Electricity Flashcards
what is electric current?`
the flow of electrical charge. its measured in ampere, A
current will only flow…
round a complete circuit if there is a source of potential difference.
when does the current have the same value everywhere in a circuit?
in a single, closed loop
what is potential difference?
(voltage) its the driving force that pushes the charge round. measured in volts, V
what is resistance?
anything that slows the flow down. measured in ohms, Ω
what does the current depend on?
the current flowing through a component depends on the potential difference across it and the resistance of the component.
(greater the resistance, smaller the current, for a given pd)
what is the size of the current?
its the rate of flow of charge. when current flows past a point in a circuit for a length of time then the charge = the current X the length of time it was passing
when does more charge pass round the circuit?
when a larger current flows
how to find PD
PD= I * R
the ammeter
measures current running through wire in amps
must be placed in series with whatever is being investigated
the voltmeter
measures potential difference across the wire in volts
must be placed in parallel around whats being investigated
resistance practical
attach 2 crocodile clips to a wire a length away from each other and record the length
close the switch record current and PD then open it again
move 1 crocodile clip and repeat a number of times.
calculate the resistance
draw a graph for length of wire and resistance
how should the graph for length of wire vs resistance look?
the line of best fit should be a straight line through the origin. resistance is directly proportional to length. the longer the wire the greater the resistance.
if the line of best fit for length vs resistance doesn’t go through the origin, whats wrong?
it could be because the first clip isn’t attached the right distance from the second 1. messing up all the results, its a systematic error
whats the resistance like for ohmic conductors?
it doesn’t change with the current. at a constant temperature the current in an ohmic conductor is directly proportional to the PD across it. the resistance is constant
what resistors and components have resistance that doesn’t change?
a filament bulb or a diode
what happens to resistance in a filament bulb?
when charge flows through a filament bulb, it transfers some energy to the thermal energy store of the filament, which is designed to heat up. as the current increases, the filament heats up and the resistance increases
what happens to resistance in a diode
it depends on the direction of a current. it will let current flow in 1 direction, but have a very high resistance if it is reversed
What is I - V characteristics?
a graph which shows how the current changes as the pd increases.
IV characteristics of linear components:
linear components have an IV characteristic thats a straight line, fixed resistor
IV characteristic of non-linear components:
non-linear components have a curved IV characteristic (filament bulb, diode)
How to make an IV plot
the current and PD has to be measured in a circuit with a varying variable resistance, plot a graph then reverse the wires and do the same this way a graph will be plotted on both positive and negative to make an IV plot
IV plot of an ohmic conductor (resistance at a constant temperature)
the current through an ohmic conductor(at constant temperature) is directly proportional to pd so u get a straight line
IV plot of a filament lamp
as current increases , temperature increases, so resistance increases.
this means less current can flow per unit pd, so the graph gets shallower hence the curve
IV plot of a diode
current will only flow in 1 direction. it has very high resistance in the opposite direction
what is an LDR
a resistor that is dependent on the intensity of light
how does an LDR react in different intensities of light?
bright light: resistance falls
darkness: resistance is highest
applications for LDRs
automatic night light, outdoor lighting, burglar detectors
what is a thermistor?
a temperature dependant resistor
how does the thermistor react in different situations?
hot conditions: resistance drops
cool conditions:resistance increases
how are thermistors used?
they are useful temperature detectors. car engine temperature sensors, electronic thermostats
what are sensing circuits for?
they can be used to turn on or increase the power to components depending on the conditions they are in
in a sensing circuit for a fan, there is a thermistor and a fixed resistor with a fan attached in parallel to it:
the fixed resistor and the fan will always have the same resistance as they’re in parallel. the pd of the power supply is shared out between all the components (according to their resistances - the bigger the resistance the more pd it takes)
as a room gets hotter what will the sensing circuit with a fan do?
the resistance of the thermistor decreases and takes a smaller share of pd so pd across fan and fixed resistor increases, making fan go faster
in a series circuit how are the components connected?
in a line, end to end. between +ve and -ve of the power supply. voltmeters are connected in parallel but they don’t count as part of the circuit
disadvantage of series circuit
if 1 thing is disconnected or removed the whole circuit stops working
in a series how is the pd distributed amongst the components?
it is shared between the various components. so the pd round a series circuit always adds up to equal the source pd
in a series how is current distributed amongst the components?
the same current flows through all of the components.
the current depends on the pd of the cells and resistance of the current.
in a series how is resistance distributed?
the resistance in each component is added to find the total resistance, this is because by adding a resistor the resistors have to share the total pd.
how is pd and current affected by adding a resistor in series?
the pd across each resistor is lower, so current through each resistor is lower. in series the current is the same everywhere so the total current is reduced when a resistor is added, total resistance increases.
