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
