CP10.17(b): Series and Parallel Lamp Circuits

Trace the circuit before connecting it. In a series arrangement there is one complete path, so the steady current is the same at different positions. The sum of the potential differences across components equals the supply potential difference in the simple model. Place a voltmeter across the two points of interest and an ammeter within the path whose current is needed.

In a parallel arrangement, branches share the same pair of junctions and therefore have the same potential difference. Current divides at a junction and the branch currents sum to the supply current. Lines crossing on a diagram do not necessarily form a junction: use the connection marks. Check for a complete route and for unintended wires that bypass a component.

Measure supply voltage and relevant currents in a series circuit, then in a parallel circuit using the specified components. Keep a record of the actual arrangement, meter positions and supply setting. A real supply can change its terminal voltage under a different load, so “same knob position” does not guarantee exactly the same measured voltage.

For identical filament lamps on an appropriate fixed-voltage supply, parallel branches can each receive the full terminal potential difference, while series lamps share it. Brightness depends on the power at each lamp’s operating condition. Because filament resistance changes with temperature, use measured V and I and P = VI rather than assuming an unchanged lamp resistance in every connection.

CP10.17(b) requires parallel filament-lamp circuits with ammeters and voltmeters, compared with the initial series arrangement. Record potential difference and current, include V = IR analysis and do not replace the lamp comparison with only ideal fixed resistors.

Small differences between a measured supply current and summed branch currents can reflect instrument resolution, timing or changed conditions if the circuit was rewired between readings. Use appropriate simultaneous measurements or repeat under stable conditions. Do not claim charge is consumed to explain a series-current difference; check the connections and measurement process.

Adding a parallel branch increases total current for an ideal fixed-voltage source with positive-resistance loads. A supply’s current limit or internal resistance can alter the observed voltage and brightness. Record these limitations and distinguish the ideal prediction from the actual source. Switch off before rewiring and follow component ratings and current limits.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.