09 Circuits And Mains Electricity Flashcards
1
Q
Why would an appliance NOT need an earth wire
A
- its double insulated
- has plastic casing or the electrical components are incased in plastic
2
Q
Power equation
A
Power = current x voltage
3
Q
Power
A
The rate of energy transfer
OR
the amount of energy transferred per second
4
Q
Purpose of a fuse - simple
A
- designed to cut off the flow of electricity to an plainte if current becomes too large due to a fault/surge
5
Q
Fuse circuit symbol
A
Rectangle with line through it
6
Q
Purpose of a fuse
A
- if the current in a wire becomes too large (exceeds stated value)
- fuse heats up and melts
- breaks the circuit and stops the current
- makes sure more current doesn’t flow through the circuit and cause a fire/spark/explosion
7
Q
Equation to find right fuse for appliance
A
Current = power/voltage
8
Q
Why should a fuse always be slightly higher than current needed by appliance
A
- always choose next size
- if the fuse current rating is too low, it will break the circuit even when an acceptable current is flowing
- if the fuse current rating is too high, it will not break the circuit in enough time before damage
9
Q
Energy transferred equation
A
Energy transferred = current x voltage x time
10
Q
Common electrical safety hazards
A
Damaged insulation - is someone touches an exposed piece of wire they can experience lethal shock
Overheating of cables - when too much current is in a small wire it can melt the insulation and expose live wires leading to a fire
Damp conditions - if someone comes in contact with live wires it can cause electrocution or fire
11
Q
Insulation
A
- electrical wires are covered with an insulating material e.g. rubber
12
Q
What are wires usually made of
A
Copper
- better conductor
13
Q
Double insulation
A
- insulation around wires
- non-metallic case that acts as a second layer of insulation
14
Q
Risk of no earth wire
A
- many electrical appliances have metal cases -> potential electrical safety hazard
- if live wire comes into contact with case, the case would be electrified and can be a risk of electrocution to people who touch it
- earth wire reduces this risk
15
Q
Earth wire
A
- provides a low resistance path to earth
- causes a surge of current in the earth wire and hence also in the live wire
- the high current through the fuse causes it to melt and break
- this cuts off the supply of electricity to the appliance making it safe
16
Q
Circuit breaker and its advantage
A
Consists of an automatic electromagnet switch that breaks the circuit if current exceeds a certain value
Adv - doesn’t melt/break so it can be reset and used again, works faster
17
Q
Current passing through a resistor(or wire) results in
A
Electrical transfer of energy
18
Q
Current
A
Rate of flow of charge
19
Q
Current in a resistor results in.. and examples of when its useful
A
Current passing through results in electrical transfer of energy
Temperature of resistor increases due to collisions of the free electrons within the wire
Some of the energy is dissipated into surroundings by heat
Used for electric heaters/ovens/hobs, toasters and kettles
20
Q
Mains electricity can be supplied by
A
Alternating current or direct current from a cell/battery
21
Q
Direct current
A
A steady current, constantly flowing in the same direction in a circuit from positive -> negative
- potential difference across a cell travels in one direction only
- has fixed positive terminal and a fixed negative terminal
- produced by cells and batteries
22
Q
Examples of direct current
A
Electric cells or batteries
23
Q
Things in a dc circuit
A
Cell/battery, bulb, resistor and voltmeter in parallel
24
Q
Alternating current
A
A current that continuously changes direction, going back and forth
- travels from positive terminal to negative terminal (two identical terminals)
- produced by electrical generators e.g. mains electricity
25
Mains electricity frequency
50 Hz
26
Mains electricity potential difference
230V
27
Plastic/polythene rod being charged by rubbing with a cloth
- both the rod and cloth are insulating material
- polythene rod may be given a charge by rubbing it witha cloth
- the occurs because negatively charged electrons are transferred from one material to another
- the rod gain electron
- since electrons are negatively charged the rod becomes negatively charged
- as result the cloth loses electrons and is left with an equal positive charge
28
In components in series circuits have
Same current
29
Current is the same at
