Ohm's Law is the fundamental relationship between Voltage (V), Current (I), and Resistance (R) in an electrical circuit. Simply put: Voltage = Current × Resistance, or V = I × R.

The Ohm's Law Triangle (Memory Tool)

Quick way to remember the formula:

- Need Current? Cover I → I = V ÷ R

- Need Resistance? Cover R → R = V ÷ I

Ohmic devices (resistors, wires) — Current increases linearly with voltage. They follow Ohm's Law.

Non-ohmic devices (diodes, LEDs, transistors) — Current does NOT increase linearly with voltage. They do NOT follow Ohm's Law.

Power is the rate at which electrical energy is transferred or converted.

The Power Formulas:

Why Power Ratings Matter:

Every component has a power rating — the maximum power it can handle without overheating or failing.

• 1/4W resistor — handles up to 0.25 watts

• 1W LED — handles up to 1 watt

• 100W light bulb — handles up to 100 watts

Power is the rate of energy transfer. Energy is the total amount of work done.

Energy Formulas:

• E = P × t (Energy = Power × Time)

• E = V × I × t (Alternative form)

• kWh (Kilowatt-hour — utility billing unit)

Practical Example:

A 100W light bulb running for 10 hours:

Energy = 100W × 10h = 1,000 Wh = 1 kWh

Unit Conversions:

• Horsepower to Watts: 1 hp = 746 W

• Milliwatts to Watts: 1 mW = 0.001 W

• Kilowatts to Watts: 1 kW = 1,000 W

Example 1: LED Current-Limiting Resistor

The problem: An LED typically needs 20mA. Connect directly to 5V, it burns out.

Calculation:

I = 20mA = 0.02A
V_source = 5V
V_LED = 2V
V_R = 5V - 2V = 3V
R = 3V ÷ 0.02A = 150Ω

Example 2: Resistor Power Rating

Given: V_source = 12V, LED Vf = 2V, I = 20mA

V_R = 12V - 2V = 10V
R = 10V ÷ 0.02A = 500Ω
P = I² × R = 0.2W

Use a 1/2W resistor (2x safety margin)

Example 3: Voltage Divider

V_out = V_in × (R2 ÷ (R1 + R2))

Example: R1=1K, R2=2K → V_out = 3.33V

Example 4: Battery Life Estimation

Battery: 2000mAh, Circuit current: 100mA

Run time = 2000mAh ÷ 100mA = 20 hours

1. Forgetting LED forward voltage

2. Confusing Volts with Amps

3. Ignoring power ratings

4. Wrong units (not converting mA to A)

5. Assuming ideal components

6. Ignoring temperature effects

7. Applying Ohm's Law to LEDs without forward voltage

Q: Does Ohm's Law work for AC circuits?

A: For pure resistive loads, yes. But for inductors and capacitors, impedance (Z) replaces resistance. V = I × Z.

Q: What's the difference between Ohms and Volts?

A: Volts measures the "push," Ohms measures the "resistance to that push."

Q: Can resistance be zero?

A: Yes — in superconductors at cryogenic temperatures.

Q: Why do resistors get hot?

A: Because electrical energy is converted to heat. P = I² × R

Master V = I × R, add Power (P), and you can debug circuits, calculate component values, understand power consumption, and design battery-powered systems.

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