Electricity and Magnetism Flashcards
1
Q
What is magnetism?
A
the force exerted by magnets when they attract or repel each other
2
Q
What happens to magnets when they are free to move?
A
they will line up between the Earth’s magnetic poles
3
Q
What two materials are magnetic material?
A
Iron and steel
4
Q
How do you call iron and steel?
A
ferrous
5
Q
How do you call all other metals which are non-magnetic?
A
non-ferrous
6
Q
Is IRON magnetically “soft” or “hard”?
A
magnetically “soft”
7
Q
What does it mean for something to be magnetically “soft”?
A
to lose its magnetism easily: easy to magnetise/demagnetise
8
Q
What does it mean for something to be magnetically “hard”?
A
to retain its magnetism: hard to magnetise/demagnetise
9
Q
What are permanent magnets made from?
A
steel
10
Q
Why are permanent magnets made from steel?
A
it is magnetically “hard”, meaning it retains its magnetism
11
Q
How can you make a temporary magnet?
A
putting it close to a permanent magnet (induced magnetism)
12
Q
What is induced magnetism?
A
making something into a magnet by putting it close to a permanent magnet
13
Q
What are TWO ways in which a magnet may be demagnetised, without using a coil?
A
- heating
- hitting with a hammer
14
Q
What will happen to a magnet if it is heated?
A
it will be demagnetised
15
Q
What will happen to a magnet if it is hit with a hammer?
A
it will be demagnetised
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
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
18
Q
What is a temporary magnet?
A
a magnet with no permanent N-S poles, only magnetised when there is a permanent magnet
19
Q
What is a magnetic field?
A
a region where a magnetic pole experiences a force
20
Q
How can you show a relatively stronger magnetic field in a drawing?
A
field lines closer together
21
Q
How can you make a permanent magnet?
A
- place a steel rod inside a coil
- use high DC current for a time
- leave steel inside, turn off current
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
23
Q
How can the direction of magnetic field in an electromagnet be reversed?
A
changing direction of current flow
24
Q
What are uses of permanent magnets?
A
guitar pickups, speakers, cupboard latches
25
What are uses of electromagnets?
electric bells, loudspeakers, electric door locks, relays, MRI machines
26
How can you make an electromagnet?
wrap coil around an iron nail, connect to a cell and turn the current on
27
How can you test the strength of an electromagnet?
- 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
28
How can you increase the strength of an electromagnet?
- use iron core
- more current
- more turns of wire (coil)
29
What material is used for an electromagnet?
a soft magnetic material
30
Are hard magnetic materials easy to magnetise/demagnetise or not?
difficult
31
What happens when a permanent magnet attracts a magnetic material?
it induces a magnetic field in the material
32
Can magnetic materials attract each other?
only when a permanent magnet is present
33
What is a permanent magnet?
a magnet that always has a magnetic field, can induce magnetic field in magnetic material
34
What causes magnetic forces?
interactions between magnetic fields
35
What is the direction of a magnetic field at a certain point?
direction of the force on the N pole of a magnet at that point
36
What are the differences between a temporary and a permanent magnet?
- 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
37
What is the difference between magnetic and non-magnetic materials?
magnetic materials will be attracted to magnets, non-magnetic materials will not
38
How can you plot magnetic field lines?
using a compass or iron filings
39
How can you determine the direction of a magnetic field?
with a compass
40
What is a compass useful for?
determining the direction of a magnetic field
41
what does charging of solids by friction involve
transfer of negative charge (electrons)
42
Unit for charge?
Coulombs
43
What is an electric field?
a region in which an electric charge experiences a force
44
what is the direction of an electric field at a point
direction of force on a positive charge at that point
45
What is electric current related to?
flow of charge
46
Why are metals good electrical conductors?
movement of free electrons
47
What is electric current?
the charge passing a point per unit time
48
in conventional current, what is the flow of electrons?
conventional current is from positive
to negative but the flow of free electrons is from negative to positive
49
Formula for charge?
current x time
50
Formula for current?
current charge/time
51
What is conventional current? How is this different from the actual flow of free electrons?
conventional current - positive to negative
flow of free electrons - negative to positive
52
Define 'electromotive force'.
the electrical work done by a source in moving a unit charge around a complete circuit
53
What is e.m.f (electromotive force) measured in?
volts (V)
54
Define 'potential difference'.
energy needed per charge to flow between two points in a circuit
55
What is the p.d between two points measured in?
volts (V)
56
What does an electric circuit do?
transfer energy from a source of electrical energy (eg electrical cell) to the circuit components and then into the surroundings
57
What always have magnetic fields around them?
current carrying conductors or moving electrons
58
What is the pattern of a magnetic field in straight wires?
circular and perpendicular to the wire, getting weaker with distance
59
How can you find the direction of a magnetic field in straight wires?
