The first circuit has two light bulbs in series. If one of the light bulbs is removed, the other light bulb goes out because the circuit is no longer complete. As more light bulbs are inserted, the resistance increases, and the current decreases. If the light bulbs have different resistances, then the voltage is different at each point. The lights are not as bright as one light because the voltage is half as what one light bulb could be, because the voltage is divided between the two.
The second circuit is in parallel. Both of the lights are equally bright, because the same amount of volts is sent across both light bulbs. For example, if there was a 12 V energy source, then 12 V would be sent through each light bulb. The resistance through the current is the same throughout the whole parallel of the circuit. If one of the light bulbs was to be taken out, the other would still be lit because it still had a complete circuit, and it would still be as bright. The value of the current is the reciprocal of the resistance.
The light bulbs that are in parallel are in series with the other light bulb. The single light bulb is much brighter that the ones in series, which are the same brightness. The voltage of the series is equal to the voltage of the bulbs in parallel. The current of the complex circuit is the total voltage divided by the combination of both currents through the series and parallel circuits. The equivalent resistance is found by combining the resistance of the parallel light bulbs with the light bulb in series. As more light bulbs are added, the resistance could increase or decrease, depending on weather the light bulbs are in series or parallel.
