Comparing series and parallel circuits

Practical method

Aim

Compare the current and potential difference in series and parallel circuits, and the total resistance of resistors in series and parallel.

Variables

Independent variable: circuit arrangement (series or parallel) and number of resistors
Dependent variable: current and potential differences
Control variables: supply potential difference (6.0 V); resistor values (10 Ω each); the same meters.

Apparatus

• 6.0 V power supply
• Two or three 10 Ω resistors
• Ammeters
• Voltmeter
• Switch
• Connecting leads

Method

1. Connect two 10 Ω resistors in series with an ammeter and the 6.0 V supply.
2. Measure the current at different points in the circuit and the potential difference across each resistor and across both.
3. Rebuild the circuit with the two resistors in parallel. Measure the current in each branch and in the main circuit, and the potential difference across each branch.
4. Calculate the total resistance R = \(\dfrac{V}{I}\) for each arrangement.
5. Repeat with three resistors.

Sample results

Supply 6.0 V; each resistor 10 Ω.

Analysis

Series: the current is the same everywhere, the supply potential difference is shared, and the total resistance is the sum (R = R₁ + R₂). Parallel: the potential difference across each branch equals the supply, branch currents add up to the total current, and the total resistance is less than the smallest resistor. Adding resistors in series increases total resistance; adding them in parallel decreases it.

Errors and improvements

Risk assessment

Common mistakes

• Thinking current is used up in a series circuit.
• Adding parallel resistances as if they were in series.
• Putting the voltmeter in series.

Worked analysis

Two 10 Ω resistors in series with 6.0 V: R = 10 + 10 = 20 Ω; I = \(\dfrac{V}{R}\) = 6.0 ÷ 20 = 0.30 A; each resistor has 0.30 × 10 = 3.0 V across it.

Common misconception

Current is not used up by components. In a series circuit the current is the same at every point.

Variations required by some exam boards

Using lamps (OCR): repeat with two identical filament lamps. In series each lamp is dimmer than a single lamp, because the supply pd is shared. In parallel each lamp has the full supply pd and is as bright as a single lamp, but the total current from the supply doubles. Record brightness qualitatively alongside the meter readings.