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Parallel circuits for OCR A-level Physics

Parallel circuits

This page covers the following topics:

1. Current in parallel circuits
2. Potential difference in parallel circuits
3. Resistance in parallel circuits
4. Parallel circuits

The current in a parallel circuit will be divided up into the different branches, depending on the resistance of each branch. The current will then all join together and the total of all of them will flow through the cell.

Current in parallel circuits

Every branch in a parallel circuit will have the same potential difference across it.

Potential difference in parallel circuits

The reciprocal of the total resistance in a prallel circuit is the sum of the reciprocals of the individual resistances in the circuit.

Resistance in parallel circuits

All equations for the current, potential difference and resistance in a parallel circuit can be used together for calculations.

Parallel circuits

1

A circuit is made up of two identical lamps connected in parallel to each other and a cell. Given that the current through one of the lamps is 2.1 A, calculate the current through the cell.

Since the lamps are identical, the current through both lamps is 2.1 A. The current will all join together and the total of all of them will flow through the cell. Therefore, the current through the cell is 2.1 A + 2.1 A = 4.2 A.

A circuit is made up of two identical lamps connected in parallel to each other and a cell. Given that the current through one of the lamps is 2.1 A, calculate the current through the cell.

2

Calculate the current through the cell in the given circuit.

Ohm's Law can be used to calculate the current through the resistor. Using Ohm's Law, 4 V = I ร— 10 ฮฉ, so I = 4 V/10 ฮฉ = 0.4 A. The current will all join together and the total of all of them will flow through the cell. Therefore, the current through the cell is 0.4 A + 1.4 A = 1.8 A.

Calculate the current through the cell in the given circuit.

3

Two lamps of equal resistance are connected in parallel with a cell. The current through the cell is 0.8 A. State the current through each.

Since the circuit is a parallel one and the resistance of the two lamps is equal, the current will divide equally between the two branches. Therefore, the current through each lamp is 0.8 A/2 = 0.4 A.

Two lamps of equal resistance are connected in parallel with a cell. The current through the cell is 0.8 A. State the current through each.

4

Describe how resistance acts in a parallel circuit.

The reciprocal of the total resistance in a prallel circuit is the sum of the reciprocals of the individual resistances in the circuit.

Describe how resistance acts in a parallel circuit.

5

A 25 ฮฉ resistor is connected in parallel to a lamp and a cell of voltage 5 V. Calculate the current through the resistor.

Every branch in a parallel circuit will have the same potential difference across it, therefore the potential difference across the resistor is 5 V. Using Ohm's Law, 5 V = I ร— 25 ฮฉ, so I = 5 V/25 ฮฉ = 0.2 ฮ‘.

A 25 ฮฉ resistor is connected in parallel to a lamp and a cell of voltage 5 V. Calculate the current through the resistor.

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