OCR - P3: Electric circuits

P3.3 How do series and parallel circuits work?
Background to the topic What you should be able to do:

When electric charge flows through a component (or device), work is done by the power supply and energy is transferred from it to the component and/or its surroundings.

Potential difference measures the work done per unit charge.

In a series circuit the charge passes through all the components, so the current through each component is the same and the work done on each unit of charge by the battery must equal the total work done by the unit of charge on the components.

The potential difference (p.d.) is largest across the component with the greatest resistance and a change in the resistance of one component will result in a change in the potential differences across all the components.

In a parallel circuit each charge passes through only one branch of the circuit, so the current through each branch is the same as if it were the only branch present and the work done by each unit of charge is the same for each branch and equal to the work done by the battery on each charge.

The current is largest through the component with the smallest resistance, because the same battery p.d. causes a larger current to flow through a smaller resistance than through a bigger one.

When two or more resistors are placed in series the effective resistance of the combination (equivalent resistance) is equal to the sum of their resistances, because the battery has to move charges through all of them.

Two (or more) resistors in parallel provide more paths for charges to move along than either resistor on its own, so the effective resistance is less.

Some components are designed to change resistance in response to changes in the environment

e.g. the resistance of an LDR varies with light intensity, the resistance of a thermistor varies with temperature; these properties used in sensing systems to monitor changes in the environment.

1. Relate the potential difference between two points in the circuit to the work done on, or by, a given amount of charge as it moves between these points

potential difference (V) = work done (energy transferred) (J) รท charge (C)

2. a) describe the difference between series and parallel circuits: to include ideas about how the current through each component and the potential difference across each component is affected by a change in resistance of a component

b) describe how to practically investigate the brightness of bulbs in series and parallel circuits.

Be able to draw circuit diagrams for the circuits used

3. explain, why, if two resistors are in series the net resistance is increased, whereas with two in parallel the net resistance is decreased (qualitative only - no maths any more!)

4. solve problems for circuits which include resistors in series, using the concept of equivalent resistance

5. explain the design and use of d.c. series circuits for measurement and testing purposes including exploring the effect of:

a) changing current in filament lamps, diodes, thermistors and LDRs

b) changing light intensity on an LDR

c) changing temperature of a thermistor (NTC only)