Section 2 - Electricity Flashcards
current
rate of flow of charge (carried by electrons) in a circuit
(AMPS - A)
voltage
what drives current round circuit (also known as potential difference)
Volts - V
resistance
anything that slows flow down
ohm (Ω)
ammeter
measures current in amps flowing through a component
- must be placed in series
voltmeter
measures voltage (in V) across a component
- must be placed in parallel
mains supply
in uk 230v alternating current - so constantly changing direction
battery supplt
direct current - always in same direction
Voltage formula
V=IR
voltage = current x resistance
I-V graph
shows how current varies as you alter the voltage
I-V graph working out
gradient = resistance
steeper gradient = low resistance
straight line graph = constant gradient therefore resistance
wire i-v graph
/ shape
current through a wire at constant temp is proportional to the voltage
different resistors i-v graph
/ varying
current through a resistor at a constant temperature is proportional to voltage.
- different resistors have different resistances therefore slightly different slopes
metal filament lamp i-v graph
S kinda shape
as the temperature of the metal filament increases the resistance increases hence the curve
diode i-v graph
—l shape
current will only flow through a diode in one direction as shown
Light Emitting Diodes
indicate the presence of current in a circuit. (light up when current goes through in forward direction)
Often used by appliances to show that they are switched on and for digital clocks, traffic lights, and remote controls
advantages - no filament that will burn out
Light Dependant Resistor
a special type of resistor that alters its resistance depending on how much light falls on it
bright light - resistance falls
darkness - resistance highest
useful for electronic circuits like burglar detectors
Thermistor
temperature-dependant resistor
hot - resistance drops
cold - resistance builds
useful temp detectors for car engines, thermostats, and fire alarms
series circuits
- different components are connected in a line, end to end, between the +ve and -ve of power supply
- if you remove/disconnect one they all stop working (so impractical in reality)
series circuit Characteristics:
- there’s a bigger voltage supply when more cells are in series
- current is the same everywhere
- voltage is distributed between components according to their resistance
- total resistance is sum of resistance in each component
parallel circuits
each component is separately connected to +ve and-ve of the supply.
- if you remove or disconnect a component it will hardly affect the others
components on same branch share series rules
parallel circuits Characteristics:
- voltage is the same across all branches
- current is shared between branches
- total cuurent is the same as addition of current in separate components
- junctions - current either splits or rejoins (total current going in a junction = total current going out)
- current through a branch depends on resistance in branch
charge (C) through a circuit formula
current x time
Q = It
charge drps blah blah
energy transferred (J)
charge x voltage
E=QxV
or
charge x current x resistance
QxIxR
a plug
three wires - live, neutral and earth
(earth only needed in emergencies)
Live wire (brown) - around 230v a.c.
neutral wire (blue) - 0v
electricity flows through both
earth wire (green/yellow) there for safety
safety measures in a plug
1) all appliances with metal cases must be “earthed” to reduce the danger of electric shock - an earthed conductor can never become live
2) if the appliance has a plastic casing and no metal parts showing then it is said to be double insulated (if so it doesn’t need an earth wire)
how do earth wires and fuses work
- if something happens and live wire accidentally touches a metal case then because it is earthed a big current passes through the live wire, case and earth wire
- surge in current melts the fuse which cuts off the live supply
- this isolates the whole appliance, making it impossible to get an electric shock and prevents risk of fire from heating effect of a large current
what are circuit breakers
alternatives to fuse
- break current by opening a switch. the circuit can later easily be reset by flipping the switch (more convenient and cheap than fuses)
Residual Current Circuit Breaker
- normally same current goes through live and neutral, but if someone touches live wire then current will flow through them to earth, meaning neutral wire carries less current. The RCCB detects the difference and cuts off power supply by opening a switch
Residual Current Circuit Breaker
Residual Current Circuit Breaker advantages
1) safer - quick as it doesnt have to wait for fuse to melt
2) work for small current charges not detected by fuses, so more efffective
Resistors
when there is ressistance there is a energy transfer which heats the resistor.
- this happens because electrons collide with ions in lattice that makes up resistor as they flow through it giving the ions energy with causes them to vibrate and heat up
- heating effect increases ressistance so less current can flow (or greater voltage needed for same current)
- heating effect could cause components to melt - causing the circuit to stop working . Fuses use this effect to protect circuits
- heating effect has other advantages - toaster contains coil with really high ressistance and when current passes through temp inreases so much it glows and goved of infrared radiation which cooks the bread
electrical power defenitions
electrical power is the rate at which an appliance transfers energy
This energy comes from the current flowing through it so an appliance with a high power rating will draw a large current from the supply
fuese have curent ratings
electrical power formula
P = IV
electrical power = current x voltage
electrical appliances - energy transferred
energy transfered = current x voltage x time
E=IxVxt
