor voltage fluctuations
  • less electrical interference
  • less friction
  • stabilised rate of rotation
  • no sparking
110
Q

resistor in a circuit is replaced with a coil. Resistance of circuit does not change. What is effect on current flowing in circuit?

A

current is reduced

bc back emf induced in coil / coil has a self-inductance

111
Q

factors that affect force on a current-carrying conductor placed near a magnet

A
  • magnetic flux density
  • curent
  • length
112
Q

function of commutator in a dc motor

A

allows current to always flow in same end of coil and out the other end /or/ so that torque is always in same direction

113
Q

how are slip rings connected to an external circuit in an ac generator?

A

brushes

springs / carbon / metal

114
Q

difference in operation of a transformer and an induction coil

A

transformer: ac input, ac output, current flows through secondary

induction coil: dc input, dc output, sparks across gap in secondary

115
Q

factors that affect efficiency of a transformer

A
  • heat losses in coils
  • poor flux linkage
  • poor core design
  • coil resistance
116
Q

differences between a capacitor and a battery

A
  • capacitor discharges faster than a battery
  • capacitor stores electrostatic potential energy while a battery stores chemical energy
  • battery gives a constant current
  • battery stores more energy
117
Q

application of capacitors

A
  • flash of a camera
  • tuning circuits
  • defibrillator
118
Q

how to charge an electroscope by induction

A
  • charged object adjacent to electroscope
  • earth electroscope briefly
  • remove charged object
119
Q

demonstrate that the capacitance of a parallel plate capacitor depends on distance between its plates

A

arrangement: connect electroscope to parallel plate capacitor
method: earth one plate + place charge on other using h.t. power supply. Slowly separate plates
observation: leaves diverge indicating capacitance is lowered

120
Q

diagram to show electric field strength pattern between two oppositely charged parallel plates

A

diagram

121
Q

how does an electron in an atom of the fluorescent coating emit a photon?

A

electron absorbs UV photon and jumps to higher energy level 4 (then) drops back (to lower level) emitting a photon

122
Q

Why does the fluorescent coating in a CFL get warm during use?

A

due to absorption // energy difference 4 (of) UV photons / energy // between incident and emitted photons

123
Q

A light sensor attached to a datalogger indicates that the light emitted from a CFL used in the home is not continuous, but flickers at a frequency that is not detected by the eye. What is the cause of the flickering in the light?

A

(house) power supply is alternating / a.c.

124
Q

Draw a circuit diagram of a diode in forward bias

A

.

125
Q

How can LEDs be used to produce white light?

A

mixture of leds

red, green and blue leds

126
Q

List the factors that affect the heat produced in a current-carrying conductor. r

A

esistance, current (squared), time, (any valid answer)

127
Q

electromagnetic induction definition

A

conductor / wire /coil / loop cuts magnetic flux 3 emf / voltage induced

128
Q

The pendulum in the diagram is 8 m long with a small bob of mass 6 kg at its end. It is displaced through an angle of 30° from the vertical (position A) and is then held in position B, as shown. Calculate the height through which the bob has been raised and the potential energy that it has gained.

A

h = l(1 – cos θ)

or use pythagoras theorem

129
Q

metre bridge formula

A

R1/R1 = L1/L2

R is resistance, L is length

130
Q

electric field diagrams

A

know and how they interact

131
Q

x ray diagram

A

diagram

LC Physics (25 decks)

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