Why does the light get dimmer and dimmer as more light bulbs are being added to a parallel circuit?

A light bulb glows when a voltage is applied across the bulb. The greater the voltage, the higher the current and the brighter the bulb will glow. The brightness is related to the power dissipated by the bulb and power is calculated by multiplying the voltage by the current.

If a second bulb is added, the voltage across each bulb will be half of the voltage across a single bulb. The current through each bulb will be lower because the voltage across each bulb is also lower. Therefore, the power dissipated in each bulb will be lower and the bulbs will be dimmer.

There are two equations that are used for the calculation:

Ohms Law - Voltage = current x resistance

Power calculation - Power = current x voltage

Here is a worked example:

A bulb with a resistance of 3 Ohms has a voltage of 6V across it. Ohms Law says that it will have a current of 2Amps. The power dissipated by the lamp will be 2Amps x 6V = 12Watts.

If two bulbs are in the circuit, each will have a voltage of 3 volts across it. The current through each of them will be 3V / 3Ohms = 1A. The power dissipated in each bulb will be 1Amp x 3V = 3Watts.

So, with two bulbs in circuit, the total power dissipated will be 2 x 3Watts = 6Watts. Therefore, not only does each bulb get dimmer but the total power is also reduced.

Alternative AnswerA lamp will only operate at its rated power at its rated voltage, which is normally the supply voltage. Lamps in parallel are always subject to their rated voltage, whereas this is not the case with lamps in series. So lamps connected in parallel always operate at their rated power. Furthermore, a small decrease in supply voltage will bring about a much greater decrease in the power of a lamp (they are not proportional), so placing lamps in series produces an enormous drops in their power (higher wattage lamps will be the least bright, and lower wattage lamps will be the brighter).

Unfortunately, the original answer fails to mention that there is a significant change in the resistance of an incandescent lamp between its 'cold' and 'hot' temperatures (around 15 times!), so the worked example isn't really valid for incandescent lamps.