Electricity Flashcards
Current
Flow of electric charge round a circuit
Unit: ampere A
Size of current is rate of flow of charge
Potential difference
The driving force that pushes the current around
Unit: volt V
Energy transferred per unit charge
Resistance
Anything which slows the current down
Unit: ohm
Which direction does conventional current flow
Positive to negative
Charge (Q)
Coulombs C
Ammeter
Placed in series
Voltmeter
Placed in parallel around the component being measured (not battery or variable resistor)
Resistors IV graph
Current is directly proportional to the voltage
Filament lamp IV graph
Like an x2 curve as temp increases so does resistance
Diode IV graph
Current only flows in one direction so current is 0 in x < 0
Why does resistance increase with temp
Ions are vibrating more as they have more energy
This makes it more difficult for the electrons to flow through as they are moving more and ‘getting in the way’
Why are LED useful
Used as lighting as they use less current than most forms
They indicate presence of a current so can be used to show devices are switched on
Use of diodes
Can rectify ac surrender through half wave recirfication to turn it into dc
Uses of LDRs
In bright light resistance falls
In low light resistance increases
Used in automatic night lights (become bright when it is dark)
Used in burglar detectors
(If burglar breaks a beam of light resistance shoots up and turns on bright light)
Resistance in thermistors
Decreases in hot temp
Increases in low temp
Used as temperature sensors and thermostats
In a series circuit
PD is shared i.e. PDs add together to make the source voltage
Current is the same everywhere
Resistance adds up to make total resistance (larger the resistance of each component the more PD is takes up)
Cell voltages add up to make total
Parallel circuits
PD is the same across all components
Current is shared between branches
Mains supply
AC current or 230V
Frequency is 50Hz
Oscilloscope traces
In DC voltage is always the same so it is a straight line
In AC it is always changing so the line is wavy (vertical height at any point shows voltage then)
Frequency =
1/time period
Brown wire
Live wire which alternates between high positive and low negative voltage
Blue wire
Neutral wire stays at 0V
Green and yellow wire
Earth wire is for safety and is attached to metal casing of appliance and carries electricity to the earth if the live or neutral wires touch the metal casing
features of 3 pin plug
Plastic casing of plug Wires covered in plastic Order is blue yellow and green brown Thicker cables have less resistance Brass and copper and good conductors so used for pins
Advantages of circuit breakers
Can be reused
Faster
Work for even small current changes
But expensive
RCCB
Normally same current flows through live and neutral wires
If someone touches the live wire a current flows through them meaning neutral wire carries less current than live wire
RCCB detects this difference and cuts off a switch
National grid
In order to transfer large amounts of electricity (power) current or voltage must be v high
High current loses lists of energy through heating wires so voltage is increased to
400,000 V using a transformer
Step down transformers in towns take it down to a safe level of 230V
