Electricity

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Current and Charge

Electric current is the rate of flow of electric charge. No current can flow if the circuit is broken - for example, when a switch is open.

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Useful equations

E=Pt

P=IV

P=I2R

P= V2 / R

E=Pt=ITV

Total resistance in series: R1 + R2

Total resistance in parallel: 1/R (total) = 1/R1 + 1/R2

Potential dividers

V out = V in x R2 / R1+R2

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kWh

Energy = power x time

J=kW x 3600 seconds

If question is asking for answer in joules, Multiply the power in watts by the amount of time in seconds. If it is asking for the answer in kWh multiply the power in kW by the time in hours.

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Transformers

Formula

N2 / N1 IS EQUAL TO

VS / VP IS EQUAL TO

IP / IS

N1 IS PRIMARY COIL

N2 IS SECONDARY COIL

V IS VOLTAGE

I IS CURRENT

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AC AND DC

AC rapidlty switches the direction of flow of electrons (in the UK we have AC at 50Hz)

DC travels only in one direction

use a diode to convert AC to DC

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Current rules

Current in series

In a series no matter where you put the ammeter, the current will always be the same. this is because current is not used up.

Current in parallel 

The current in a parallel circuit splits into different branches then combines again before it goes back into the supply. When the current splits, the current in each branch after the split adds up to the same as the current just before the split.

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Voltage rules

Voltages in series

  • The supply voltage is shared between components in a series circuit.
  • The sum of the voltages across components in series is equal to the voltage of the supply.
  • The voltages across each of the components in series is in the same proportion as their resistances. This means that if two identical components are connected in series, the supply voltage divides equally across them.

Voltage in parallel

The voltage across components in parallel is the same for each component

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resistivity

R= (p x L) / (A)

where R= resistance

P=resistivity

L=legnth of wire

A= cross sectional area m2

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