Electricity Flashcards
Current
Rate of flow of charge
When can current only flow
When there’s a source of potential difference
What is a complete circuit
A closed one
Potential difference
Driving force that pushes charge round
Unit of current
amp (A)
Unit of potential difference
volts (V)
Voltage
Potential difference
Resistance
Anything that slows flow down
Unit of resistance
ohm (Ω)
Effect of resistance on current
The greater the resistance across a component, the smaller the current that flows
Cell
Store of chemical energy
Battery
2 or more cells together
Open switch
Breaks a circuit
Closed switch
Joins a circuit
Filament lamp
Lights up when current flows because wire gets hot
Fuse
Melts when current gets to hot - safety device
LED
Light emitting diode - emits light when current flows in 1 direction
Fixed resistor
Restricts amount of current flowing
Variable resistor
Allows current to be varied
Ammeter
- measures current, counts number of charges per second
- goes in series
Voltmeter
- measures potential difference, measures difference in energy
- goes in parallel to component it’s measuring
Diode
Only allows current to flow 1 way, resistance very high in reverse
LDR
Light dependent resistor, at light intensity increases, resistance decreases
Thermistor
As temperature increases, resistance decreases
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
Ohmic conductor
Conductor that obeys Ohm’s law
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
Ohm’s law exceptions
- resistance of some components not constant
- e.g - diode/filament lamp
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
Applications of thermistors
- car engine temperature sensors
- electronic thermostat
Purpose of sensing circuits
Turn on or increase power to components depending on conditions they’re in
Where are components connected in series circuits
In a line, end to end
What happens if you remove a component from a series circuit
The circuit breaks and all components stop
Series circuit - current
Same everywhere
Series circuit - pd
Shared
Series circuits - resistance
Increases as you add more resistors
Parallel circuits
Components (except ammeters) separately to supply
What happens if you disconnect a component from parallel circuit
There is little effect on others
Parallel circuits - current
Shared
Parallel circuits - pd
Same everywhere
Parallel circuits - resistance
Decreases as you add more resistors
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
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
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
Types of electricity supplies
- alternating current (ac)
- direct current (dc)
Alternating current
- current is constantly changing direction
- produced by alternating voltages - positive/ negative ends alternate
UK mains supply type
ac
Voltage of UK mains supply
230V
Frequency of UK mains supply
50Hz
Direct current
- current always flowing in same direction
- created by direct voltage
Cells/batteries type of electricity supply
dc
How are most electrical appliances connected to mains supply
Three-core cable
What are wires in three-core cable made up of
Each have core of insulating copper with coloured plastic coating
Wires in three-core cable
- live
- neutral
- earth
Live wire colour
Brown
Live wire function
Provides alternating potential difference at 230V from mains electricity
Neutral wire colour
Blue
Neutral wire function
- completes circuit - when appliance is operating, current flows through live and neutral wires
- 0V pd
Earth wire colour
Green and yellow
Earth wire function
- safety - stops appliance casing becoming live
- protects wiring
- only carries current when there’s a fault
- 0V pd
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, still danger
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
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 blow - no further current flows
What determines amount of energy transferred
- how long appliance is on for
- power of appliance
Power ratings
Maximum amount of energy transferred between stores per second when appliance is in use
What happens when an electrical charge goes through a change in pd
Energy is transferred
Why do batteries with larger pds supply more energy to circuit for every coulomb of charge flowing around it
Charge is raised up higher at start
The National Grid
System of cables and transformers, covering the UK, linking power stations to consumers
When does electricity demand increase
- when people get up in morning
- when people come home from school/work
- when it gets dark/cold outside
- when popular evens are shown on TV
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
Why does national grid have high pd
To transmit a lot of power
Why does national grid have low current
High current would heat up wires, causing energy transfer to thermal store of surroundings
Pd of national grid
400,000V
Why is the national grid an effective way of transferring energy
For a given power, increasing the pd decreases the current, decreasing energy lost to surroundings
What are transformers made up of
Primary and secondary coil joined with an iron core
Types of transformer
- step-up
- step-down
Step-up transformers
Increase the pd from the power station to the transmission cables, causing current decrease
Where do step-up transformers have more turns
Secondary coil
Step-down transformers
Decrease pd from transmission cables for domestic use, causing current increase
Where do step-down transformers have more turns
Primary coil
Efficiency of transformers
Nearly 100%
