Understanding Ohm's Law

Ohm's Law is the fundamental relationship in electrical circuits, discovered by German physicist Georg Ohm in 1827. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.

The Basic Formula: V = IR

The formula V = IR relates voltage (V in volts), current (I in amperes), and resistance (R in ohms). This can be rearranged to solve for any variable: I = V/R or R = V/I.

Power Calculations

Electrical power can be calculated using several equivalent formulas:

• P = VI - Power equals voltage times current
• P = I²R - Power equals current squared times resistance
• P = V²/R - Power equals voltage squared divided by resistance

The Ohm's Law Wheel

The Ohm's Law wheel (or pie chart) is a useful tool that shows all twelve formulas relating V, I, R, and P. Given any two values, you can calculate the other two using the appropriate formulas.

Practical Applications

• LED circuits: Calculate resistor values for current limiting
• Power consumption: Determine energy usage of devices
• Wire sizing: Calculate voltage drop over wire lengths
• Circuit design: Design voltage dividers and loads

Unit Prefixes

Electrical values often use metric prefixes for convenience:

• Milliamps (mA): 1 mA = 0.001 A
• Microamps (μA): 1 μA = 0.000001 A
• Kilohms (kΩ): 1 kΩ = 1,000 Ω
• Megohms (MΩ): 1 MΩ = 1,000,000 Ω
• Milliwatts (mW): 1 mW = 0.001 W
• Kilowatts (kW): 1 kW = 1,000 W

Common Examples

• USB charger: 5V, 2A = 10W power output
• LED with 220Ω resistor at 5V: ~23mA current
• 100W light bulb at 120V: ~0.83A, 144Ω