Right hand grip rule n1: your thumb is conventional current, and the rest of your fingers are the magnetic field direction
60
How do you know if the conventional current is going into or out of the paper?
cross: current into the page
dot: current out of the page
61
In a single wire, how can you increase the strength of the magnetic field?
greater current
62
What is the direction/pattern of the magnetic field in a solenoid?
the same as in a bar magnet
63
resistance in terms of proportionality
(a) resistance is directly proportional to length
(b) resistance is inversely proportional to
cross-sectional area
64
What is the relationship between current and direction of magnetic field on a solenoid?
the general direction of the current flow indicates the direction of the magnetic field
65
How can you find out the direction of the magnetic field in a solenoid?
right hand grip rule n2: your thumb points to the the north pole, the rest of your fingers are conventional current
66
How can you reverse magnetic field direction in a solenoid?
- reverse current/cell
- wrap wire in opposite direction
67
How can you increase the strength of the magnet in a solenoid?
- more turns of coil
- more current
- wrap wire on an iron core
68
Describe an experiment to identify the pattern/direction of a magnetic field in straight wires and solenoids.
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
69
What is a relay?
electromagnetic switch, a circuit with a small current turns on another circuit with a large current
70
How does a relay work?
1. switch is closed
2. small current magnetised the coil
3. iron reeds are attracted and close
4. large current flows
71
What is the energy conversion in a loudspeaker?
it converts electrical energy into sound energy
72
How does a loudspeaker work?
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
73
If you use high frequency ac in a loudspeaker, what will happen?
high frequency sound waves
74
If you use large voltage ac in a loudspeaker, what will happen?
loud sound waves
75
Why are relays useful?
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
76
What happens if you supply a solenoid with AC?
the current will change, so the direction of the magnetic field will also change every half cycle
77
What is the variation of magnetic field strength in straight wires?
strongest closest to wire
78
What is the variation of magnetic field strength in solenoids?
uniform magnetic field inside coil
79
What happens if you increase the size of the current in a straight wire?
results in a stronger magnetic field
80
What happens if you increase the size of the current in a solenoid?
results in a stronger magnetic field
81
What happens if you reverse the current in a solenoid?
reverses magnetic field direction
82
What happens if you reverse the current in a straight wire?
reverses magnetic field direction
83
What happens whenever a current is at a right angle to a magnetic field?
it experiences a force, perpendicular to both the current and magnetic field
84
Why is a force experienced when a current carrying conductor is in a magnetic field?
interaction between the current's magnetic field and the permanent magnetic field
85
How can we work out the direction of the force when there is a current carrying conductor in a magnetic field?
Fleming's left hand rule:
First finger is magnetic field (N to S)
Second finger is conventional current
Thumb is the thrust/force
86
How can you reverse the force direction (current carrying conductor in a magnetic field)?
reverse current or reverse magnetic field direction
87
How can you increase the strength of the force (current carrying conductor in a magnetic field)?
stronger magnets, magnets closer together or increase current
88
What happens if you reverse the current in a current-carrying conductor in a magnetic field?
you reverse the force
89
What happens if you reverse the direction of the magnetic field in a current-carrying conductor in a magnetic field?
you reverse the force
90
How do you make a permanent magnet?
1. place steel rod inside coil
2. use high dc current for a time
3. leave steel inside, turn off current
91
How can you demagnetise a magnet?
1. place magnet inside coil
2. use ac current
3. slowly remove magnet with ac current still on
92
Describe an experiment to show the force in a current-carrying conductor in a magnetic field.
