2- ELECTRICITY Flashcards
what is electrical current
the flow of electrical charge, - charge will only flow around a closed circuit if theres a potential difference, current only flows if theres a source of potential difference, unit of current is ampere (A)
- current flowing through component depends on potential difference across it and resistance of component
what is the current linke in a single closed loop
current has the same value everywhere in the circuit
what is potential difference
the driving force that pushes the charge round- unit is voltage (V)
what is resistance
anything that slows the flow down , until is ohm Ω-
the greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component)
what does the size of current depend on
size of current is the rate of flow of charge- when current flows past a point in circuit for a length of time- charge that has passed is given from
charge(C -coulombs)= current(A) x time(s)
more charge passes aorund circuit when a larger current flows
how do you calculate potential difference
current x resistance
what can the resistance of a circuit depend on
how can you measure this
- whether components are in parallel or series circuit
- length of wire used in circuit
ammeter-measure current in amps flowing around test wire, ammeter should be placed in circuit with what youre investigating
voltmeter- measure potential difference across test wire, voltmeter must be parallel around what youre investigating, not around anything
practical investigating resistance
1-attach crocodile clip to wire level with 0cm on ruler
2-attach second crocodile clip to wire, like 10 cm away from first clip, write down length of wire between clips
3- close switch then record current through wire and the pd across it
4- open switch , move second crocodile clip (another 10 cm along wire) close switch again, record new length, current and pd
5- repeat for a number of different lengths for the test wire
6- use measurements of current and pd to calculate resistance for each length of wire (resistance = voltage / current)
7- plot graph of resistance against wire length - draw line of best fit
8- resistance should be directly proportional to length - longer the wire- greater the resistance
9- if graph does not go through origin - first clip may have not been attached properly causing readings to be off. = systematic error
explain resistance of ohmic conductors
it doesnt change with current.at a constant temp, current flowing through ohmic conductoris directly proportional to potential difference across it
(diagram is straight line through graph)
explain resistance in a filament lamp
when electrical charge flows throught the filament lamp, it transfers some energy to thermal store of filament, which is designed to heat up, resistance increases with temperature so as current increases, filament lamp heats up more and resistance increases
explain resistance through diodes
reistance depends on direction of the current, it will allow current to flow through one direction but has a very high resistance if it is reserved
what does the term I-V characteristic mean
the graph which shows how
the current flowing through a component changes as potential difference across it is increased
linear components have a curved I-V characteristic thats a straight line(fixed resistor)
non linear components have a curved I-V characteristic (filament lamp or diode)
practical to find out a components I V characteristics
1-
————–i|I|—————— | |
| |
| [vr]
| |
|—(A)—[|COMPONENT]—
|———-(V)———–|
1- step up circuit as shown above
2- vary the variable resistor, this alters the current flowing through circuit and pd across component
3- take several pair of readings from ameter and voltmeter to see how pd across component varies as current changes, repeat each reading twice to get average pd at each current
4-swap over the wires connected to cell so direction of current is reversed
5- plot a graph of current against voltage for componet
6- IV characteristics you get should look like this
| /
____ |/____ = OHMIC
/|
/ |
| ( \_\_\_\_\_|/\_\_\_\_\_= FILAMENT
/|
) |
| /
| / = \_\_\_\_\_|\_\_\_)\_\_
|
| =curved line - diode
explain current through an ohmic conductor
at a constant temp- is directly proportioanl to potential difference = straight line
explain current through filament lamp
as current increases, temp of filament increases so resistance increases. less current can flow per unit pd, so graph gets shallower - giving a curve
explain current through a diode
current will only flow through a diode in 1 direction - diode has a high resistance in the reverse direction
circuit devices- explain LDRs
light dependent resistor- dependent on the intensity of light
in bright light, resistance falls
in darkness, resistance is highest,
they have lots of applications including automatic night lights, outdoor lighting and burgular detectors
circuit devices- thermistor
the resistance of a thermistor depends on temperature
a thermistor is a temperature dependent resistor - in hot conditions, resistance drops
in cool conditions, the resistance goes up
thermistor make useful temperature detectors e.g. car engine temperature sensors and electronic thermostats
what are sensing circuits
they can be used to turn on or increase power to components depending on the conditions that they are in
how is a sensing circuit used to control a fan in a room
1 - the fixed resistor and the fan will always have the same pd across them. (parallel)
2- pd of power supply is shared out between thermistor and loop made up of the fixed resistor and fan according to their resistances- bigger a components resistance , the more pd it takes
- as the roo gets hotter- resistance of thermistor decreases, takes a smaller share of the pd from power supply so pd across fixed resistor and fan rises making fan go faster
how can you use variable resistors in sensing circuits
if you connect the bulb in parallel to an LDR.
