Electricity Flashcards
(100 cards)
1
Q
Current
A
Rate of flow of charge
2
Q
When can current only flow
A
When there’s a source of potential difference
3
Q
What is a complete circuit
A
A closed one
4
Q
Potential difference
A
Driving force that pushes charge round
5
Q
Unit of current
A
amp (A)
6
Q
Unit of potential difference
A
volts (V)
7
Q
Resistance
A
Anything that slows flow of charge down
8
Q
Unit of resistance
A
ohm (Ω)
9
Q
Effect of resistance on current
A
The greater the resistance across a component, the smaller the current that flows
10
Q
Units for charge
A
Coloumbs (C)
11
Q
Cell
A
Store of chemical energy
12
Q
Battery
A
2 or more cells together
13
Q
Open switch
A
Breaks a circuit
14
Q
Closed switch
A
Joins a circuit
15
Q
Filament lamp
A
Lights up when current flows
16
Q
Why does filament lamp light up when current flows
A
Wire gets hot
17
Q
Fuse
A
Melts when current gets to hot - safety device
18
Q
LED
A
Light emitting diode - emits light when current flows in only 1 direction
19
Q
Fixed resistor
A
Restricts amount of current flowing
20
Q
Variable resistor
A
Allows current to be varied
21
Q
Ammeter
A
Device that measures current
22
Q
How does ammeter measure current
A
Counts number of charges per second
23
Q
Where do ammeters go
A
In series
24
Q
Voltmeter
A
Device that measures potential difference
25
How does voltmeter measure pd
Measures difference in energy
26
Where does voltmeter go
In parallel to component it's measuring
27
Diode
Only allows current to flow 1 way, resistance very high in reverse
28
LDR
Light dependent resistor, at light intensity increases, resistance decreases
29
Thermistor
As temperature increases, resistance decreases
30
How to investigate effect of wire length on resistance
- attach crocodile clip at 0cm on ruler
- attach 2nd crocodile clip at length (e.g-10cm) from first clip
- close switch and record current through wire + p.d across it
- open switch and move 2nd clip a length up wire (e.g-10cm), close switch and rerecord
- repeat test for different lengths of wire
- use measurements to calculate resistance (V=IR)
- plot resistances on graph, draw line of best fit
- graph should be directly proportional, if not, there is systematic error
31
Ohmic conductor
Conductor that obeys Ohm's law
32
Ohm's law
- at constant temperature, current flowing through ohmic conductor is directly proportional to p.d across it
- resistance remains constant as current changes
33
Ohm's law exceptions
- resistance of some components not constant
- e.g - diode/filament lamp
34
I-V characteristics practical
- set up test circuit with: variable resistor, ammeter, voltmeter parallel to component being measured
- begin to vary resistor, altering current flowing through circuit and p.d across component
- take simultaneous readings from ammeter/voltmeter to see how p.d varies as current changes
- repeat readings twice to get average p.d at each current
- swap over wires in battery to reverse direction of current
- plot graph of current against p.d
35
Name of I-V graph
I-V characteristic graph
36
Applications of thermistors
- car engine temperature sensors
- electronic thermostat
37
Purpose of sensing circuits
Turn on or increase power to components depending on conditions they're in
38
Ways of joining electrical components
- in series
- in parallel
39
Where are components connected in series circuits
In a line, end to end
40
What happens if you remove a component from a series circuit
The circuit breaks and all components stop
41
Series circuit - current
Same everywhere
42
Series circuit - pd
Shared between components
43
Series circuits - resistance
- **total increases as you add more resistors**
- total resistance of 2 components is sum of resistance of each component
44
Explanation of series circuits resistance
- add resistor = these resistors have to share total pd
- lower pd across each resistor = lower current
- current is same everywhere = total current reduced
- as pd is constant and current has decreased, total resistance increases
45
Do bigger components in series take more/less pd
More - bigger share
46
Parallel circuits
Components (except ammeters) separately to supply
47
What happens if you disconnect a component from parallel circuit
There is little effect on others
48
Parallel circuits - current
Shared between branches
49
Parallel circuits - pd
Same everywhere
50
Parallel circuits - resistance
- **total decreases as you add more resistors**
- total resistance of 2 resistors less than resistance of smallest resistor
51
Explanation for resistance in parallel circuits
- each resistor added has same pd
- add another loop = current has more directions to go
- increases total current that can flow around circuit
- current increase and pd same = total resistance decrease
52
How to investigate effect of adding resistors to circuits
- build circuit with battery + resistor + ammeter
- measure current with ammeter
