How to Solve: Ohm’s Law

How to Solve: Ohm’s Law

(For Students Who Want to Ace Their Exam & Teachers Who Need a Ready-to-Record Script)

Introduction

"If you can’t solve Ohm’s Law problems, you can’t fix a blown fuse, design a phone charger, or even pass your circuit exam—let’s fix that in 10 minutes."

What You Need To Know First

Before diving into Ohm’s Law, you must understand: 1. Electric current (I): The flow of charge (measured in amperes, A). 2. Voltage (V): The "push" that makes charges move (measured in volts, V). 3. Resistance (R): How much a material opposes current (measured in ohms, Ω).

If these terms confuse you, pause here and review them first.

Key Vocabulary

Formulas To Know

1. Ohm’s Law (MEMORISE THIS)

Formula: V = I × R - V = Voltage (volts, V) - I = Current (amperes, A) - R = Resistance (ohms, Ω)

Why it matters: This is the only formula you need for basic circuit problems.

2. Rearranged Forms (MEMORISE THESE TOO)

• To find current: I = V / R
• To find resistance: R = V / I

Exam tip: If you forget, cover the variable you want in the triangle below and the formula appears!

      V

    -----

   I  ×  R

Step-by-Step Method

Follow these exact steps for every Ohm’s Law problem:

1. Read the question carefully. Underline what’s given and what’s asked.
2. Draw a simple circuit diagram (even if it’s not required). Label all known values.
3. Write down Ohm’s Law: V = I × R.
4. Rearrange the formula to solve for the unknown variable.
5. Plug in the numbers with units (V, A, Ω).
6. Calculate and box your answer with the correct unit.
7. Check if it makes sense:
8. Current should be smaller if resistance is higher (for the same voltage).
9. Voltage should be higher if current or resistance increases.

Worked Example Using the Steps

Question: A 12 V battery is connected to a resistor. If the current is 3 A, what is the resistance?

1. Given: V = 12 V, I = 3 A. Find: R.
2. Diagram:
   [Battery 12 V] ----[Resistor R]----
3. Ohm’s Law: V = I × R
4. Rearrange: R = V / I
5. Plug in: R = 12 V / 3 A = 4 Ω
6. Answer: R = 4 Ω
7. Check: Higher resistance (4 Ω) with 12 V gives 3 A—makes sense!

Worked Examples

Example 1 – Basic

Question: A toaster has a resistance of 20 Ω. If it’s plugged into a 120 V outlet, what current flows?

1. Given: R = 20 Ω, V = 120 V. Find: I.
2. Diagram:
   [Outlet 120 V] ----[Toaster 20 Ω]----
3. Ohm’s Law: V = I × R
4. Rearrange: I = V / R
5. Plug in: I = 120 V / 20 Ω = 6 A
6. Answer: I = 6 A
7. What we did: Used Ohm’s Law to find current when voltage and resistance are known.

Example 2 – Medium

Question: A circuit has a current of 0.5 A and a resistance of 100 Ω. What voltage is supplied?

1. Given: I = 0.5 A, R = 100 Ω. Find: V.
2. Diagram:
   [Battery V] ----[Resistor 100 Ω]----
3. Ohm’s Law: V = I × R
4. No rearrangement needed.
5. Plug in: V = 0.5 A × 100 Ω = 50 V
6. Answer: V = 50 V
7. What we did: Multiplied current and resistance to find voltage. Note: Units matter—amperes × ohms = volts!

Example 3 – Exam Style

Question: A student measures the current through a resistor as 250 mA when connected to a 6 V battery. What is the resistance? Give your answer in ohms (Ω).

1. Given: V = 6 V, I = 250 mA. Find: R.
2. Trick: Current is in milliamperes (mA), not amperes!
3. Convert units: 250 mA = 0.25 A (because 1 A = 1000 mA).
4. Diagram:
   [Battery 6 V] ----[Resistor R]----
5. Ohm’s Law: V = I × R
6. Rearrange: R = V / I
7. Plug in: R = 6 V / 0.25 A = 24 Ω
8. Answer: R = 24 Ω
9. What we did: Converted mA to A first, then applied Ohm’s Law. Exam trap: Always check units!

Common Mistakes

Exam Traps

1-Minute Recap

"Okay, exam tomorrow? Here’s Ohm’s Law in 60 seconds: 1. Memorise V = I × R—that’s the golden rule. 2. Rearrange it to find what you need: I = V / R or R = V / I. 3. Check units—volts, amperes, ohms. Convert mA to A and kΩ to Ω. 4. Draw the circuit—even a quick sketch helps. 5. Plug in the numbers, calculate, and box your answer. 6. Double-check: Does it make sense? Higher resistance = lower current (for the same voltage).

That’s it. You’ve got this. Now go ace that exam!