Each point in a series circuit
30
Decreasing voltage of a power source will
Reduce current
31
Increasing the number of components in a series circuit increases
The total resistance
- hence less current flows through the circuit
32
In a parallel circuit what happens to current
- current splits at a junction
- electrons have different paths to take
- sum of current on individual branches equals total current
33
Current is ___ at a junction
Conserved
34
In a series circuit the total voltage of a power supply is
Shared between the components
35
For two identical components with equal resistance, the voltage across them will be
The same
- equal to half the total voltage of the power supply
36
For two non-identical components (with different values of resistance) the voltage will be
Higher across the component with higher resistance
Lower across the component with lower resistance
37
Advantages of series circuit
- all of the components are controlled by a single switch
- fewer wires required
38
Disadvantages of series circuit
- components can’t be controlled separately
- if one component breaks, all the others stop working
39
Advantages of parallel circuit
- the components can be individually controlled using their own switches
- if one component breaks the others will continue to function
40
Disadvantages of a parallel circuit
- many more wires can be complicated to set up
- all branches has same voltage as supply making it more difficult to control the voltage across individual components
41
Why do bulbs dimmer in a series circuit when another one is added
- voltage shared between bulbs is in series
- less voltage across each bulb therefore bulb is dimmer
- adding a second bulb doubles resistance of circuit which halves the current
42
Voltage
Energy per unit charge
- or energy transferred per unit charge passes
43
What is voltage across a cell or power supply a measure of
The energy transferred from chemical store electrically to wires per coulomb
44
What happens to the total resistance and current in a circuit as more bulbs are added in parallel
Adding bulbs in parallel decreases the total resistance -> v=i/r
As resistance decreases, current increase for the same voltage
45
Fixed resistor - how would a line change for a resistor with half the resistance
- steeper
- double the gradient
- double the current
46
Total resistance equation
R = R1 + R2
47
Components connected in series summary
- current is the same at a;; point and in each components
- voltage of the power supply is shared between components
- total resistance is the sum of the resistances of each components
48
Components connected in parallel summary
- current from supply splits at branches
- voltage across each branch is the same
- total resistance is less than that of each component
49
Components with linear IV graphs
Fixed resistors, wires
50
Components with linear IV graphs
Fixed resistors, wires
51
Components with non-linear IV graphs
Filament lamps, diodes, LDR, thermistors
52
Components with non-linear IV graphs
Filament lamps, diodes, LDR, thermistors
53
Describe IV graph for wire/resistor
- relationship is linear/directly proportional
- resistance is constant
- current is directionally proportional as graph is a straight line through origin
54
Describe IV graph for filament bulb
- relationship is non-linear and not directionally proportional
- not straight line, so voltage and current do not increase for decrease by same amount
- resistance changes as slope is not constant
- as voltage increase the current increases at a proportionally slower rate
55
Describe why the IV graph for filament lamp is non linear
As current through a filament lamp increases, the resistance increases because
- higher current causes the temperature of the filament to increase
- higher temperature causes atoms in a metal lattice to vibrate more
-causes increase in resistance -> more difficult for free electrons (current) to pass through
- since resistance opposes current, it causes it to increase at a slower rate
56
Describe IV graph for a diode
Forward bias - shown by sharp increase in voltage and current on right of graph
Reverse bias - shown by 0 reading of current and voltage on left of graph
- shows resistance is very high
57
What do lamps and LEDs indicate
Presence of current in a circuit
58
Voltage equation
Voltage = current x resistance
59
Charge equation
Q = I x T
Charge = current x time
60
Electric current in a solid metallic conductor
Flow of negatively charged electrons
61
