Unit 4 - Electricity and magnetism Flashcards
(125 cards)
1
Q
Attract
A
A force that pulls objects together
2
Q
Repel
A
A force that pushes objects apart
2
Q
Poles of a magnet
A
North & South poles
3
Q
Magnetic field
A
A region of space where another magnet or magnetic material experiences a force
4
Q
Attraction displayed in a diagram
A
Field lines point in the same direction (N to S) and flow between the two magnets
5
Q
Repulsion displayed in a diagram
A
Field lines point in opposite directions and bend away from each other
6
Q
Attraction of poles
A
Opposite poles attract e.g. (N&S) hence same poles repel
7
Q
Non-magnetic materials
A
Do not experience a force when in a magnetic field
8
Q
Property of field lines that represent the field strength
A
Density of the lines
9
Q
Permanent magnet
A
Always has a magnetic field
10
Q
Induced magnetism
A
When a magnet material, in the presence of a permanent magnet becomes attached to a magnet and temporarily induces a North and South Pole until later becoming demagnetized
11
Q
Materials that keep their magnetism after the external field is removed
A
- Hard magnetic material e.g. steel - hard to magnetize and demagnetize - used to make permanent magnets
- Soft magnetic material e.g. iron - easy to magnetize and demagnetize - good for temporary magnetics
12
Q
Properties of a permanent magnet
A
- Constant magnetic field
- Cannot be switched on or off
- North and south poles cannot be swapped
- Used for guitar pickups, speakers & cupboard latches
13
Q
Properties of an electromagnet
A
- Variable strength magnetic field
- Can be switched on and off quickly
- North and south poles can be changed by changing the direction of the current flow
- Used for electric door locks, relays & MRI machines
14
Q
Insulators
A
Do not let electrical charge / electrons to move freely
15
Q
Conductors
A
Allow electrical charge / electrons to move freely
16
Q
Charge carriers in electrical devices
A
Electrons
17
Q
Two types of electrical charge
A
- Positive
- Negative
18
Q
Unit of electrical charge (Q)
A
Coulombs (C) - one electron carries the charge of 1.6x10^-19 C
19
Q
Condition for static electricity
A
Occurs when friction between two insulators causes electrons to be transferred from one surface to another making one insulator gain electrons and become negative and the other to become positive through the loss
20
Q
Electric field
A
The space in which an electric charge experiences a force
21
Q
Direction that an electric field points
A
in the direction that a positive charge experiences a force
22
Q
Uniform field
A
When the field lined are parallel between parallel plates
23
Q
Pattern of field lines used around conducting spheres or point charges
A
Radial patterns
24
Current
A measure of the flow of charge
25
Ways to increase current
- Making each charged particle move faster
- Increasing the number of charged particles
- Increasing the amount of charge each particle carries
26
Unit of current
Amps (A)
27
Device used to measure current
Ammeter
27
Current equation
- Current (I) = Charge (Q) ÷ Time (T)
- Amps = Coulombs ÷ Seconds
28
Direction of conventional current
Flowing out of the positive terminal of the battery
29
Correlation between conventional current and electron flow
both act in the opposite direction to each other
30
Alternating current (a.c.)
Electrons continuously change direction
31
Direct current (d.c.)
Electrons flow in one direction only
32
Condition of voltage source to make alternative current (a.c.)
- Source must cause electrons to move back and forth
33
Condition of voltage source to make direct current (d.c.)
Electrons must flow in one direction
34
Voltage
The unit of potential difference
35
Unit of measurement for voltage
Volts (V)
36
Electromotive force (e.m.f)
The work done by a source in moving a unit charge around a complete circuit
37
Unit of measurement for electromotive force (e.m.f)
Volts (V)
38
Potential difference (p.d.)
