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VCE Systems Engineering (Year 11) · Unit 1 — Electrotechnological Systems Design · 13m read

Electrical Fundamentals — Voltage, Current, Resistance and Ohm’s Law

Every electrotechnological system relies on a few core relationships. Master these calculations now — they appear in every unit and in the Year 12 examination.

Learning objective: Use Ohm’s law, the power equation and series and parallel rules to calculate values in DC circuits.

By the end of this lesson you can:

  • I can define voltage, current and resistance with units.
  • I can apply Ohm’s law and the power formula.
  • I can calculate total resistance in series and parallel.

Water in pipes. Voltage is like water pressure, current is the flow rate, and resistance is a narrow section of pipe that restricts flow.

Quantities

VOLTAGE (V, volts) — electrical "push" or potential difference.
CURRENT (I, amperes) — rate of flow of charge. 1 mA = 0.001 A.
RESISTANCE (R, ohms Ω) — opposition to current. 1 kΩ = 1000 Ω.
POWER (P, watts) — rate of energy use.

SERIES: one path; the same current everywhere; voltages add up to the supply.
PARALLEL: several paths; the same voltage across each branch; currents add up.

Circuit formulas

  • Ohm’s law: V = IR
  • Power: P = VI = I^2R = \dfrac{V^2}{R}
  • Series resistance: R_T = R_1 + R_2 + \dots
  • Parallel resistance: \dfrac{1}{R_T} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dots
  • Voltage divider: V_{out} = V_{in} \times \dfrac{R_2}{R_1 + R_2}

Worked example (Medium)

A 100 Ω and a 200 Ω resistor are in series across a 6 V supply. Find the current, the voltage across the 200 Ω resistor and the total power.

  1. Total resistance. Series resistances add. R_T = 100 + 200 = 300\ \Omega
  2. Current. Ohm’s law. I = \dfrac{V}{R_T} = \dfrac{6}{300} = 0.02\ \text{A} = 20\ \text{mA}
  3. Voltage across 200 Ω. Same current flows through each resistor. V = IR = 0.02 \times 200 = 4\ \text{V}
  4. Power. Total power from the supply. P = VI = 6 \times 0.02 = 0.12\ \text{W}

Answer: I = 20 mA; 4 V across the 200 Ω resistor; total power 0.12 W.

Common mistakes

  • Forgetting to convert mA to A 20 mA is 0.02 A, not 20 A. Fix: Convert to base units before calculating.
  • Adding parallel resistors directly Parallel total is always less than the smallest resistor. Fix: Use the reciprocal formula.

Practice questions

  1. A 12 V supply drives 0.5 A through a resistor. What is its resistance?

    1. 6 Ω
    2. 24 Ω
    3. 12.5 Ω
    4. 0.04 Ω

    (1 marks)

  2. Two 100 Ω resistors are connected in parallel. Find the total resistance.

    (2 marks)

  3. A 9 V motor draws 300 mA. Calculate its power.

    (2 marks)

Summary

  • V = IR and P = VI.
  • Series: resistances add; same current.
  • Parallel: reciprocals add; same voltage.
  • Convert mA and kΩ to base units.

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