In a parallel circuit with bulbs, what happens to the brightness of existing bulbs if another bulb is added in parallel?

In a parallel circuit with bulbs, what happens to the brightness of existing bulbs if another bulb is added in parallel?

• A. They remain the same
• B. They become brighter
• C. They burn out
• D. They become dimmer

Answer: (A)

In a parallel circuit, each bulb has the full supply voltage across it. Adding another bulb in parallel creates another path for current, but it doesn’t change the voltage across the existing bulbs (assuming the power source can supply the extra current without dropping voltage). Since the voltage across an existing bulb and its resistance don’t change, the power it dissipates stays the same, so its brightness remains the same. The new bulb simply adds its own brightness in addition, without dimming the others. This differs from a series arrangement, where adding more bulbs changes the current through all of them and can make them dimmer.

In a parallel circuit, each bulb has the full supply voltage across it. Adding another bulb in parallel creates another path for current, but it doesn’t change the voltage across the existing bulbs (assuming the power source can supply the extra current without dropping voltage). Since the voltage across an existing bulb and its resistance don’t change, the power it dissipates stays the same, so its brightness remains the same. The new bulb simply adds its own brightness in addition, without dimming the others. This differs from a series arrangement, where adding more bulbs changes the current through all of them and can make them dimmer.