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Electricity and Magnetism Flashcards

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1
Q

What is magnetism?

A

the force exerted by magnets when they attract or repel each other

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2
Q

What happens to magnets when they are free to move?

A

they will line up between the Earth’s magnetic poles

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3
Q

What two materials are magnetic material?

A

Iron and steel

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4
Q

How do you call iron and steel?

A

ferrous

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5
Q

How do you call all other metals which are non-magnetic?

A

non-ferrous

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6
Q

Is IRON magnetically “soft” or “hard”?

A

magnetically “soft”

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7
Q

What does it mean for something to be magnetically “soft”?

A

to lose its magnetism easily: easy to magnetise/demagnetise

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8
Q

What does it mean for something to be magnetically “hard”?

A

to retain its magnetism: hard to magnetise/demagnetise

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9
Q

What are permanent magnets made from?

A

steel

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10
Q

Why are permanent magnets made from steel?

A

it is magnetically “hard”, meaning it retains its magnetism

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11
Q

How can you make a temporary magnet?

A

putting it close to a permanent magnet (induced magnetism)

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12
Q

What is induced magnetism?

A

making something into a magnet by putting it close to a permanent magnet

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13
Q

What are TWO ways in which a magnet may be demagnetised, without using a coil?

A
  • heating
  • hitting with a hammer
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14
Q

What will happen to a magnet if it is heated?

A

it will be demagnetised

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15
Q

What will happen to a magnet if it is hit with a hammer?

A

it will be demagnetised

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16
Q

What is another way a magnet can be demagnetised, using a.c current?

A

place magnet inside a coil, using a.c current, then slowly remove magnet

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17
Q

How can you test for a (permanent) magnet? Why is this the only sure test?

A

repulsion: temporary magnets (magnetic material) can attract only

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18
Q

What is a temporary magnet?

A

a magnet with no permanent N-S poles, only magnetised when there is a permanent magnet

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19
Q

What is a magnetic field?

A

a region where a magnetic pole experiences a force

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20
Q

How can you show a relatively stronger magnetic field in a drawing?

A

field lines closer together

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21
Q

How can you make a permanent magnet?

A
  1. place a steel rod inside a coil
  2. use high DC current for a time
  3. leave steel inside, turn off current
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22
Q

What are the differences between a permanent magnet and an electromagnet?

A
  • permanent magnet has a constant magnetic field, electromagnet variable magnetic field strength
  • permanent magnet cannot be switched on/off, electromagnet can quickly do so
  • permanent magnet cannot switch N and S poles, electromagnet can change N and S
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23
Q

How can the direction of magnetic field in an electromagnet be reversed?

A

changing direction of current flow

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24
Q

What are uses of permanent magnets?

A

guitar pickups, speakers, cupboard latches

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25
Q

What are uses of electromagnets?

A

electric bells, loudspeakers, electric door locks, relays, MRI machines

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26
Q

How can you make an electromagnet?

A

wrap coil around an iron nail, connect to a cell and turn the current on

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27
Q

How can you test the strength of an electromagnet?

A
  • set up circuit with coil surrounding iron nail and a variable resistor.
  • change current using variable resistor, and change number of turns of wire
  • see how many paper clips it picks up
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28
Q

How can you increase the strength of an electromagnet?

A
  • use iron core
  • more current
  • more turns of wire (coil)
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29
Q

What material is used for an electromagnet?

A

a soft magnetic material

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30
Q

Are hard magnetic materials easy to magnetise/demagnetise or not?

A

difficult

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31
Q

What happens when a permanent magnet attracts a magnetic material?

A

it induces a magnetic field in the material

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32
Q

Can magnetic materials attract each other?

A

only when a permanent magnet is present

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33
Q

What is a permanent magnet?

A

a magnet that always has a magnetic field, can induce magnetic field in magnetic material

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34
Q

What causes magnetic forces?

A

interactions between magnetic fields

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35
Q

What is the direction of a magnetic field at a certain point?

A

direction of the force on the N pole of a magnet at that point

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36
Q

What are the differences between a temporary and a permanent magnet?

A
  • a temporary magnet can ONLY attract, not repel
  • a permanent magnet is harder to magnetize
  • a temporary magnet is only a magnet when near a magnet
  • a permanent magnet has its own, permanent magnetic field
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37
Q

What is the difference between magnetic and non-magnetic materials?

A

magnetic materials will be attracted to magnets, non-magnetic materials will not

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38
Q

How can you plot magnetic field lines?

