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Alvl P1: electricity (ch9) Cheat Sheet by

aqa A-level physics Year 1 (yr12) electricity : chapter 9

Defini­tions

potential difference
the measure of how much energy is transf­erred by each coloumb of charge
current
the rate of flow of charge
resistance
the measure of how much a component resists the flow of current
resist­ivity
how resistive a material is to the flow of charge
emf
energy supplied to each unit charge

Series and Parallel circuits

 
series
parallel
pd
shared across compon­enets
equal
current
same for all components
split at branches
resistance
sum of resist­ances
recipo­racal

Variable reisistors

Themistor:
T increases = R decreases

LDR:
Light increases = R decreases


--both of these can be used a potential dividers

Power, AC/DC, rms

AC - altern­ating current
eg mains electr­icity

DC - direct current
eg. a battery

rms:
root means squared- average of variables

P av = V rms I rms

mains uk: V rms = 230V

X rms = Xo / 21/2

EMF

Emf is the total energy a battery has however the measured value will be smaller

this is due to internal resist­ance.

V = W (by the charge) / Q

E = W (on the charge) / Q
 

Laws

Kirchhoffs 1st law
charge and current is conserved at any junction in a circuit
Kirchhoffs 2nd law
the sum of the emfs must equal the sum of the pd drop in a closed loop

resistance

ohms law:
current and pd in an ohmic conductor held under constant physical conditions are directly propor­tional (resis­tance is the constant of propor­tio­nality)

V = IR <<for a fixed resistor only

resistance is not constant for objects such as filament lamps
- this is due to the deloca­lised electrons colliding with the ironic lattice
- this causes them to vibrate more and increase temper­ature

you can reverse the cell to obtain negative values for I and V




diodes only let current flow in one direction-
> low resistance = forward direction
> high resistance = backward direction

no current flows until it reaches breaking voltage on either side (-ve/+ve)


superc­ond­uctors- material that resistance decreases to 0 at the critical temper­ature




Resist­ivity:
how to work it out
1. measure the diameter of the wire with a micrometer and calculate the cross-­sec­tional area
2. change the L of the wire by moving one crocodile clip
3. use wire of material for which resist­ivity does not change much eg nichrome
4. calculate R from V/I for each length


Variable resistors:
rheostat > change the current, can never turn the bulb off (perma­nently connected)
potent­iometer > change the voltage, can turn the bulb off (doesnt have to be connected)


situat­ions:

if you have two different identical circuits with a resistor each, one has 20R and the other R, what is the simila­rities and differ­ences:
S- voltage is the same at the end of both
D- current is different, R would have more current as R is lower than 20R
D- physical difference would be R is hottoer as its being hit by more current, quicker


if you have a circuit with parallel resistors, with two in series, if the propor­tions between the resistors on each side of the parallel circuit is the same then no current flows as theres no voltage
-> no potential difference


parallel circuit. one branch has an ideal voltmeter and resistor, other branch has two resistors, battery has 5 V. as it is an ideal voltmeter is has infinite resist­ance. this means one side of the branch has 5V and the otherside has 0V, this means the resistor has next to it has no voltage passing through it therefore is not included when working out total resistance of the circuit.
               
 

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