Voltage across 2 components connected in parallel is
The same
62
Volt
Joule per coulomb
63
Energy transferred equation
En ergo transferred = charge x voltage
64
Conductor
A material that allows charge (electrons) to flow through it easily
65
Examples of conductors
Copper, steel, aluminum
66
Metals conduct electricity well because
Current is the rate of flow of electrons
So the more easily electrons are able to flow the better the conductor
67
Insulator
Has no free charges so it doesn’t allow the flow of charge through easily
68
Examples of insulators
Rubber, plastic, wood, glass
69
Examples of insulators
Rubber, plastic, wood, glass
70
Investigating how insulating materials can be charged by friction
- rub both ends of a polythene rod with a cloth
- rub both ends of an acrylic rod
- bring each end of the rods together
- record observation of the motions of the rod
- repeat 3x
71
What happens to electrons when two insulating materials are rubbed together
Electrons will transfer from one insulator to the other insulator
72
Polythene rod is given a negative charge by rubbing it with a cloth because
Electrons are transferred to the polythene rod from cloth
Electrons are negatively charged so the rod becomes negatively charged
73
Acetate rod is given a positive charge when rubbed with a cloth because
Electrons are transferred away from acetate to the cloth
Electrons are negatively charged so when the acetate loses negative charge, it becomes positively charged
74
If the material is repelled by the polythene rod
The materials have the same charge
75
If the material is attracted to the polythene rod
They have opposite charges
76
Neutral atom
Number of electrons equals number of protons
- equal charges cancel out, overall charge is 0
77
Opposite charges
Attract
78
Same charges
Repel
79
Charging by friction
When certain insulating materials are rubbed against each other they become electrically charged
80
The object the electrons are transferred TO becomes
Negatively charged
81
The object the electrons transfer FROM becomes
Positively charged
82
Use of electrostatic charges in photocopiers
- image of document is projected onto a positively charged copying plate
- plate loses its charge in light areas and keeps the positive charge in dark areas
- a negatively charged ink is applied to the plate and sticks to where there’s positive charge as positive and negative charges attract
- ink is transferred to white paper
- paper is heated so the powder sticks
83
Dangers of electrostatic charge
- risk of electrocution
- risk of fire and explosion due to a spark
84
Earthing
Used to prevent the dangerous build-up of charge
- this is done by connecting the vehicles to the earth with a conductor
85
Fuelling vehicles
- build up of static charge is a potential danger when refuelling aeroplanes
- fuel runs through pipes at a fast rate and fuel is flammable
- friction between fuel and pipe causes the fuel to gain charge
- is the charge causes a spark the fuel could ignite and cause explosion
86
How to prevent spark when fuelling plane
Fuel tank is connected to earth with a copper wire called the bonding line
Conductor earths the plane by carrying the charge through to the earth which removes risk of any sparks
87
Thermistor
Resistor with two lines
88
Variable resistor
Resistor with line through it
89
LDR
Light dependant resistor
- lets light in -> circle and rectangle
90
LED
Diode in circle
- light emitting diode
91
Resistance increases
Current decreases
92
Use of variable resistor
To vary current in circuit
93
How to measure resistance of component circuit
- lamp or resistor
- ammeter
- voltage
- variable resistor
- battery
94
Resistor graph
Resistance is constant
95
Filament lamp
Curved line
Resistance is highest at high voltage because of high temperature
96
Diode graph
Line goes up on right
Resistance = infinite when negative and only allows current in one direction
97
LDR graph
Doesn’t go through y axis but decreases
Resistance decreases as light intensity increases
High resistance in dark
98
Thermistor graph
Starts on y axis and decreases
Resistance decreases as temp increases
High resistance in cold
99
Bright light
High current
100
Role of resistors
Total resistance of circuit increases because pathways becomes harder for current to flow through
101
Static electricity in photocopiers
Image of document to be copied is projected on positively charged image plate
—> light areas of image lose charge, dark areas keep charge
Negatively charged toner is attracted to positive regions on plate