The energy needed per charge to flow between two points in a circuit / the work done per unit charge passing through a component
39
Device used to measure volts
Voltmeter
40
Type of circuit voltmeters are used in
Parallel circuits
41
Voltage equivalent in joules
-1 V is equivalent to giving 1 J of energy to 1 C of charge
- 1 V = 1 J/C
42
Electromotive force (e.m.f) equation
- Electromotive force (V) = Work done (J) ÷ Charge (C)
- E = W/Q & V = W/Q
43
Resistance
Measure of how much opposition there is to the flow of current in a circuit
44
Resistance equation
- Resistance = Voltage ÷ Current
- R = V/I
45
Unit of measurement of resistance
Ohms (Ω)
46
Correlation of resistance and the length of a wire
- Directly proportional e.g. if length doubles resistance doubles
47
Correlation of resistance and the width of a wire
- Inversely proportional e.g. if the cross-section doubles in width resistance is halved
48
Ohm's law
- V = IR
- Relationship between p.d. and current is directly proportional e.g. if voltage doubles current doubles
49
Negative current or voltage
Achieved by changing the direction of the battery
50
Plotting of I-V graphs
Current and voltage are plotted for both positive and negative values
51
Ohmic device
A device where under constant temperature the current and voltage are directly proportional e.g. a resistor or wire
52
Non-ohmic device
e.g. a filament lamp where when large voltage is applied there is only a small increase in current causing an increase in heat and atoms gaining kinetic energy and colliding, causing an increase in resistance
53
Diode
- A circuit component that only allows current to flow in one direction but requires a small voltage to 'switch on'
- Non-ohmic device since there is always zero negative current
- Only works when more than 0.7 V is applied
- In circuit, diode symbol points in the direction of conventional current flow
54
Power
A measure of how quickly energy is transferred
55
Unit of power
Watt (W) = 1J/s
56
Electrical power equation
- P = IV
- or:
- P = V^2 ÷ R
- or:
- P = I^2R
57
Energy equation
- E = Pt
- E = IVt
58
Common unit of energy
Kilowatt hours
- Energy (kWh) = P(kW) x t(h)
59
Cost of electricity equation
Cost of electricity = number of kilowatt hours x cost per kilowatt-hour / cost per unit
60
Conventions for a circuit diagram
- Straight lines for wires
- Placing voltmeters in parallel with the components
- Placing ammeters in series with components
- using conventional electrical symbols
61
Switch symbol
Straight line with an opening
62
Cell symbol
Straight line with two vertical lines in the middle, a longer one on the left with a positive sign and a shorter on the right
63
Battery symbol
Straight line with two vertical lines alike a cell on the right, and another two alike a cell on the left, with a dotted line connecting them
64
Lamp symbol
A circle with an X inside
65
Fuse symbol
A rectangle with a straight line running through it
66
Voltmeter symbol
a circle with a V inside it
67
Ammeter symbol
A circle with an A inside it
68
Fixed resistor symbol
An empty rectangle
69
Variable resistor symbol
a rectangle with a diagonal arrow running through pointing to the top right
70
Thermistor symbol
A rectangle with a diagonal line running through to the top left, with a straight hook at the bottom left
71
Heater symbol
A rectangle divided into four squares
72
Light dependent resistor (LDR) symbol
A rectangle with two arrows pointing at it from the top left
73
Relay coil symbol
A rectangle cutting through a vertical line
74
Transformer symbol
Two rectangles with vertical coils separated by a vertical line
75
Variable potential divider (potentiometer) symbol
A rectangle with an arrow pointing in the middle in a straight direction
76
Magnetizing coil symbol
A coil
77
a.c. power supply symbol
Two white dots with a squiggle in between
78
Motor symbol
A circle with an M inside of it
79
Generator symbol
A circle with a G inside of it
80
Diode (extended only) symbol
A straight line with a 90˚ clockwise rotated equilateral triangle pointing at it
81
Light-emitting diode (LED) (extended only) symbol
A straight line with a 90˚ clockwise rotated equilateral triangle pointing at it and two arrows pointing at it from the top right
82
Light-emitting diodes (LED)
- Same features as a regular diode but produces a single color of light
- Current flow from battery and direction of the diode are the same - other way around current cannot flow because resistance is too high
83
Property of a thermistor that changes with temperature
Resistance increases with increased temperature
84
Properties of a series circuit
- Current flows in one loop - os same at all points
- Voltage is shared - hence multiple lamps are dimmer than one
- Total resistance is sum of resistors
- Electromotive forces of cells added together to find total e.m.f.
