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Physics

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Series and Parallel Circuits

**Demonstrate understanding that the current at every point in a series circuit is the same.**

Current is the flow of electric charge within a circuit. Current does NOT ever change or run out in a series circuit. It is constant at ANY point in the circuit; it is the same at the beginning and at the end of a circuit.

**Recall and use the fact that the sum of the potential differences across the components in a series circuit is equal to the total potential difference across the supply.**

Since we know that all the potential energy is used up by the components in the circuit, the potential difference at the end of a circuit is always 0.

If the potential at the end of the circuit is 0, then we can say that the sum of the potential differences across all components in a series circuit is equal to the electromotive force (EMF) the supply provides or the potential difference across the supply.

**Calculate the combined resistance of two or more resistors in series.**

For a series circuit, the total resistance is equal to the sum of the resistance of each component:

RT = R1 + R2 + …

For example, if there are two resistors, each with 6Ω of resistance, then the total resistance will be:

6Ω + 6Ω = 12Ω

**State that, for a parallel circuit, the current from the source is larger than the current in each branch.**

In a parallel circuit, the current in each 'branch' of the circuit is less than the current at the beginning or end.

Why?

In order for electricity to flow through multiple branches in a parallel circuit, the current has to split up. This means that the current is weaker in each branch compared to the source. Imagine a river when it splits. The amount of water flowing in each river branch is less than the original, larger river.

**Recall and use the fact that the current from the source is the sum of the currents in the separate branches of a parallel circuit.**

When a circuit splits up into parallel, so does the current. However, remember that the current at the beginning and at the end of the circuit is constant.

When the circuit rejoins again, the current before and after is the same.

Therefore, we can say that the sum of the currents in the separate branches of the circuit is equal to the current from the source.

**State that the combined resistance of two resistors in parallel is less than that of either resistor by itself.**

It might seem illogical that when you add another branch to a circuit, the total resistance of the circuit decreases. You would expect it to remain the same.

The best way to explain it would be that by adding an additional branch to the circuit, the total current flowing in the circuit increases. This is because adding another branch gives the current another path through which it can flow.

Using the equation:

R = V/I

Voltage (V) remains the same since our power source does not change in a parallel circuit.

However, by adding more branches, our total current (I) increases in the circuit.

And if current increases, our resistance (R) therefore decreases.

**Calculate the effective resistance of two resistors in parallel.**

The total resistance in a parallel circuit is given by the equation:

1/RT = 1/R1 + 1/R2 + …

For example:

The total resistance of the circuit would be:

1/RT = 1/2 + 1/3 + 1/4

1/RT = 6/12 + 4/12 + 3/12

1/RT = 13/12

RT = 12/13Ω or 0.92Ω

**State the advantages of connecting lamps in parallel in a lighting circuit.**

In a series circuit, connecting two lamps together results in decreased brightness for both lamps.

The lamps are not giving off light at their maximum brightness. This is because the electrical energy that the current carries is split between both lamps.

However, connecting two lamps in parallel results in both lamps outputting at maximum brightness.

This is because the energy carried by the current is not shared between the two lamps. The current splits up and powers each lamp individually.

If one lamp blows, the other will remain on.

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