An electrical circuit can be series, parallel (the basic configurations) or a combination of series/parallel devices. When finding the current through a circuit one first need to derive its equivalent resistance. For a series circuit the equivalent resistance is

`R_(eq) = R_1+R_2+...+R_n`

while for a parallel circuit the equivalent resistance is

`1/R_(eq) =1/R_1 +1/R_2 +....+1/R_n`

For a complex series-parallel circuit the equivalent resistance is found by grouping together the resistances first in series and parallel combinations then finding the total resistance of these groups.

The total current in circuit is given by the Ohm law:

`I_("tot") =U_("tot")/R_(eq)`

For a series circuit the current is the same in all the components while the different voltage drops on the components add together to give the total supply voltage.

For a parallel circuit the voltage is the same in all components (and equal to the supply voltage) while the different currents through components add together to give the total current.

For a complex series-parallel circuit the voltages and currents split accordingly the above rules on to different existent series and parallel resistor groups.

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