the pd across both the LDR and bulb will be high when its dark and the LDR’s resistance is high. the greater the pd across a component, the more energy it gets.
so a bulb connectd across an LDR would get brighter as the room got darker
how do the components work in a series circuit
components are connected ina line end to end between the positive and the negative of the power supply(except voltmeters)
if you remove 1 component, circuit is broken and they all stop.
what is potential difference like in a series circuit
the total pd of the supply is shared between various components, - pd around series circuits always adds up to equal the source pd
Vtotal = V1 + V2 + V3 etc
there is a bigger potential difference when there are more cells in series, if they are all connected in the same way.
e.g. when two cells with a pd of 1.5 V are connected in series, they supply 3V between them
what is current like in a series circuit
current is the same everywhere, same current flows through all components
I1=I2=I3 etc
size of current is determined by the total pd of the cells and the total resistance of circuit (I=V / R )
what is resistance like in a series circuit
the total resistance of 2 components is the sum of their resistances
R1+R2= Rtotal
this is because by adding a resistor, 2 resistors have to share the total pd
pd across each resistor is lower so current through each resistor is also
=total current is reduced when resistor is added = total resistance increases
bigger the components resistace, bigger its share of potential difference
what are components like in parallel circuits
each component is separately connected to the positive and negative of the supply (except ammeters which are always connected in series)
if you remove one of them it will hardly affect any of the others
(this is how most things are connected eg in cars and household electrics) (switch everything on and off separately)
what is potential difference like across a parallel circuit
all compopnents get the full source pd so the potential difference is the same across all components
meaning that identical bulbs connected in parallel will all be at the same brightness
v1 =v2
what is current like in a parallel circuit
current is shared, the total current flowing around the circuit is equal to the total of all the currens through the separate componens.
there are junctions where current either splits or rejoins -
total current going into a junction = total current leaving
if 2 identical components are connected in parallel - same current will flow through each component
what does adding a resistor to a parallel circuit do
if you have 2 resistors in parallel- their total resistance is less than the resistance of the smallest of 2 resistors
in parallel = both resistors have same pd - so pushing force making current flow is same as the source pd for each resistor you add
by adding another loop - current has more than 1 direction to go in
this increases total current that can flow around the circuit
using v=ixr= an increase in current means a decrease in the total resistance of the circuit
how do you investigate resistance in a series circuit (parallel)
you need 4 identical resistors.