- p.d. across resistors is shared - sum of p.d.s across the resistors is qual to the battery power (e.m.f.)
85
Properties of a parallel circuit
-More than one loop of current - current is shared - total current flowing into a junction is equal to the sum of the currents flowing out of a junction
- Voltage is the same through each loop - hence p.d. of each lamp is equal to electromotive force (e.m.f.) of the battery
- Loops can be turned on or off independently of each other
- Combined resistance of two resistors in parallel is less than that of either resistor by itself - 1/Rtotal = 1/R1 + 1/R2 - each resistor experiences the same p.d. as the power supply
86
Advantages of parallel
- One light can go out without others going out - one but in series would cause the whole circuit to fail
- Less resistance than in series
87
Variable potential divider use
- Long resistor that can be split in two parts
- Arrow in the centre represents an adjustable wiper
- Wiper can be moved to adjust ratios of resistance - e.g. if wiper is moved to the left right side has more resistance
88
Potential divider use
- Splits up voltage
- Seen as two resistors in series
- R1/R2 = V1/V2
89
Electrical hazards
- Damaged insulation - damaged insulation leads to live wires that can electrocute - repair with insulating tape
- Overheating of cables and applianced - can cause fire- switch off any appliance that feels hot
- Overloading sockets - can draw too much current to one socket - can result in a fire
- Damp conditions - Water conducts electricity there can electrocute - keep applianced dry and have dry hands
90
Circuit used for a house
Mains supply
91
Safety devices with electricity
- Earthing metal cases - Earth wire is connected to the outer metal casing of an appliance - can prevent a lethal shock because current flows to earth because resistance is lower than a person
- Fuses - protect a circuit - thin wire inside that connects to the live wire - too much current and the wire melts and circuit breaks - rating of a fuse must be the lowest value greater than the current needed for an appliance
- Trip switches - circuit breaker - safety device - if too much current is flowing between the live and neutral wires a switch opens to break the circuit - relay in reverse - can be used more than once - value is lowest value that is greater than the current needed for the appliance
91
Three wires of a three-core electric cable
- The Live Wire - brown - carries the current from the mains supply and an on/off switch would be connected
- The neutral wire - blue - completes the full circuit - does not supply current
- The earth wire - Green & yellow - safety feature to prevent electrocution
92
Faraday's Law
A wire close to a changing magnetic field will experience an induced electromotive force
93
Two ways to induce e.m.f
- Moving a magnet so that its field lines are cut by a wire
- Moving a wire across a magnetic field
94
Fleming's right-hand rule
- First and second finger and thumb are perpendicular
- Line up hand with the diagram with and the two quantities given
- Observe the direction of the third quantity by:
- The direction of the motion of the wire is represented by the thumb
- The direction of the magnetic field is represented by the first finger
- The direction of the current is represented by the second finger
95
Lenz's law
The direction of the induced e.m.f opposes the change that creates it
96
Components of an a.c. generator
- Magnets - provide a constant magnetic field across the coil
- Coil (or armature) - made from many turn of wire - rectangular so that its sides are perpendicular to the magnetic field
- Slip rings - cylindrical conductors that make constant contact with the coil during rotation - allow the direction of induced e.m.f. to alternate and cause an alternating current
- Carbon brushes - make an electrical connection between the rotating coil and a circuit - avoiding the wires becoming twisted
97
Stages of rotating generator coil
1 - top and bottom sides of the coil are moving parallel to the magnetic field and no e.m.f is induced
2 - Long sides of the rectangular coil move exactly perpendicular to the magnetic field and maximum e.m.f. is induced
3 - Again the top and bottom sides of the coil are moving parallel to the magnetic field and no e.m.f is induced
4 - Again the long sides move perpendicular to the field in the opposite direction - maximum e.m.f.
