Does Ohm's Law apply to all electronic components?

No — Ohm's Law only applies to ohmic (linear) resistors at constant temperature. It does not apply to semiconductors (diodes conduct only above ~0.6V threshold), light bulbs (resistance increases with temperature), thermistors (resistance varies with temperature), or reactive components like capacitors and inductors in AC circuits.

What is the difference between resistance and impedance?

Resistance (R) is the opposition to DC current, measured in ohms, independent of frequency. Impedance (Z) is the total opposition to AC current, including resistance and reactance (from capacitors and inductors). Impedance varies with frequency. For pure DC circuits, impedance equals resistance; for AC circuits, Z = √(R² + X²).

How do I choose the right resistor for an LED?

Use the formula R = (Supply voltage − LED forward voltage) ÷ LED current. For a standard red LED (forward voltage ~2V, current 20mA) on a 9V supply: R = (9 − 2) ÷ 0.020 = 350Ω. Use the nearest standard value — 360Ω or 390Ω. Also calculate power: P = I² × R = 0.02² × 390 = 156mW — use a ¼W resistor.

What is a short circuit?

A short circuit occurs when a very low resistance path forms between two points in a circuit, typically due to a wire touching an unintended connection. By Ohm's Law, near-zero resistance means near-infinite current (I = V/R). This causes extreme heat, potential fire, and component failure. Fuses are rated to blow before this damage occurs.

Ohm's Law Calculator: Voltage, Current & Resistance Explained

Ohm's Law is one of the most fundamental principles in electronics and electrical engineering. Whether you're designing a circuit, troubleshooting a wiring problem, or working through a physics exam, understanding the relationship between voltage, current, and resistance is essential. This guide explains what each quantity means, how to apply the formula in every direction, and walks through practical examples from real electronics.

The Three Quantities

Voltage (V) — measured in Volts (V): The electrical "pressure" that drives current through a circuit. Like water pressure in a pipe. A 9V battery, a 230V mains socket, a 5V USB charger — all express the potential difference driving current.
Current (I) — measured in Amperes/Amps (A): The flow of electrical charge. Like the rate of water flow. A typical LED draws 20mA; a household kettle draws about 10A.
Resistance (R) — measured in Ohms (Ω): The opposition to current flow. A resistor slows current down, like a narrow section of pipe. Measured in Ohms, kilohms (kΩ = 1,000 Ω), or megaohms (MΩ = 1,000,000 Ω).

Ohm's Law

V = I × R (Voltage = Current × Resistance)

I = V ÷ R (Current = Voltage ÷ Resistance)

R = V ÷ I (Resistance = Voltage ÷ Current)

Ohm's Law states that in a linear (ohmic) conductor, voltage is directly proportional to current, with resistance as the constant of proportionality. The formula triangle: write V on top, I and R on the bottom. Cover what you want to find — the remaining two show whether to multiply or divide.

Worked Examples

Example 1: Finding Current (LED circuit)

You connect a 9V battery to a 470Ω resistor in series with an LED (the LED has negligible resistance for this approximation). What current flows?

I = V ÷ R = 9 ÷ 470 = 0.01915 A = 19.15 mA

This is slightly under 20 mA — the typical LED forward current. The 470Ω resistor is a standard choice for driving LEDs from a 9V supply.

Example 2: Finding Voltage (motor circuit)

A DC motor with 15Ω coil resistance draws 0.8 A. What voltage is being applied?

V = I × R = 0.8 × 15 = 12 V

This matches a typical 12V car battery — consistent for a small automotive motor.

Example 3: Finding Resistance (household wiring)

Your 230V mains socket powers a lamp that draws 0.43 A. What is the lamp's resistance?

R = V ÷ I = 230 ÷ 0.43 = 535 Ω

This is a typical filament lamp resistance when hot (cold resistance is much lower — tungsten's resistance increases with temperature).

The Power Formula (Watt's Law)

Ohm's Law extends to power calculations. Electrical power (in Watts) is:

P = V × I (Power = Voltage × Current)

P = I² × R (using substitution)

P = V² ÷ R (using substitution)

Example: What power does our LED circuit (9V, 19.15 mA) dissipate?
P = V × I = 9 × 0.01915 = 0.172 W = 172 mW

This is why resistors must be rated correctly — a ¼W (250 mW) resistor is fine for this application; a 1/8W would overheat.

Series and Parallel Circuits

Series (resistors in a line):

Total resistance = R₁ + R₂ + R₃...
Current is the same everywhere in the series circuit.

Parallel (resistors on branches):

1/R_total = 1/R₁ + 1/R₂ + 1/R₃...
For two equal resistors: R_total = R ÷ 2
Voltage is the same across each parallel branch.

Limitations of Ohm's Law

Ohm's Law applies to ohmic (linear) conductors at constant temperature. It doesn't apply directly to:

Semiconductors (diodes, transistors): These have non-linear V/I relationships. A silicon diode doesn't conduct at all below ~0.6V, then conducts dramatically above it.
Inductors and capacitors: In AC circuits, these introduce reactance (frequency-dependent resistance). You need complex impedance (Z) rather than simple resistance (R).
Temperature-sensitive components: Thermistors and light bulbs change resistance with temperature, so their current/voltage relationship isn't linear.

Practical Safety: Fuse Ratings

Fuses are rated in amps to protect wiring from overcurrent. To choose a fuse for a 230V appliance rated at 700W:

Current = P ÷ V = 700 ÷ 230 = 3.04 A

You'd choose a 3A or 5A fuse (the next standard fuse rating above the calculated current). Never fit an oversized fuse — it defeats the purpose of overcurrent protection.

Summary

Ohm's Law: V = IR. Use it in three directions — find voltage, current, or resistance given the other two. Extend with P = VI for power calculations. Works for ohmic conductors but not semiconductors or reactive components. Our Ohm's Law calculator solves any combination instantly and calculates power at the same time.