Skip to content
Exam Cat

Study guides & original practice

Ohm’s law practice problems

For an ohmic component under stable conditions, voltage, current and resistance satisfy V = IR. Decide which quantity is unknown before rearranging.

By ExamCat · Original study lesson · Published September 24, 2026

Understand the method

Current is measured in amperes, voltage in volts and resistance in ohms. Milliamperes must be converted when using volts and ohms directly. Many real components do not maintain a constant resistance across all operating conditions.

Worked example, step by step

A 12 V source is connected across a 6 Ω ideal resistor. Find current and power.

  1. Use I = V/R = 12/6 = 2 A.
  2. Calculate power with P = VI = 12 × 2 = 24 W.
  3. Check using I²R: 2² × 6 = 24 W.
  4. Power is the rate of energy transfer, so over 5 s this ideal resistor receives 24 × 5 = 120 J.

A mistake to watch for

Watts and joules are not interchangeable: watts are joules per second. Also distinguish the source voltage from the voltage across one resistor in a multi-component circuit.

Try it yourself: three questions

Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.

1. A 3 Ω resistor carries 0.5 A. Find voltage.

Show answer and explanation

1.5 V, using V = IR.

2. A resistor has 10 V across it and carries 20 mA. Find resistance.

Show answer and explanation

20 mA = 0.020 A, so R = 10/0.020 = 500 Ω.

3. At fixed resistance, voltage doubles. What happens to power?

Show answer and explanation

Power quadruples because P = V²/R. Current doubles too; both factors in VI increase.

Build your next practice session

Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.

Use the free flashcard maker for your own question-and-answer pairs, or bring your course material into PDF-to-flashcards practice.

Further reading

OpenStax Physics provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.

Related lessons

Browse this subject in the Blog ↗

Practice your own class material

Generate flashcards, notes and quizzes from your lecture notes in Exam Cat.

Try 7 days free ↗

Yearly plan: 7 days free, then $39.99/year. Cancel before the trial ends to avoid a charge.

APUSH study guide · Biology study guide · APUSH practice · All study tools