A

using a compass or iron filings

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39
Q

How can you determine the direction of a magnetic field?

A

with a compass

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40
Q

What is a compass useful for?

A

determining the direction of a magnetic field

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41
Q

what does charging of solids by friction involve

A

transfer of negative charge (electrons)

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42
Q

Unit for charge?

A

Coulombs

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43
Q

What is an electric field?

A

a region in which an electric charge experiences a force

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44
Q

what is the direction of an electric field at a point

A

direction of force on a positive charge at that point

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45
Q

What is electric current related to?

A

flow of charge

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46
Q

Why are metals good electrical conductors?

A

movement of free electrons

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47
Q

What is electric current?

A

the charge passing a point per unit time

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48
Q

Formula for charge?

A
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49
Q

What is conventional current? How is this different from the actual flow of free electrons?

A

conventional current - positive to negative
flow of free electrons - negative to positive

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50
Q

Define ‘electromotive force’.

A

the electrical work done by a source in moving a unit charge around a complete circuit

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51
Q

What is e.m.f (electromotive force) measured in?

A

volts (V)

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52
Q

Define ‘potential difference’.

A

the work done by a unit charge passing through a component

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53
Q

What is the p.d between two points measured in?

A

volts (V)

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54
Q

What does an electric circuit do?

A

transfer energy from a source of electrical energy (eg electrical cell) to the circuit components and then into the surroundings

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55
Q

What always have magnetic fields around them?

A

current carrying conductors or moving electrons

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56
Q

What is the pattern of a magnetic field in straight wires?

A

circular and perpendicular to the wire, getting weaker with distance

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57
Q

How can you find the direction of a magnetic field in straight wires?

A

Right hand grip rule n1: your thumb is conventional current, and the rest of your fingers are the magnetic field direction

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58
Q

How do you know if the conventional current is going into or out of the paper?

A

cross: current into the page
dot: current out of the page

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59
Q

In a single wire, how can you increase the strength of the magnetic field?

A

greater current

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60
Q

What is the direction/pattern of the magnetic field in a solenoid?

A

the same as in a bar magnet

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61
Q

What is the relationship between current and direction of magnetic field on a solenoid?

A

the general direction of the current flow indicates the direction of the magnetic field

62
Q

How can you find out the direction of the magnetic field in a solenoid?

A

right hand grip rule n2: your thumb points to the the north pole, the rest of your fingers are conventional current

63
Q

How can you reverse magnetic field direction in a solenoid?

A
  • reverse current/cell
  • wrap wire in opposite direction
64
Q

How can you increase the strength of the magnet in a solenoid?

A
  • more turns of coil
  • more current
  • wrap wire on an iron core
65
Q

Describe an experiment to identify the pattern/direction of a magnetic field in straight wires and solenoids.

A
  1. use a compass to determine the direction of a field at a single point
  2. draw an arrow in the direction of the field
  3. move the compass to different points and repeat
  4. you will have a diagram of the magnetic field
66
Q

What is a relay?

A

electromagnetic switch, a circuit with a small current turns on another circuit with a large current

67
Q

How does a relay work?

A
  1. switch is closed
  2. small current magnetised the coil
  3. iron reeds are attracted and close
  4. large current flows
68
Q

What is the energy conversion in a loudspeaker?

A

it converts electrical energy into sound energy

69
Q

How does a loudspeaker work?

A
  1. alternating current in coil creates an alternating magnetic field in coil
  2. this alternating magnetic field will interact with the permanent magnetic field
  3. this interaction will make the papercone move in/out producing sound waves
70
Q

If you use high frequency ac in a loudspeaker, what will happen?

A

high frequency sound waves

71
Q

If you use large voltage ac in a loudspeaker, what will happen?

A

loud sound waves

72
Q

Why are relays useful?

A

the turn on larger current circuit by turning on a smaller current circuit: this way, we do not need to use thick, heavy, expensive cables to carry the large current

73
Q

What happens if you supply a solenoid with AC?

A

the current will change, so the direction of the magnetic field will also change every half cycle

74
Q

What is the variation of magnetic field strength in straight wires?

A

strongest closest to wire

75
Q

What is the variation of magnetic field strength in solenoids?

A

uniform magnetic field inside coil

76
Q

What happens if you increase the size of the current in a straight wire?

A

results in a stronger magnetic field

77
Q

What happens if you increase the size of the current in a solenoid?

A

results in a stronger magnetic field

78
Q

What happens if you reverse the current in a solenoid?

