If two lightbulbs are parallel to each other, they have the same brightness. If the same two lightbulbs are put in series with each other, they also have the same brightness but are dimmer than when they are in parallel. Explain why.

The bulbs becomes dimmer because effective resistance of the circuit increases since the resistance of the circuit in series connection involves adding the individual resistance compared to in parallel connection where the effective resistance = 1 / R1 + 1/R2 = R1 R2 / (R2 + R1). The brightness remains the same because the lightbulbs must be of the same resistance. Also using ohm's law

V = IR

I = V / R

I is inversely proportional to the current in the circuit so as R, effective resistance of the circuit increases, I, current decreases and vice versa