Power Stations WORLDWIDETHE GLOBAL EDITION 01
LESSON 1 OF 6 · ORIGINAL EDUCATIONAL EXAMPLE

Voltage, current and resistance

Distinguish volts from amperes, apply Ohm’s law and calculate power in a hypothetical resistor.

Learning goal: Identify voltage across an element and current through it, then use V = IR with consistent units.

Prepared by Power Stations Worldwide. Sources checked on 30 September 2026; not independently engineering peer reviewed.

The essentials

Across versus through

Voltage is a potential difference between two points, measured in volts (V). Current is the rate of charge flow through a branch, measured in amperes (A). A voltage value needs two reference points; a current value needs a branch and direction.

A resistor is a specific model

For an ideal resistor, V = IR. This is not a complete model of a motor, capacitor or converter. Power absorbed by a DC resistor is P = VI = I²R, in watts (W). Resistance is measured in ohms (Ω).

Keep AC and DC distinct

The calculation here is DC. For a sinusoidal AC resistor, the same power calculation works with RMS voltage and RMS current. Peak voltage must not be substituted for RMS voltage in that calculation.

Voltage across, current through

Voltage across, current throughA conceptual closed circuit has a 24 V DC source and a 12 ohm resistor. A current arrow through the top branch is labelled 2 A; plus and minus marks define 24 V across the resistor.+−I = 2 A24 V DCR = 12 Ω+−24 V
Original circuit abstraction. Connections, protection and equipment construction are omitted; this is not an installation drawing.
HYPOTHETICAL VALUES · NOT PLANT RECORDS

Worked example

A 24 V DC source and a 12 Ω resistor

Given

  • Voltage across resistor V = 24 V DC
  • Resistance R = 12 Ω
  • Ideal source and resistor; negligible connecting-wire resistance

Formula toolbox

  • I = V / R
  • P = VI = I²R = V² / R

Calculate step by step

  1. Current: I = 24 V / 12 Ω = 2 A.
  2. Power: P = 24 V × 2 A = 48 W.
  3. Cross-check: I²R = (2 A)² × 12 Ω = 48 W.

2 A current · 48 W absorbed power

This is an analysis of hypothetical values, not a resistor power-rating recommendation or instructions for connecting a circuit.

Results are rounded only for display. Reproduce the calculations using unrounded intermediate values.

Quick quiz

Choose one answer for each question, then check your answers. This is a practice check, not a qualification. Answers stay in this page only; refreshing or leaving resets them.

1. An ideal 8 Ω resistor has 16 V DC across it. What is the current?
2. Which statement correctly distinguishes voltage and current?

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Show answers and explanations (also works without JavaScript)
  1. 2 A. I = V / R = 16 / 8 = 2 A.
  2. Voltage is across two points; current is through a branch. Voltage is a potential difference in volts; current is charge flow in amperes.

Curriculum connection: MIT 6.061 (Spring 2011). Its readings include 2007 notes; some equations use peak rather than RMS amplitudes. This lesson states its own value conventions and uses original diagrams, numbers and quiz questions. No MIT affiliation or endorsement is claimed.

Textbook references

Suggested technical background for this topic. These books are further reading, not proof that every explanation was checked against every edition. See the full bibliography and review scope.

  1. Fundamentals of Electric Circuits (7th ed.) — Alexander, C. K. & Sadiku, M. N. O., McGraw-Hill, 2021 · ISBN 9781260226409Basic circuit quantities, elements and circuit laws.
  2. Electric Circuits (11th ed.) — Nilsson, J. W. & Riedel, S. A., Pearson, 2019 · ISBN 9780134746968Circuit variables and systematic circuit analysis.