Electrical Foundations

Module 1 / 8
Concept20 min· Path total 3 h 40 min

Voltage, current, resistance, and Ohm's law

ORIENT

What this module is for

Predict how current changes when voltage or resistance changes, and read a simple series circuit.

Voltage is a difference in electrical potential between two points — the push. Current is the rate of charge flow past a point — the movement. Resistance is how strongly a material opposes that flow. None of the three exists alone: a voltage across a resistance produces a current, and that relationship is Ohm's law, V = I × R.

Two consequences matter for everything later. First, current is set by the circuit, not by the source alone: a 120 V source across 12 Ω pushes 10 A, and across 1200 Ω pushes 0.1 A. Second, any current through any resistance produces a voltage drop and heat — power dissipated as P = I² × R. Conductor sizing, voltage drop, and thermal limits are all downstream of that single fact.

In series, current is common and voltages add. In parallel, voltage is common and currents add. Most real distribution problems reduce to recognising which of those two situations you are looking at.

PREDICT

Commit to an answer first

Predicting before you experiment is what makes the result stick. There is no penalty for being wrong here.

If the load current doubles to 40 A on the same conductors, what happens to the power lost in the wire?
EXPLAIN

Voltage drop on a small feeder run

GIVEN

  • Load current: 20 A
  • Conductor loop resistance for the run: 0.25 Ω
  • Source voltage: 120 V

  1. 1. Drop across the conductors

    V = I × R = 20 A × 0.25 Ω = 5.0 V

  2. 2. Voltage at the load

    120 V − 5.0 V = 115 V

  3. 3. Percent drop

    5.0 V ÷ 120 V = 4.2 %

  4. 4. Heat in the conductors

    P = I² × R = 20² × 0.25 = 100 W lost in the run

The load sees 115 V, not 120 V, and 100 W is being spent heating the wire. Both effects grow with the square of current for power and linearly with current for drop.

PROFESSIONAL DISCLAIMER

For education, training, and preliminary support only. Verify results against applicable codes, standards, manufacturer data, project specifications, AHJ requirements, and qualified engineering judgment.

CHECK

Quick retrieval check

A 240 V source feeds a 24 Ω heater. What current flows?
Two resistors are in series. Which quantity is the same in both?
APPLY

Put it in your own words

Describe one circuit you work with (or expect to) and name what sets the current in it: the source, the load impedance, or the conductor. Explain your answer in a sentence.

Finish this module

Still outstanding before this module can be marked complete:

  • Prediction recorded
  • Knowledge check submitted
  • Reflection confirmed

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