electricity (inc mains + static) Flashcards
current (I)
the RATE of flow of charge
measured by ammeter (A)
- series = same throughout
- parallel = splits at a junction
ammeter symbol
A
voltmeter symbol
V
variable resistor
square with arrow theough
thermistor symbol
square with _/line through
ldr symbol
square with circle round and arrows pointing towards
diode symbol
play button with circle round
led
diode symbol with arrows pointing away
resistor symbol
sauare
current equation
Q = It
- charge (C) = current (A) x time (secs)
actual current flow directions
->+
electrons attracted to + terminal
conventional direction of current flow
+>-
voltage (potential difference)
measure of how much energy (E) the charges (Q) are carrying
measured with voltmeter (V)
- series = splits between components
- parallel = each branch receives same voltage
voltmeter
measured voltage
compares energy charges carried into/out of a component and reads the differences
voltage equation
E= VQ
- energy transferred (J)= voltage (V) x charge (C)
charge (Q)
made up trillions of electrons in wires flowing round circuit
gain energy from battery and deliver to components
measured in coulombs (C)
equation linking VIR
V = IR
voltage (V) = current (A) x resistance (ohms)
resistance
limits the flow of current
hugh R = low I
low R = high I
measured using ohmmeter (in ohms)
all components have some resistance
factors that affect resistance
material
length
thickness
temperature
measuring resistance
can measure resistance using a circuit R = V/I
VIR circuit problems solving
- write down all values you know / work out
- see if you can use V = IR
- series - Rt = R1 + R2 total resistance INCREASES
- parallel - Rt = Vt/ If total resistance DECREASES
fixed resistor graph
straight line - through origin
- linear relationship between current and voltage
- current is proportional to voltage
fixed resistor
resistor is Ohmic conductor
- follows Ohms law
OHMS LAW - current is proportional to the voltage providing temp of resistor doesn’t change
filament lamp graph
S - curved graph
for each interval increase in V the I increases by a smaller amount
filament lamp
doenst obey ohms law
- resistance of lamp increases due to it getting hot
- atoms in filament vibrate faster causing more collisions w electrons which slows them down
diode / LED graph
_/ cruved graph going up
diode /LED
only allows current to flow in 1 direction
when connected the wrong way have infinite R
voltage must exceed threshold voltage before conducting
sensitive to large currents so a resistor is used to prevent the LED from blowing if too much current flows
thermistors /LDRs
made up of semi- conducting material that changes resistance depending on the environment
thermistor
resistor changes with temperature
useful for temperature control
eg ovens kettles fridges thermostats
thermistor graph
as temp. increases resistance decreases non- linear inverse relationship (curve) because…
- heat energy releases more free electrons to flow
LDR
light dependent resistor
resistance changes with brightness of light (light intensity)
useful for sending light changes
eg control night lights
LDR graph
as brightness increases resistance decreases
non- linear inverse relationship because…
- light energy releases more free electrons to flow
power
rate of transfer of energy
measured in watts (W)
power (Et) equation
E = Pt
Energy /work done = power x time
power (IV) equation
P = IV
energy (Itv) equation
E = ItV
why do appliances get hot
appliances transfer electrical energy into heat
due to electrons colliding with atoms in the wire
appliances left running transfer electrical energy onto other types
power of lamp experiment
connect big and little lamp in series then in parallel
note brightest then calculate power to see if it matches
battery electricity
provide low voltage d.c electricity
dc is direct current (flows in one direction +>-)
mains electricity
provide 230V ac electricity
ac is alternating current (continuously changes direction)
frequency of 50Hz
plug
earth - green
live - brown
neutral - blue
fuse
flex (cable)
flex grip
plug mistakes
wires connected wrong
exposed copper wire
flex not under grip
wrong fuse
what makes a plug / wire safe to use
double insulated with plastic casing
ohms law
current is proportional to voltage providing temp. of resistor doesn’t change
fuse symbol
rectangle w line theough
variable resistors
can change size of current and resistance
can control the speed of motor and be used as dimmer switches
CANNOT INCREASE AMOUNT OF CURRENT FLOWING
way diode is connected
flows +>-
fuse
connected to live wire
protects APPLIANCE
from too high current which could damage it
fuse rating
maximum current it will allow through before it blows
work out using P= IV
- fuse too low - would blow as soon as use appliance
- fuse too high - could allow a high current into appliance and damage it
fuse blows
fuse wire melts and breaks circuit
live wire
brown
carried current into appliance
neutral wire
blue
carried current away from appliance
earth wire
green/ yellow
- protects USER from electric shock is fault occurs and appliance becomes live
- connected to metal casing of appliance away from ground
normal operation of kettle
current flow in live wire and out neuteal
fault occurs in kettle
- if something breaks + occurs metal casing of appliance to carry current
- would cause electric shock if touches
- earth wire is connected to casing and carries current to ground outside
- earth has low R so high I flows which blows the fuse and stops I flowing so appliance is safe
double insulated
has a plastic insulator casing so doenst need earth wure
alternative to fuse - circuit breaker
will trip a switch is current exceeds particular value
can be reset instead of replaced and reacts quicker
electromagnet attracts open switch if currents too high
alternative to fuse - RCD or RCCB
detects if there’s a difference between the current in live and neutral wires - fault must’ve occurred and trips the circuit breaker
what type of material can be charged up
insukators
how to charge up
- friction - causes electron transfer (like a cloth and rod rubbing against each other)
- induction - already charged object will induce a charge if brought near to another object (like a balloon sticking to wall)
friction / electron transfer
if gain - charge
if loose + charge
the direction of transfer depends on objects
what happens if an already charged object goes near another object
it will induce a charge onto the other object
like charges
repel
opposite charges
attract
how does charged objects lose charge
to become neutral it must DISCHARGE
has to connect object to EARTH with conductor
can static charge move
no
static charge
builds up in one place and is not free to move (on an insulator )
as charge builds up so does …
voltage
pd between charged objects and earth (0V) increases
spark
if voltage is large electrons can jump between object and earth
investigating static electricity
- rub balloon against hair - balloons charged- balloon can pick up little bits of paper
- rub rod with cloth - rod is charged - rod can bend a stream of water
- rub two rods of diff. materials with cloth - both rods will have charge - rods will either attract or repel depending if they have diff. or similar charges
van de graaff generatir
electrostatic charge is built up on the metal dome
if you stand on an insulated block put your hands on the dome your body gets a charge
each hair has the same charge so they repel and spread
dangers of static electricity
lighting refuelling a plane refuelling vehicle
uses of static electricity
ink jets
printers
photocopiers
chimney filter
electrostatic crop sprayer
lightning - how to prevent danger
tall buildings have lightning conductors on top which earths the building
refuelling a plane - how to prevent danger
have earthing cable (conductor ) to prevent charge building up and allow it to be discharged
electrical hazards
- water + electricity
- could reach high temp
- damaged equipment is a hazard
- overloaded sockets
- trip hazard of wires
advantage of series/ parallel
series - one switch controls all
parallel - if one breaks other is fine