1. Place a wire carrying current in between two permanent magnets
2. turn on power supply
3. observe force on wire
93
Will any moving charged particle in a magnetic field experience a force?
yes
94
Determine the direction of the force on charged particles in a magnetic field when the field is into the page.
positive particle: up
negative particle: down
95
Determine the direction of the current on charged particles in a magnetic field when the field is into the page.
positive particle: left to right
negative particle: right to left
96
What is the basic principle of a d.c motor?
a current + a magnetic field will create a force
97
How does a d.c motor work?
the sides of the coil have a magnetic field, which interacts with permanent magnetic field to produce a force
98
How can we identify the force direction in a d.c motor?
Fleming's left hand rule: first finger is field, second finger is current and thumb is thrust
99
What causes the turning effect in a d.c motor?
a current-carrying coil in a magnetic field
100
Why do we have split-ring commutators on a d.c motor?
to reverse the direction of the current every half turn so that the coil rotates in the same direction
101
What would happen without split-rings?
the coil would flip backwards and forwards (the changing direction of current caused by the coil rotating through 180 would cause this)
102
Why do we need (carbon) brushes in d.c motors? Why are they good for this?
connect commutator to the circuit: low friction, good conductors
103
What do split rings do?
reverse the direction of the current every half turn
104
What do brushes ensure?
that the wires don't get twisted
105
How can the turning effect in a d/c motor be increased?
- increasing number of turns on coil
- increasing current
- increasing strength of magnetic field
106
What may happen if a conductor moves rapidly across a magnetic field?
creates induced emf or current or p.d
107
What is the basic principle of electromagnetic induction?
movement of a conductor + magnetism -> induced emf/current/pd
108
What would happen if a conductor cutting a magnetic field is not moving?
there will be no induced current
109
When is there a maximum of induced current in electromagnetic induction?
when the conductor is at 90 degrees to magnetic fields
110
What happens when a conductor is moving parallel to a magnetic field?
there will be no induced current
111
the sum of the currents entering a junction
in a parallel circuit is...
equal to the sum of the
currents that leave the junction
112
the total p.d. across the components in a series circuit is...
equal to the sum of the individual p.d.s across each component
113
the p.d. across an arrangement of parallel resistances is...
the same as the p.d. across one
branch in the arrangement of the parallel resistances
114
What are two situations in electromagnetic induction when there will be no induced current?
1. when there is no movement
2. when the conductor is moving parallel to the magnetic field
115
How can you reverse the current direction in electromagnetic induction?
- reverse wire movement direction
- reverse magnetic field direction
116
How can you increase the size of induced current in electromagnetic induction?
- stronger magnetic field
- move wire faster
- use a coil of wire
117
Describe an experiment to demonstrate electromagnetic induction.
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
118
What is a galvanometer?
a very sensitive ammeter
119
Why does electromagnetic induction work?
the conductor cuts the magnetic field lines, forcing electrons in the wire to move as a current
120
How do we identify the induced current direction in electromagnetic induction?
Fleming's right hand rule: first finger is field, second finger is current and thumb is thrust
121
What are two ways to induce an e.m.f (electromagnetic induction)?
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
122
What is Lenz's Law?
the direction of the induced emf (electromagnetic induction) opposes the change that creates it
123
How does the a.c generator work?
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
124
What is the simple idea behind ac generators?
movement of coil + magnetic field -> induced emf/current
125
How is the graph of induced emf vs time of an ac generator?
it goes up and down
126
What does a peak in an induced emf vs time graph of an ac motor mean?
long sides of coil are perpendicular to magnetic field, maximum emf is induced
127
What do the zeros in an induced emf vs time graph of an ac motor mean?
top and bottom sides of coil are parallel to magnetic field, so no emf is induced
128
What does a trough in an induced emf vs time graph of an ac motor mean?
long sides of coil move perpendicular to field in opposite direction: induced emf is maximum but in opposite direction
129
What is the function of slip rings in ac generators?
make constant contact with the coil during rotation
allow direction of induced emf to alternate, ergo causing an alternating current
connect coil to brush
130
What are slip rings?
cylindrical conductors that make constant contact with the coil during rotation
131
What is the function of carbon brushes in ac generators?
make electrical connection between rotating coil and circuit, avoiding wires becoming twisted
132
What does a transformer do?
increases or decreases alternating p.d
133
Describe the construction of a simple transformer.
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.
134
How does a transformer work?