the bigger a components resistance…
the bigger its share of the total pd
in a parallel circuit how are components connected?
they are separately connected to the +ve and -ve of the supply (except ammeters which are always connected in series)
what happens if something is removed in parallel?
hardly effect the others at all.
how are most circuits connected?
parallel is obviously how most things must be connected. we have to be able to switch everything on and off individually. everyday circuits include a mixture of series and parallel
in parallel how is pd distributed?
the potential difference is the same across all components. so identical bulbs in parallel will all have the same brightness
in parallel how is current distributed?
the total current equals the total of the current of each component together
in parallel how is current split?
there are junctions where the current splits, the total current going into a junction is the same coming out when it rejoins
what happens if 2 identical components are in parallel
the same current will flow through each component
in parallel how is resistance distributed?
if there is 2 resistors in parallel, their total resistance is less than the the resistance of the smallest of the 2 resistors
why is total resistance in parallel smaller than the resistors in the circuit?
both resistors have same pd as source
but by adding another loop, current has more directions to flow in
this increases total current which means a decrease in the resistance
investigation for adding resistors in series circuits
make a circuit with an ammeter, battery and resistor. measure the resistance using the amps and volts of battery
keep adding resistors and measuring resistors until your done
plot a graph
investigation for adding resistors in parallel circuits
make a circuit with an ammeter, battery and resistor.
measure the resistance using the amps and volts of battery
keep adding resistors in parallel and measuring resistance until your done
plot a graph
what should the graph for resistors in a series look like?
adding a resistor increases total resistance, adding a resistor decreases the total current
the more resistors you add the larger the resistance of the circuit. the graph is a positive directly proportional straight line
what should the graph for resistors in a parallel look like?
when resistors are added total current increases, so total resistance decreases
more resistors added the smaller overall resistance becomes.
the line is a negative curved line that dips not rises
what are the 2 types of electricity supplies?
alternating current(ac) and direct current(dc)
ac supplies
current is constantly changing direction. alternating currents are produced by alternating voltages, where positive and negative ends keep alternating
is the UK mains supply ac or dc?
ac supply at around 230 V
what is the frequency of the ac mains supply?
50 cycles per second, 50 Hertz (Hz)
what kind of electrical supply do cells and batteries have?
direct current
direct current
its a current that is always flowing in the same direction, its created by a direct voltage
what connects electrical appliances to the mains supply?
three-core cables, which means they have 3 wires inside them with a core of copper and a coloured plastic coating
what does the colour of the insulation on cables mean?
it shows its purpose. the colours are the same for every appliance, so that its easy to tell the different wires apart
what is the live wire?
brown. provides alternating potential difference from the mains supply(230 V)
what is the neutral wire?
blue. it completes the circuit and carries away current - electricity usually flows in through the live wire and out through the neutral wire. its around 0 V
what is the earth wire?
green and yellow. its for protecting the wiring, and for safety - it stops the appliance casing from becoming live. doesn’t usually carry a current, only when there’s a fault. at 0 V
what happens when a person touches a live wire?
your body is at 0 V. if you touch the live wire, a large pd is produced across your body and a current flows through you. this causes a large electric shock that could injure or kill.
is a live wire still dangerous when turned off or disconnected?
there is still a danger. there’s no current but there’s still pd. if you touch a live wire your body will create a link between the supply and earth, so a current would flow through you.
what happens if the wires in an appliance become exposed to each other?
any connection between live and earth can be dangerous. if the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
why does a moving charge transfer energy?
the charge does work against the resistance of the circuit. (work done is the same as energy transferred)
what are electrical appliances designed for?
to transfer energy to components in the circuit when a current flows
how do kettles transfer energy?
electrically, from the mains ac supply to the thermal energy store of the heating element inside the kettle
how do handheld fans transfer energy?
electrically, from the battery to the kinetic energy store of the fans motor
can any appliance transfer all energy completely usefully?
no
how can more energy be transferred to thermal energy stores of the components?
the higher the current, the more energy that is transferred to the thermal energy stores of the components, and then the surroundings
what does the total energy transferred depend on?
how long the appliance is on for and its power.
what is power?
the power of an appliance is the energy that it transfers per second. so the more energy it transfers in a given time, the higher its power.
energy transferred =
power X time
E=Pt
what is a power rating?
appliances are often given a power rating. they’re labelled with the maximum safe power that they can operate at. this can be taken as their maximum operating power
what does the power rating tell you?
it tells you the maximum amount of energy transferred between stores per second when the appliance is in use
how are power ratings useful?
helps a customer chose between models. the lower the power rating, the less electricity an appliance uses in a given time and so the cheaper it is to run.
what does a higher power not necessarily mean?
that it transfers more energy usefully. an appliance may be more powerful than another, but less efficient, meaning that it might still only transfer the same amount of energy(or even less) to useful stores
what happens when an electrical charge goes through a change in potential difference?
