Question

You have three circuits that include a power supply and one (or two) lightbulbs. The only variation among these circuits involves the number of lightbulbs and how the lightbulbs are wired (if there are more than one). Circuit 1: one lightbulb; Circuit 2: two lightbulbs in series; Circuit 3: two lightbulbs in parallel. Which circuit has the greatest current?

a) 1

b) 2

c) 3

d) all have the same current.

Answer 1

Assuming that the power supply (V) remains same in all three circuits and all the bulbs (R) are identical.

Circuit 1:

Current = Voltage/Resistance =V/R

Circuit 2:

Current = Voltage/ Equivalent resistance of circuit

In series connection, the equivalent resistance is simply the addition of individual resistances

==> Current = V/2R

Circuit 3 = Voltage/ Equivalent resistance of circuit

In parallel connection, Req = R1*R2 / R1+R2  

Here Req = R/2

Current = 2V/R

Hence the circuit with parallel connection of Resistors has greatest current.

Answer = c) 3