A

reverses magnetic field direction

79
Q

What happens if you reverse the current in a straight wire?

A

reverses magnetic field direction

80
Q

What happens whenever a current is at a right angle to a magnetic field?

A

it experiences a force, perpendicular to both the current and magnetic field

81
Q

Why is a force experienced when a current carrying conductor is in a magnetic field?

A

interaction between the current’s magnetic field and the permanent magnetic field

82
Q

How can we work out the direction of the force when there is a current carrying conductor in a magnetic field?

A

Fleming’s left hand rule:
First finger is magnetic field (N to S)
Second finger is conventional current
Thumb is the thrust/force

83
Q

How can you reverse the force direction (current carrying conductor in a magnetic field)?

A

reverse current or reverse magnetic field direction

84
Q

How can you increase the strength of the force (current carrying conductor in a magnetic field)?

A

stronger magnets, magnets closer together or increase current

85
Q

What happens if you reverse the current in a current-carrying conductor in a magnetic field?

A

you reverse the force

86
Q

What happens if you reverse the direction of the magnetic field in a current-carrying conductor in a magnetic field?

A

you reverse the force

87
Q

How do you make a permanent magnet?

A
  1. place steel rod inside coil
  2. use high dc current for a time
  3. leave steel inside, turn off current
88
Q

How can you demagnetise a magnet?

A
  1. place magnet inside coil
  2. use ac current
  3. slowly remove magnet with ac current still on
89
Q

Describe an experiment to show the force in a current-carrying conductor in a magnetic field.

A
  1. Place a wire carrying current in between two permanent magnets
  2. turn on power supply
  3. observe force on wire
90
Q

Will any moving charged particle in a magnetic field experience a force?

A

yes

91
Q

Determine the direction of the force on charged particles in a magnetic field when the field is into the page.

A

positive particle: up
negative particle: down

92
Q

Determine the direction of the current on charged particles in a magnetic field when the field is into the page.

A

positive particle: left to right
negative particle: right to left

93
Q

What is the basic principle of a d.c motor?

A

a current + a magnetic field will create a force

94
Q

How does a d.c motor work?

A

the sides of the coil have a magnetic field, which interacts with permanent magnetic field to produce a force

95
Q

How can we identify the force direction in a d.c motor?

A

Fleming’s left hand rule: first finger is field, second finger is current and thumb is thrust

96
Q

What causes the turning effect in a d.c motor?

A

a current-carrying coil in a magnetic field

97
Q

Why do we have split-ring commutators on a d.c motor?

A

to reverse the direction of the current every half turn so that the coil rotates in the same direction

98
Q

What would happen without split-rings?

A

the coil would flip backwards and forwards (the changing direction of current caused by the coil rotating through 180 would cause this)

99
Q

Why do we need (carbon) brushes in d.c motors? Why are they good for this?

A

connect commutator to the circuit: low friction, good conductors

100
Q

What do split rings do to the current?

A

reverse the direction of the current every half turn

101
Q

What do brushes ensure?

A

that the wires don’t get twisted

102
Q

How can the turning effect in a d/c motor be increased?

A
  • increasing number of turns on coil
  • increasing current
  • increasing strength of magnetic field
103
Q

What may happen if a conductor moves rapidly across a magnetic field?

A

creates induced emf or current or p.d

104
Q

What is the basic principle of electromagnetic induction?

A

movement of a conductor + magnetism -> induced emf/current/pd

105
Q

What would happen if a conductor cutting a magnetic field is not moving?

A

there will be no induced current

106
Q

When is there a maximum of induced current in electromagnetic induction?

A

when the conductor is at 90 degrees to magnetic fields

107
Q

What happens when a conductor is moving parallel to a magnetic field?

A

there will be no induced current

108
Q

What are two situations in electromagnetic induction when there will be no induced current?

A
  1. when there is no movement
  2. when the conductor is moving parallel to the magnetic field
109
Q

How can you reverse the current direction in electromagnetic induction?

A
  • reverse wire movement direction
  • reverse magnetic field direction
110
Q

How can you increase the size of induced current in electromagnetic induction?

A
  • stronger magnetic field
  • move wire faster
  • use a coil of wire
111
Q

Describe an experiment to demonstrate electromagnetic induction.

A
  1. connect a wire to an ammeter or a galvanometer
  2. place two permanent magnets on either side of the wire
  3. move the wire
  4. watch what happens to ammeter/galvanometer
112
Q

What is a galvanometer?

A

a very sensitive ammeter

113
Q

Why does electromagnetic induction work?