energy is transferred
energy is supplied to …………………………. to ‘raise’ it to it through a potential
the charge at the power source to raise it through a potential. the charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit.
energy transferred=
charge flow X potential difference
E =Q * V
a battery with bigger pd will supply more energy to the circuit for…..
for every coulomb of charge which flows around it, because charge is raised up higher at the start
how else can power be found other than through energy?
power= pd X current
P=V * I
how to find power with resistance?
use V = IR and put it in P=VI
P=(IR)I
P=I2 *R
what is the national grid?
a giant system of cables and transformers that covers the UK and connects power stations to consumers (anyone who is using electricity)
what does the national grid do?
transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the grid where it’s needed (the demand) e.g. homes and industry
throughout the day, electricity use…
changes. power stations have to produce enough electricity for everyone to have it when they need it
what do power stations predict?
they can predict when the most electricity will be used.
when is the most electricity used?
demand increases when people get up in the morning, come home from school or work, when it gets dark or cold, and when popular events are being shown on tv
what do power stations work below?
they often run well below their maximum power output, so there’s spare capacity to cope with a high demand, even if another power station unexpectedly shuts down
what is kept in stand by for high electricity demand?
lots of smaller power stations that can start up quickly
what is needed to transmit the huge amount of power needed for a power station?
either a high potential difference or a high current
what is the problem with a high current in a power station?
you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings
what is done in power stations instead of high current?
its much cheaper to boost the pd up really high (400 000 V) and keep current as low as possible
for a given power, increasing the pd does what?
decreases the current, which decreases the energy lost by heating the wires and the surroundings. this makes the national grid an efficient way of transporting energy
what is required to get the potential difference to 400 000 V to transmit power?
to get to the pd needed to transmit power transformers are required, as well as big pylons with huge insulators(but its still cheaper than big current)
what do transformers do?
they have to step the pd up at 1 end, for efficient transmission, and then bring it back down to safe, usable levels at the other end
what does a step - up transformer do?
increases the potential difference
what does a step-down transformer do?
reduce the pd for domestic use
what happens when certain materials a re rubbed together?
when certain insulating materials are rubbed together, negatively charged electrons will be scraped off 1 and dumped on the other
what is the result of electrons travelling from 1 material to another?
the materials will be electrically charged, with a positive static charge on 1 and an equal negative static charge on the other
which way electrons are transferred depends on…
the materials involved
e.g. polythene and acetate rods being rubbed with a cloth duster. leaving negative polythene and positive acetate
both +ve and -ve electrostatic charges are only produced by the movement of….
electrons. the positive charges definitely do not move.
how is a positive static charge caused?
by electrons moving away. the material that loses the electrons loses negative charge, and is left with an equal positive charge
what happens as electric charge builds on an object?
the pd between the object and the earth(0 V)increases
what happens if the pd gets large enough?
electrons can jump across the gap between the charged object and the earth - this is called a spark
what can sparks jump to?
the earth and any earthed conductor that is nearby - which is why you can get static shocks getting out of a car, a charge builds up on its metal frame and when you touch it, charge travels through you to the earth
when do sparks usually occur?
usually when there is a small gap. but lightning is 1 big spark
what will 2 things with opposite electric charges do?
attract
what will 2 things with the same electric charges do?
repel
forces between magnets are weaker when…
the 2 things are farther apart
what is it called if the electric force causes an object to move?
it is known as electrostatic attraction/repulsion and is a non-contact force
how to show electrostatic attraction/ repulsion?
suspend a rod with a known charge (so it is free to move) place an object with the same charge nearby and the rod will move away. an oppositely charged rod will attract the rod
an electric field is created around what?
any electrically charged object, the closer to the object you get the stronger the field is (and the further the weaker)
how to show an electric field?
use field lines.
properties of electric field lines?
they go from positive to negative
they’re always at a right angle to the surface
the closer together the lines are, the stronger the field is - the further from a charge you go the further apart the lines are and so the weaker the field
what happens when a charged object is placed in an electric field of another object?
it feels a force.
this force causes attraction or repulsion
what happens if you increase the distance between charged objects?
the strength of the field decreases and the force between them gets smaller
explain what happens when P (positively charged) and N (negatively charged) interact
the electric field of P interacts with N’s electric field
this causes forces to act on P and N
these forces move P and N closer together
what is the force felt by an object in an electric field caused by?
it is caused by the electric fields of each charged object interacting with each other.
what is the force on an object linked to?
the force on an object is linked to the strength of the field
potential difference causes a strong electric field between
a charged object and an earthed object.
the strong electric field causes electrons in the air particles to be removed (known as ionisation)
what happens when air particles are ionised?
air is normally an insulator, but when it is ionised it is much more conductive, so a current can flow through it. this is a spark
