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# Alvl P1: electricity (ch9) Cheat Sheet by MostAncientDream

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 < 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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