A

the conductor cuts the magnetic field lines, forcing electrons in the wire to move as a current

114
Q

How do we identify the induced current direction in electromagnetic induction?

A

Fleming’s right hand rule: first finger is field, second finger is current and thumb is thrust

115
Q

What are two ways to induce an e.m.f (electromagnetic induction)?

A
  1. moving a magnet so its field lines are cut by a wire
  2. moving a wire across a magnetic field so that it cuts it
116
Q

What is Lenz’s Law?

A

the direction of the induced emf (electromagnetic induction) opposes the change that creates it

117
Q

How does the a.c generator work?

A
  1. coil made to rotate
  2. sides of coil cut magnetic field lines, inducing emf
  3. emf changes direction every half turn, because for half a turn one side moves up and for the other half turn the same side moves down
  4. the emf changes size because there is a max. induced emf when the coil is perpendicular
118
Q

What is the simple idea behind ac generators?

A

movement of coil + magnetic field -> induced emf/current

119
Q

How is the graph of induced emf vs time of an ac motor?

A

it goes up and down

120
Q

What does a peak in an induced emf vs time graph of an ac motor mean?

A

long sides of coil are perpendicular to magnetic field, maximum emf is induced

121
Q

What do the zeros in an induced emf vs time graph of an ac motor mean?

A

top and bottom sides of coil are parallel to magnetic field, so no emf is induced

122
Q

What does a trough in an induced emf vs time graph of an ac motor mean?

A

long sides of coil move perpendicular to field in opposite direction: induced emf is maximum but in opposite direction

123
Q

What is the function of slip rings in ac generators?

A

allow direction of induced emf to alternate, ergo causing an alternating current
connect coil to brush

124
Q

What are slip rings?

A

cylindrical conductors that make constant contact with the coil during rotation

125
Q

What is the function of carbon brushes in ac generators?

A

make electrical connection between rotating coil and circuit, avoiding wires becoming twisted

126
Q

What does a transformer do?

A

increases or decreases alternating p.d

127
Q

Describe the construction of a simple transformer.

A

a primary circuit, including the a.c power pack, with a primary coil attached to a soft iron core.
a secondary circuit with a secondary coil attached to the soft iron core.

128
Q

How does a transformer work?

A
  1. alternating p.d/current in primary circuit
  2. produces an alternating magnetic field in primary coil
  3. alternating magnetic field travels in iron core
  4. secondary coil cuts the alternating magnetic field
  5. induces an alternating p/d/current in secondary circuit
129
Q

What does a step up transformer do?

A

increases voltage (in secondary circuit), decreases secondary current

130
Q

What does a step down transformer do?

A

decreases voltage (in secondary circuit), increases secondary current

131
Q

What does Np mean?

A

number of turns on primary coil

132
Q

What does Ns mean?

A

number of turns on secondary coil

133
Q

What does Vp mean?

A

voltage applied to primary coil

134
Q

What does Vs mean?

A

voltage induced on secondary coil

135
Q

What does Ip mean?

A

current in primary coil

136
Q

What does Is mean?

A

current in secondary coil

137
Q

What is the physical difference between a step up and step down transformer?

A

step up: more turns on secondary coil
step up: less turns on secondary coil

138
Q

How can you find out a missing number of turns in the primary/secondary coil?

A

Np/Ns = Vp/Vs

139
Q

Vp/Vs =

A

Np/Ns

140
Q

Primary power =

A

secondary power

141
Q

Why is primary power = secondary power?

A

principle of conservation of energy

142
Q

Power =

A

IV

143
Q

Rate of heat energy loss from wires of resistance R:

A

P = Iˆ2 x R

144
Q

How are transformers used in transmission of electricity?

A
  1. generator carries voltage to step up transformer
  2. wire will carry a high voltage and a low current, resulting in low heat loss
  3. it will then be brought to a step down transformer so the p.d can be used
145
Q

State the advantages of high-voltage transmission.

A
  • low heat loss (low current)
  • most of electrical energy reaches destination
  • thin, cheaper, less heavy cables can be used
146
Q

Np/Ns =

A

Vp/Vs

147
Q

IpVp =

A

IsVs (100% efficiency)

148
Q

Power input = power output

A

IpVp = IsVs

149
Q

Why are power losses in cable smaller when voltage is greater?

A

P = I^2 x R (where R is rate of heat energy loss)

150
Q

Power input =

A

power output

151
Q

Power =

A

current x voltage

COPY IGCSE Physics (9 decks)

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