Parallel Circuits

In a parallel circuit, there is more than one way for electrons to flow. The components are in different loops, which we call branches.

Below is an example of a parallel circuit:

Two different representations of the same parallel circuit: on the left, a simplified representation using electrical circuit symbols. Showing a battery connected to two parallel resistors; on the right, a more detailed representation with a battery, arrows indicating the direction of current, and two glowing bulbs as resistors. Both are labelled "Parallel Circuit".

In this parallel circuit, we have a closed switch in the first branch, an open switch in the second branch, and a closed switch in the third branch. The circuit has parallel lines and components parallel to each other, which is why we call it a parallel circuit.

Advantages of Parallel Circuits

In a parallel circuit, when a component in a branch breaks, the components in other branches keep working. For example, let’s look at the diagram below.

Simple diagram of a parallel circuit: Starting from the top, there's an electrical cell. Below the cell, there's a broken lamp depicted by a circled cross with no light rays. Further down, there are two lamps, both represented by circled crosses with light rays emanating, suggesting they are glowing or in working condition.

The first lamp is broken but the other two lamps are still brightly lit. In the same way, if the second or third lamp breaks, the other lamps will still light up. This is why parallel circuits are much more common than series circuits, as they are much more useful.

Switches in Parallel Circuits

We can use switches to control which components we want to turn on and off. For example, let’s look at the diagram below.

Simple diagram of a parallel circuit with three branches. Starting from the top, there's an electrical cell. Moving downwards, each branch consists of a switch followed by a lamp. The first and third branches have switches in the closed position with illuminated lamps, represented by a circled cross with radiating lines. The middle branch has its switch in the open position, so the lamp is not illuminated, represented by a simple circled cross.

In this parallel circuit, We have a closed switch in the first branch, an open switch in the second branch and a closed switch in the third branch.

In the second branch, the switch is open, so we have an incomplete circuit. Even though the switch is open, the first and third lamps are still lit. We can control which lamps are on and which lamps are off by using switches. If we wanted to turn the lamp on, then we close the switch and if we wanted to turn it back off, we open the switch.

Current in a Parallel Circuit

In a parallel circuit, the current is shared between the different branches. We measure the current using an ammeter, which is placed in series with the components.

An analogue ammeter in a beige casing, with a clear display showing a needle pointing between 4 and 6. The scale ranges from 0 to 8 with the letter 'A' indicating amperes at the centre. Below the display, there are two adjustment knobs with blue centres; the left knob has a minus sign and the right knob has a plus sign.

  • Current is measured in amperes (A)

As the current is shared between different branches, the current in each branch adds up to the total current. For example, if the total current is 6 amperes, all the branches must add up to 6 amperes. Each branch is 2 amperes and there are three branches, which adds up to 6 amperes (2 + 2 + 2 = 6).

An electrical circuit diagram featuring a main cell at the top, with an ammeter indicating 6 amps of current. Following this, there are three vertically arranged branches. Each branch contains an open switch, a bulb, and an ammeter showing a current of 2 amps.

Keep in mind that the ammeter should be placed next to the component you are measuring.

As we are using the same component, the lamp, we should expect the same current to flow through each branch. In this case, we have 2 amps for all three branches.

Rules About Potential Difference in a Parallel Circuit

In a parallel circuit, the potential difference across each circuit is the same as the potential difference across the cell or the battery. This means if we add more lamps, the lamps stay bright.

We use a voltmeter to measure the potential difference.

A blue voltmeter with a beige face displaying a scale from 0 to 8. The needle points to 4. Below the face, there are two knobs: one with a minus sign on the left and one with a plus sign on the right.

  • Potential difference is measured in volts (V)

Whether it is a series circuit or a parallel circuit, the voltmeter is not placed in the same loop as the other components. Instead, it is placed parallel to the component you are measuring, connecting a wire to either side of the component you are measuring. You can see this in the parallel circuit diagram below.

An electrical circuit diagram with three components. At the top, a cell indicates 4 volts. Below it, two parallel branches, each consisting of a light bulb and a voltmeter. Both voltmeters read 4 volts, and each branch is connected to the main circuit line.

If the potential difference going across the cell is 4 volts, then the potential difference going across each lamp will also be 4 volts. This applies to any other components in a circuit, not just lamps.

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