then build the circuit with ammeter, resistor and battery
make note of the pd of the battery
measure current through the circuit using ammeter
calculate resistance using v/I
2 )add another resistor in the series with the first resistor
3)measure current through circuit and use this and pd of battery to calculate overall resistance in circuit
repeat steps 2&3 until youve added all resistors
plot graph of number of resistors against total resistance of circuit
(it should be a straight line graph)
how to investigate resistance in a parallel circuit - practical
use same equipment used to find resistance in series so its a fair test-
build same initial circuit , measure total current through circuit and calculate resistance of circuit with v/I
2-add another resistor in parallel circuit with the first
3-measure total current through circuit and use this and potential difference of battery to calculate overall resistance in circuit
repeat steps 2 and 3 until youve added all resistors
plot a graph of the number of resistors against total resistance
(should be a downwards curved line)
what does adding resistors in a series circuit do
increases the total resistance of circuit, adding a resistor decreases the total current through the circuit, more resistors you add , the larger the resistance of the whole circuit
what does adding resistors to a parallel circuit do
total current through the circuit increases so the total resistance of the circuit has decreases
the more resisors you add, the smaller the overall resistance becomes
what type of current is the UK mains supply and what is its voltage
what is its frequency
alternatiing current
at around 230 V
frequency is 50 cycles per second or 50 Hz
what type of current is battery supply
direct current
what is the current like in alternating currents
current is constantly changing direction , it is produced by alternating voltages in which the positive and negative ends keep alternating
what is direct current
current that is always flowing in the same direcition- created by a direct voltage
what are most electrical appliances connected to
the mains supply by 3 core cables - they have 3 wires in them each with a core of copper and a coloured plastic coating
what is the colour of the neutral wire and what does it do
blue
it completes the circuit and carries away current- electricity flows in through the live wire and out through neutral wire- it is around 0 V
what is the colour of the live wire and what does it do
brown
it provides the alternating potential difference (at about 230 V) from the mains supply
what is the colour of the earth wire and what does it do
it is green and yellow
it is for protecting the wiring, and for safety, stops the appliance casing from becoming live- it doesnt usually carry a current only when theres a fault - it is at 0V
Why can the live wire be dangerous
your body is at 0V so if you touch the live wire- a large pd is produced across body and current flows through you
this causes a large electric shock which could injure or kill u-even if plug socket or light switch is turned off- there is still danger of an electric shock - current isnt flowing but theres a pd in the live wire
if you made contact with live wire, your body would provide a link between supply and earth so current would flow through you
any connection between live and earth can be dangerous - if link creates low resistance path to earth , a huge current will flow = could result in fire
how does a moving charge transfer energy
the charge does work against the resistance of the circuit
how are kettles designed to transfer energy to components in a circuit when a current flows
kettle transfers energy electrically from mains ac supply to thermal energy store of the heating element inside the kettle
how are hand held fans designed to transfer energy to components in a circuit when a current flows
energy is transferred electrically from the battery of a hand held fan to the kinetic energy store of the fan’s motor
what does a high current in an appliance mean
more energy is transferred to the thermal energy store of the component and then surroundings -
efficiency=
useful output energy transfer/ total input energy transfer
what does a high current in an appliance mean
more energy is transferred to the thermal energy store of the component and then surroundings -
efficiency=
useful output energy transfer/ total input energy transfer
what does the total energy transferred by an appliance 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- more energy it transfers in a given time= higher the power
how do you calculate the amount of energy transferred by electrical work
power x time (seconds)
what is a power rating
appliances are given this- theyre labelled with the maximum safe power that they can operate at- you take this as maximum operating power
it tells you the max amount of energy transferred between stores per second whe appliance is in use
helps customers choose between models
the lower the power rating- the less electricity an appliance uses in a given time - the cheaper it is to run
why does a higher power not necessarily mean it transfers more energy usefully
an appliance may be more powerful than another but less efficient so 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
how is it calculated
energy is transferred-]
energy is supplied to 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
this means the battery with the bigger pd will supply more energy to the circuit for every coloumb of charge that flows around it bc charge is raised up higher at the start
how can the power of an appliance be found with current and pd
current x potential difference = power
how can the power of an appliance be found with current and pd
current x potential difference = power
how can the power be found if you dont know the potential difference
current ^2 x resistance
how is electricity distributed by the national grid
what is the national grid
a giant systm of cables and transformers that covers the uk and connects power stations to consumers (anyone who is using electricity)
how does the national grid transfer electrical power
transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the grid where it is needed (the demand) e.g. the home and industry
how do the power stations in the national grid produce enough electricity for everyone
throughout the day - the usage changes. they can predict when most electricity will be used . demand increases when people get up in the morning, come home from school/work and when it gets dark and cold. popular events like sporting finals shown on tv could cause peak in demand
power stations often run at well below max power output so theres spare capacity to cope with high demand even if theres unexpected shutdown of another station
lots of smaller power stations that can start up quick are also kept standby in case
why does the national grid use a high pd and a low current
what is the problem with a high current
to transmit a huge amount of power needed, you need high pd or high current.