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
135
What does a step up transformer do?
increases voltage (in secondary circuit), decreases secondary current
136
What does a step down transformer do?
decreases voltage (in secondary circuit), increases secondary current
137
What does Np mean?
number of turns on primary coil
138
What does Ns mean?
number of turns on secondary coil
139
What does Vp mean?
voltage applied to primary coil
140
What does Vs mean?
voltage induced on secondary coil
141
What does Ip mean?
current in primary coil
142
What does Is mean?
current in secondary coil
143
What is the physical difference between a step up and step down transformer?
step up: more turns on secondary coil
step down: less turns on secondary coil
144
How can you find out a missing number of turns in the primary/secondary coil?
Np/Ns = Vp/Vs
145
Vp/Vs =
Np/Ns
146
Primary power =
secondary power
147
Why is primary power = secondary power?
principle of conservation of energy
148
Power =
IV
149
Rate of heat energy loss from wires of resistance R:
P = Iˆ2 x R
150
How are transformers used in transmission of electricity?
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
151
State the advantages of high-voltage transmission.
- low heat loss (low current)
- most of electrical energy reaches destination
- thin, cheaper, less heavy cables can be used
152
Np/Ns =
Vp/Vs
153
IpVp =
IsVs (100% efficiency)
154
Power input = power output
IpVp = IsVs
155
Why are power losses in cable smaller when voltage is greater?
P = I^2 x R (where R is rate of heat energy loss)
156
Power input =
power output
157
Power =
current x voltage
158
hazards of electrical safety
* damaged insulation: electrecuted
* overheating cables: cause fire
* damp conditions: electrecuted
* excess current from overloading of plugs extension leads, single and multiple sockets: too much current => fire
159
what is electromotive force?
The work done or energy per unit charge around the whole circuit by an energy source, such as a battery
160
formula for electromotive force
emf (volts) = work (J) / charge (C)
161
what is faraday's law
A wire close to a constantly changing magnetic field will experience an induced electromotive force
162
what are two ways to induce emf
- Moving a magnet so that its field lines are cut by a wire.
- Moving a wire across a magnetic field.
163
what does each finger do in fleming's right hand rule
- ThuMb: Motion
- First finger: magnetic Field
- SeCond finger: Current
164
when is fleming's right hand rule used
when a force acts on a conductor perpendicular to a magnetic field.
165
what is Lenz's law
The direction of the induced e.m.f. opposes the change that creates it.
166
how to increase emf induced
- Increase strength of magnet -
- Increase speed of motion
- Increase force of motion
- Increase number of turns in coil
167
how to represent current coming out of the page
circle with dot
168
how to represent current going into the page
circle with cross
169
equipment to measure electromagnetic induction
- Voltmeter
- Magnets
- Conducting wire
- (Power supply is not needed to test e.m.f. induction)
170
reading of voltmeter if wire is moving up and down between the magnet in practical to measure electromagnetic induction
changing value on the voltmeter between positive and negative depending on whether you are moving the wire up or down
171
reading of voltmeter if wire is moving left and right between the magnet in practical to measure electromagnetic induction
no change on voltmeter
172
reading of voltmeter if wire is increasing speed in practical to measure electromagnetic induction
bigger change in emf
173
what is current
Rate of flow of charge (electrons) around a circuit
174
state and describe the two types of current
- Alternating current (ac): Electrons continuously change direction
- Direct current (dc): Electrons flow in one direction only)
175
give an example of an ac generator
wind turbine
176
what do the magnets do in AC generator
Provide constant magnetic field across the coil
177
what does the coil (armature) do in AC generator
Usually made from many turns of wire, is rectangular so that its sides are perpendicular to the magnetic field
178
what do the slip rings do in AC generator
Cylindrical conductors that make constant contact with the coil during rotation, they allow the direction of the induced electromotive force (e.m.f.) to alternate and therefore cause an alternating current (a.c.)
179
what do the carbon brushes do in AC generator
Make an electrical connection between the rotating coil and a circuit, avoiding the wires becoming twisted
180
Direct current (d.c.)
Creates a constant magnetic field
181
Alternating current (a.c.)
