Electric Current, Series and Parallel Circuits: Comprehensive Study Notes

Learning Objectives

  • Good: State what an electric current is.

  • Even Better: Compare the current at different points in a parallel circuit.

  • Outstanding: Explain why the current changes at different points on a circuit.

Key Vocabulary

  • Current – rate of flow of electric charge, measured in \text{Amperes (A)}.

  • Parallel – components are arranged in branches so charges can take more than one path.

  • Ammeter – instrument used to measure current; always connected in series with the component being measured.

Concept of Electric Current

  • An electric current is a flow of electricity (mobile charges, usually electrons in a metal conductor).

  • Symbol for current: I(Amps, \text{A}).

  • Series view: Current passes through components one after another.

  • Parallel view: At each junction, the current divides; the sum of branch currents equals the current supplied by the source.

Measuring Current with an Ammeter

  1. Connect the red terminal of the ammeter to the positive battery terminal.

  2. Insert the ammeter in series at the point where current is to be measured.

  3. Never place an ammeter directly across the battery (that would create a short-circuit).

Series Circuits

  • Current is the same everywhere: I1 = I2 = I3 = \dots = I{\text{series}}.

  • Example: A single cell powering two bulbs in series will show identical ammeter readings at points 1, 2, 3.

Parallel Circuits

  • Contain more than one loop; an electron does not traverse every component.

  • House lighting circuits are wired in parallel so that if one lamp fails, the others remain lit.

  • Current rule: The current leaving the supply equals the sum of currents in individual branches: I{\text{cell}} = I1 + I2 + \dots + In.

  • Measurement tip: Place an ammeter close to the cell (supply) and compare with readings in each branch; branch readings should add up to the supply reading.

Classroom Practical – Measuring Current in a Parallel Circuit

Circuit built:

  • One cell, switch, and two identical bulbs wired in parallel.

  • Ammeter positions: A1 (before the split), A2 (branch 1), A3 (branch 2), A4 (after the junction where currents recombine).

Observed readings:

  • A_1 = 0.30\;\text{A}

  • A_2 = 0.15\;\text{A}

  • A_3 = 0.30\;\text{A} (Note: the transcript states A3 rather than A2 again; treat as branch 2 reading)

  • A_4 = 0.15\;\text{A}

Interpretation:

  • The supply current (A_1) splits roughly in half through each identical bulb: 0.30\,\text{A} \Rightarrow 0.15\,\text{A} per branch.

  • After the branches re-join at A_4, the currents converge, returning to 0.30\,\text{A}—equal to the source current.

  • Removing one bulb while the circuit is ON did not extinguish the other bulb, confirming the independence of branches in a parallel circuit.

Explanation of Results

  • Why A2 = A3: Identical resistances create equal branch currents when fed by the same voltage.

  • Why A4 equals A1: Charge is conserved; all charge that leaves the battery must return.

Voltage (Potential Difference)

  • Voltage (Potential Difference, V) measures a cell’s ability to drive current; unit is \text{Volts (V)}.

  • The value printed on a cell represents the energy difference between its positive and negative terminals.

Measuring Voltage

  • Use a voltmeter connected in parallel with the component (e.g., lamp) whose V you wish to measure.

Voltage Rules

  1. Series circuit: Total voltage equals the sum of individual drops:
    V = V1 + V2 + \dots + V_n

  2. Parallel circuit: Voltage across each branch equals the supply voltage:
    V = V1 = V2 = \dots = V_n

Numerical Illustration

  • Series example: A 12\,\text{V} supply driving three identical lamps:
    \begin{aligned}
    V &= 12\,\text{V} \
    V1 &= V2 = V_3 = 4\,\text{V} \
    12\,\text{V} &= 4\,\text{V} + 4\,\text{V} + 4\,\text{V}
    \end{aligned}

  • Parallel example: Same 12\,\text{V} source feeding three lamps in separate branches:
    V = V1 = V2 = V_3 = 12\,\text{V}

Practical & Real-World Relevance

  • Household wiring: Parallel configuration ensures each appliance receives full mains voltage and operates independently. A fault in one branch does not disrupt others—critical for safety and convenience.

  • Troubleshooting tip: If total branch currents do not sum to the supply current, look for connection errors, differing resistances, or a faulty ammeter.

Ethical / Safety Considerations

  • Always disconnect power or use low-voltage supplies when assembling student circuits.

  • Mis-wiring an ammeter across a supply can cause excessive current and potential burns or fire.

Links to Foundational Principles

  • Conservation of Charge: Underpins the current-splitting and recombining behavior observed.

  • Ohm’s Law (preview for future study): V = IR explains why equal voltage across equal resistors yields equal current.

Self-Check / Plenary Question

"Are your home lights connected in series or parallel? Explain why."
Model answer: Parallel, so each light receives full supply voltage and operates independently; a failure in one bulb does not interrupt current to others.