problem with high current =you lose lots of energy as wires heatup and energy is transferred to thermal energy store of surroundings
cheaper to boost pd up really high (400000 v) andd keep current as low as possible
for given power, increasing pd decreases current which decreases energy lost by heating wires and surronding - makes national grid an efficient way of transferring energy
how is potential difference changed by a transformer
to get pd to 400000 V to transmit power requires transformers as well as big pylons with huge insulators(but its still cheaper)
transformers have to step the potential difference up at one end for efficient transmission and bring it fown to safe usable lvel at other end
pd is increased using step up transformer
its reduced for domestic use using step down transformer
explain static electricity
its about charges that are not free to move. (e.g. in insulating materials) causes them to build up in a place - ending with a spark or shock when they move
explain how to build up of static is caused by friction
when certain insulating materials are rubbed- negative electrons will be scraped off one and dumped on another
this leaves material electrically charged which positive static charge on one and an equal negative static charge on the other
which way the electrons are transferred depends on the 2 materials involved
e.g. polythene and acetate rods being rubbed with a cloth duster
why does electrons move and never positive charges
both +ve and -ve charges are produced by movement of electrons. positive charges do not move. positive static charge is caused by electrons moving away elsewhere. material that loses the electrons loses some negative charge - left with an equal positive charge
why does too much static cause sparks
an electric charge builds on object. pd between object and earth (which is 0v) increases
if pd gets large enough , electrons can jump across gap bewteen charged object and earth - this is spark
they can also jump to any earthed conductor nearby- this is why you can get static shock getting out of car, charge builds up on cars metal frame and when you touch car , charge travels through you to earth
happens when gap is fairly small(not always tho- lightning is just a really big spark)
what happens when 2 electrically charged objects are brought close together
they exert a force on one another. 2 things with opposite electric charges are attracted. two things with same electric charge will repel.
forces get weaker the further apart the 2 things are
forces cause object to move is they can. - electrostatic attraction/ repulsion- non contact force
to see this force- suspend a rod with a known charge from a piece of string - so it is free to move- place an object with same charge nearby will repel rod. rod will move away from object. oppositely charged object will cause rod to move towards object
how do electric charges create an electric field
electric field is created around any electrically charged object, closer to the object you get , the stronger the field is, the further you are from it, the weaker it is
how can you show an electric field around an object
using field lines,
electric field go from positive to negative
they are always at a right angle to the surface
the closer together the lines are, the stronger the field is - you can see that the further from a charge you go , the further apart the lines are so the weaker the field is
why do charged objects in an electric field feel a force
when a charged object is placed in the electric field of another object, it feels a force
this force causes the attraction or repulsion
force is caused by the electric fields of each charged object interacting with each other
force on object is linked to strength of the electric field it is in
as you increase distance between charged objects, strength of field decreases & force between them gets smaller
e.g. say you have a large Q+ve and a q-ve
the electric field of Q interacts with the electric field of q- causing forces to act on Q&q- making them move bringing them closer
how can sparking be explained by electric fields
sparks are cause when there is a high enough pd between a charged object and earth(or earthed object)
high pd causes a strong electric field between charged object and earthed object
strong electric field causes electrons in air paticles to be removed (ionisation)
air is normally an insulator , but when it is ionised , it is more conductive so current can flow through it - this is spark