Creates an alternating magnetic field
182
When the wire is arranged as a coil (known as a solenoid), the resulting magnetic field is
the same as that of a bar magnet
183
General direction of the current flow (left to right) indicates
the direction of the magnetic field in the solenoid
184
The areas where the field lines are closest together indicate
where the field is strongest; in this case, inside the solenoid
185
To increase the strength of the magnetic field you can:
- Increase the current through the solenoid
- Increase the number of turns of wire in the solenoid
186
Supplying a coil with an alternating current (a.c.) has the effect of
changing the direction of the current and, therefore, the direction of the magnetic field every half cycle
187
Properties of electromagnets
- Very useful
- Can be designed to be extremely strong
- Can be turned off and on with a switch
188
where are electromagnets commonly used
in cranes to sort scrap metal, door lock, relays and school bells
189
how the electromagnet work
A flow of current results in the generation of a magnetic field around a coil.
The magnetic field will attract a magnetic material, for example an iron bar, and close the circuit.
The closed circuit will now perform an action, such as ringing a bell, turning on a different circuit or locking a door.
190
what is a relay
device used to switch between separate circuits
191
how does a relay work
- When an electromagnet is supplied with current, magnetic field is generated
- The field attracts the iron bar, which is pivoted and allows the conducting contacts at A to touch. The second circuit containing the lamp is now closed and current flows
192
how does a speaker cone work
- A speaker cone can be seen to oscillate left and right when an alternating current is supplied to the coil
- Magnetic field due to the alternating current in the coil either attracts or repels a permanent magnet around it, resulting in the vibrations necessary for sound.
193
To increase the size of force
- Increase the current in the wire.
- Increase the number of individual wires.
- Increase the strength of the magnetic field.
- Increase the length of the wire within the magnetic field.
194
fleming's left hand rule fingers
- First finger = Field (always N to S)
- SeCond finger = Current
- ThuMb = Movement or force
195
left hand rule on Positive particle
Field: into the page. Current: left to right. Force: up
196
Negative particle
Field: into the page. Current: right to left (because it is a negative particle). Force: down
197
To make dc motor spin more quickly
- Increase the strength of the magnets and thus the magnetic field
- Increase the number of turns of wire in the coil
- Increase the current to the coil from the power supply
198
coil (armature) in dc motor
Rectangular and often made up of lots of turns of current-carrying wire
199
magnets in dc motor
Bar magnets are usually used, producing a field perpendicular to the coil, from N to S
200
brushes in dc motor
Allow constant electrical contact with the inside of the split ring while the coil rotates – the wires would get twisted otherwise
201
split ring commutator in dc motor
As the coil rotates, the direction of the current needs to stay the same so that the force also acts in the same direction
This is achieved using a split ring commutator
Once the coil has rotated through 180°, current continues to flow in the original clockwise direction causing the force on the left side to be up, and the force on the right side to be down
Without the split ring, the changing direction of current caused by the coil rotating through 180° would cause the coil to flip backwards and forwards
202
how dc motor works
A turning force causes the coil to spin about the pivot. Brushes allow constant contact whilst the coil spins freely. The split ring commutator ensures that current always flows the same way around the loop. In this case, the turning force always acts in a anticlockwise direction.
203
symbol for switch
204
symbol for cell
205
symbol for battery
206
symbol for fuse
207
symbol for fixed resistor
208
symbol for variable resistor
209
symbol for thermistor
210
symbol for heater
211
symbol for light dependent resistor
212
symbol for relay coil
213
symbol for transformer
214
symbol for variable potential divider
215
symbol for magnetizing coil
216
symbol for ac power supply
217
action of potential variable divider
The resistor with the largest resistance will have a greater potential difference than the other one. If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share.
218
factors affecting magnitude of induced emf
- Magnetic strength of the core in the coil of wire. (stronger --> bigger emf)
- Number of turns of wire in the coil (more turns --> bigger emf)
- The cross-sectional area of the coil (bigger area --> bigger emf).
219
What is a non-magnetic material?
materials that do not experience a force when in a magnetic field
220
Difference between magnetic and non-magnetic material?
- non-magnetic - do not experience a force when in a magnetic field
- magnetic materials - NOT magnets, but are attracted to magnets.
- there will always be a force of ATTRACTION between a permanent magnet and a magnetic material
221
Uses of permanent magnets.
* guitar pickups
* speakers
* cupboard latches
222
Uses of electromagnets.
* electric door locks
* relays
* MRI machines
223
Properties of permanent magnets vs electromagnets.