- calc resistance - battery V ÷ I
- add resistor (in parallel with first?), measure current, calc resistance
- repeat until 4 resistors connected
- plot graph - number of resistors against resistance
53
Expected results of investigating resistance in series circuits
- adding resistors in series increases total resistance of circuit
- adding resistance decreases total current of circuit
- graph should be directly proportional
54
Expected results of investigating resistance in parallel circuits
- adding resistors increases total current of circuit + decreases total resistance of circuit
- graph should be decreasing curve
55
Types of electricity supplies
- alternating current (ac)
- direct current (dc)
56
Alternating current
- current is constantly changing direction
- produced by alternating voltages - positive/negative ends alternate
57
Types of potential difference
- direct current
- alternating current
58
UK mains supply type
ac
59
Voltage of UK mains supply
230V
60
Frequency of UK mains supply
50Hz
61
Direct current
- current always flowing in same direction
- created by direct voltage
62
Cells/batteries type of electricity supply
dc
63
How are most electrical appliances connected to mains supply
Three-core cable
64
What are wires in three-core cable made up of
- copper core
- insulating coloured plastic coating
65
Wires in three-core cable
- live
- neutral
- earth
66
Live wire colour
Brown
67
Live wire function
Carries alternating potential difference at 230V from mains electricity
68
Neutral wire colour
Blue
69
Neutral wire function
- completes circuit - when appliance is operating, current flows through live and neutral wires
70
Pd of neutral wire
0V
71
Earth wire colour
Green/yellow stripes
72
Earth wire function
- **safety**
- stops appliance becoming live
- only carries current when there's a fault
73
Earth wire pd
0V
74
How can live wire cause electric shock
- body has 0V
- touch live wire - large pd produced across body, current flows through you
- causes electric shock - injury/death
- turned off - still pd in live wire even though no current, still danger
75
How can live wire cause fire
- connection between live + earth creates low resistance path to earth
- current chooses path of lowest resistance - huge current flows - fire
76
How does earth wire protect from electric shock
- made of copper - low resistance path to ground
- current chooses path of lowest resistance - follows path to ground instead of going through person
- wire has virtually no resistance - large current flows causing fuse to melt, breaking circuit - no further current flows
77
What are everyday electrical appliances designed to bring about
Energy transfers
78
Kettle energy transfers
Transfers energy **electrically** from mains ac supply to **thermal** energy store of heating element inside kettle
79
Handheld fan energy transfers
Energy transferred **electrically** from battery of handheld fan to **kinetic** energy store of fan's motor
80
What determines amount of energy transferred
- how long appliance is on for
- power of appliance
81
Power ratings
Maximum amount of energy transferred between stores per second when appliance is in use
82
How do electrical charges transfer energy
- energy supplied to charge at power source, raising it through a potential
- charge gives up energy when it falls through a potential drop components
83
Why do batteries with bigger pd supply more energy to circuit
Charges raised up higher at start
84
The National Grid
System of cables and transformers, covering the UK, linking power stations to consumers
85
What is transferred between power stations and consumers using national grid
Electrical power
86
How does national grid make sure there's always enough electricity for demand
- power stations run below maximum power output
- smaller power stations on standby can be started up quickly
87
Why does national grid have high pd
To transmit a lot of power
88
Why does national grid have low current
High current would heat up wires, causing energy transfer to thermal store of surroundings
89
Pd of national grid
400,000V
90
Why is the national grid an efficient way of transferring energy
For a given power, increasing the pd decreases the current, decreasing energy lost to surroundings through heating wires
91
What are transformers made up of
Primary and secondary coil joined with an iron core
92
Types of transformer
- step-up
- step-down
93
Step-up transformers
Increase the pd from the power station to the transmission cables, causing current decrease
94
Where do step-up transformers have more turns
Secondary coil
95
Step-down transformers
Decrease pd from transmission cables for domestic use, causing current increase
96
Where do step-down transformers have more turns
Primary coil
97
Efficiency of transformers
Nearly 100%
98
Where is earth wire pin
Top
99
Where is neutral wire pin
Left
100
Where is live wire pin
Right, connected to fuse
