Electrical Circuit
Electricity and Magnetism

Voltage in parallel circuits

Physics Narrative for 11-14 Supporting Physics Teaching

Voltage in parallel circuits: what happens?

How do the ideas about voltage apply to parallel circuits? In this circuit, two identical bulbs are connected in parallel to a 3 volt battery and a voltmeter is connected across each bulb.

We've already seen that adding a bulb in parallel results in both bulbs being of equal, normal brightness. What happens to the voltage reading across each bulb?

When the second bulb is added in parallel, there is a voltage equal to the full battery voltage across both. Each voltmeter reads 3 volt.

As the second bulb is added, there is a current in both loops. The power in both bulbs is equal and set by the current in and the voltage across each bulb. Both bulbs are as bright as they would be in a simple loop (one battery, one bulb).

Making sense of the voltmeter readings using a rope loop

Each loop travels at constant speed, so the driving and counter forces must be equal in size (see the SPT: Forces topic). The product of the speed and the force calculates the power switched.

Voltage across components in parallel: a summary

When bulbs or resistors are connected in parallel, the full battery voltage is dropped across each.

The powerin must be equal to the powerout.

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