98
Magnetic fields created by types of currents
Direct current - constant magnetic field
- Alternating current - alternating magnetic field
99
Solenoid
When the wire is arranged as a coil the resulting magnetic field is the same as that of a bar magnet
100
Increasing the strength of a magnetic field inside a solenoid
- Increase the current through the solenoid
- Increase the number of turn of wire in the solenoid
101
How electromagnets work
- A flow of current results in the generation of a magnetic field around a coil
- The magnetic field will attract a magnetic material close to the circuit
- The closed circuit will now perform an action
102
Function of a relay
A device used to switch between separate circuits
103
How loudspeakers function
- Speaker cone oscillates left and right when an alternating current is supplied to the coil
- Magnetic field due to alternating current in the coil either attracts or repels a permanent magnet around in resulting in vibrations needed for sound
104
Corkscrew rule
- Thumb points in the direction of current flow and fingertips in the direction of the magnetic field
- Used to find out the direction of the magnetic field around a straight wire
105
Ways to increase the force experienced by a wire within another magnetic field
- 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
106
Fleming's left-hand rule
- Direction of force, current and field are perpendicular
- If field and current are parallel, no force will be acting
107
Flow of charged particles
- Conventional currents and positive charge flow from positive to negative
- Negative charge movies in the opposite direction to current flow
108
How to make a d.c. 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
109
How to increase the turning force of a d.c. motor
- Increase the current
- Increase the strength of the magnetic field
- Increase the number of turns in the coil
110
Components of a d.c. motor
- Coil (or armature) - rectangular and often made up of lots of turns of current-carrying wire
- Magnets - bar magnets used making a field perpendicular to the coil N-S
- Brushes - allow constant electrical contact with the inside of the split ring while the coil rotates otherwise wires would get twisted
- Split ring commutator - As coil rotates the direction of the current needs to stay the same so that the force also acts un the same direction - once the coil rotates through 180˚ current continues to flow in the original clockwise direction causing the force on the left side to be up and force on the right side to be down - if not present the coil would flip backwards and forwards
111
Function of a transformer
A device that can increase or decrease the size of an alternative e.m.f.
112
Two types of transformers
- Step-up transformer - increases voltage - has more turns on the secondary coil than the primary coil
- Step-down transformer - decreases voltage - has fewer turns on the secondary coil than on the primary coil
113
Parts of a transformer
- Primary coil - where a.c. is supplied - energy source of the transformer
- Soft iron core - designed to allow the transition of magnetic flux to a secondary coil
- Secondary coil - the output of the transformer
114
Transformer notations
- Primary coil:
- Np - number of turns on the primary coil
- Vp - voltage applied to the primary coil
- Ip - current in the primary coil
- Secondary coil:
- Ns - number of turns on the secondary coil
- Vs - voltage induced on the secondary coil
- Is - current in the secondary coil
115
Equation for coil turns and voltage in transformers
Vp/Vs = Np/Ns
116
Use of transformers
- Designed to increase the voltage which electricity is transmitted - current in cables is lower and less energy is wasted as heat - cables with large cross-sectional area lowers the resistance and energy wasted
117
How transformers function
- Only work with a.c.
- Primary coil is supplied with an a.c. and behaves like a bar magnet that is constantly switching its poles due to changing current
- Constantly changing magnetic field is transferred to the secondary coil via soft iron core
- Secondary coil acting as a conductor is the presence of a changing magnetic field induces e.m.f across the secondary coil
- e.m.f. induced across the secondary coil is constantly changing and therefore provides and alternating current
118
Conservation of energy in a transformer
- Power in is equal to the power out
- Pp = Ps
- VpIp = VsIs
119
Equation of the efficient of transformers
Efficiency of transformers = Pout / Pin or IsVs / IpVp
120
How to increase the efficiency of a transformer
- Using low resistance coils to reduce the power wasted due to the heating effect of the current
- Using a laminated core which consists of layers of iron separated by layers of insulation - reduces heat in the iron core and prevents eddy currents (currents induced in the core itself_
121
Main energy loss in transmission of electricity
- Heat
122
Equation to calculate power loss in a wire
P = I^2R
- Power (W) = Current ^2 (A) x resistance (Ω)