**Permanent magnets:**
* constant magnetic field
* can't be switched off/on
* north and south poles can't be swapped
**Electromagnets:**
* variable strength magnetic field
* can be switched on and off quickly
* north and south poles can be changed by changing the direction of current flow
224
Experiment to show production of electrostatic charges by friction.
1. Rub a plastic rod quickly with a dry nylon cloth to generate friction
2. Once charged, move the rod close to small pieces of paper (for example, pieces from inside a hole punch)
3. Observe what happens
225
What is static electricity? How does it occur?
- **imbalance between - and + charged objects**, often created by adding or subtracting electrons from insulators
- occurs when **friction between two insulators** causes **electrons to be transferred** from one surface to another, one becomes - charged and the other becomes positively charged
226
What are two ways an ammeter can be?
- analogue, using a needle pointer
- digital, showing current to a set number of s.f
227
What is the use of an ammeter?
- measuring electric current
- always inserted into a circuit in series to measure current flowing through a component
228
What is electric current?
- a measure of the flow of charge/rate of flow of electric charge
- charge passing a point per unit time
229
Experiment to determine resistance using a voltmeter and ammeter.
| give equipment and method
Equipment:
* ammeter
* voltmeter
* variable power supply
* fixed resistor
Method:
1. turn on the circuit, adjust the voltage so voltmeter reads 1 V
2. take a reading from both the voltmeter and the ammeter
3. gradually increase voltage from power suppl, increase it by 1 V up to a maximum of 8 V (don't touch wire, it gets hot)
4. take readings from the voltmeter and ammeter at each stage. Turn off power supply between readings.
5. Draw a graph: p.d on the x-axis and current on the y-axis.
6. Your results should produce a straight-line graph. Calculate resistance of the resistor by finding inverse of the gradient
230
What is the relationship of resistance of metallic wire with its length?
- directly proportional to length
- if length doubles, the resistance also doubles
231
What is the relationship of resistance of metallic wire with its cross-sectional area?
* inversely proportional to the cross-sectional area
* wider diameter gives more room for electrons to flow
232
What is the kilowatt-hour?
- alternative unit for energy used to measure consumption of electricity
- amount of power (in kilowatts) that would be used within one hour
- instead of watts --> kilowatts, instead of seconds --> hours
233
How do electricity companies calculate cost of using electrical appliances?
cost of electricity = number of kilowatt hours X cost per kilowatt-hour
*(this is the same thing as units of energy consumed X cost of one unit of energy, since the unit of energy used by electricity companies are kilowatt-hours)*
234
Current-voltage graph for a RESISTOR/WIRE.
- I and V directly proportional (because R is constant)
- graph goes through origin
- ohmic device when temperature is constant
- obeys Ohm’s Law so the resistance can be calculated using
235
Current-voltage graph for a FILAMENT LAMP.
- low values: I and V are directly proportional, so graph goes through origin
- large voltage applied: only a small increase in current.
- Flow of current ---> wire in filament lamp heats up. Atoms gain KE, collisions with electrons become more frequent, R increases, so I and V are not directly proportional.
- does not obey Ohm’s Law, non-ohmic
236
Current-voltage graph for a DIODE.
- diode requires a small voltage to ‘switch on’
- I–V graph: as voltage increases from zero, the current remains zero until around 0.7 V when the diode ‘switches on’
- the diode then behaves like a wire.
- there is always zero negative current - therefore a non-ohmic device
237
Simple electric field patterns around a point charge
- negative: radial pattern of lines towards point
- positive: radial pattern of lines away from point
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Simple electric field patterns around a charged conducting sphere
same radial patterns of field lines seen around a charged point can be seen around charged conducting spheres
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Simple electric field patterns around two oppositely charged parallel conducting plates
- field lines are parallel
- uniform field
240
Describe an experiment to distinguish between electrical conductors and insulators
1. Charge the plate of the GLE (gold leaf electroscope) so that the gold leaf stands clear of the rod
2. Carefully touch the plate of the GLE with the item being tested
3. Record the observations each time
Leaf falls: material is a good conductor
Leaf remains in place: object is a poor conductor (good insulator)
Leaf falls slowly: material is a poor conductor
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mains circuit consists of
live wire (line wire), a neutral wire and an earth wire
