You need to remember all the rules for each type of circuit:
In a series circuit the total of all the voltages dropped in the circuit must equal the supply.
In a parallel circuit the voltage dropped in each branch is the same.
In a series circuit the current is the same at all points.
In a parallel circuit the currents in each branch are added to give the total circuit current OR from Kirchoff's law, the current entering a node must equal the currents exiting a node. In a series circuits all the resistances are added to give the total resistance.
In a parallel circuit the reciprocal (1/Ω) are added, or if there are only two resistors in parallel the special total resistance formula can be used: Rt = Product/Sum.Lets consider this circuit.
To work out where the series and parallel parts are follow your finger around the circuit. When you reach a node (junction/join) you know that you have come to a parallel branch.
First you should work out the equivalent resistance of the parallel resistors. This, put simply, is finding out what resistance value these two have created, or working out the single value of resistor which could replace these two.
This makes it easier to see that these two parts of the circuit are in series with each other, so we can then work out the total resistance:
Knowing the total resistance we can find the total circuit current:
So going back to the circuit as it was, we can see that the circuit current flows through R1, and it then splits.
So we can work out the voltage dropped over R1 and from that work out the voltage dropped over the parallel branches (remember that this will be the same voltage in each branch):
Knowing the voltage dropped over each of the parallel resistors and the resistance, we can work out the current using ohm's law.
And finally we can check if our current calculations are correct using Kirchoff's Law.
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