TL;DR
Ohm's Law: Fundamental electrical relationship: voltage (E) = current (I) × resistance (R).
Ohm's Law
Definition
Fundamental electrical relationship: voltage (E) = current (I) × resistance (R).
Overview
Ohm's law establishes the linear relationship between voltage, current, and resistance. Current is directly proportional to voltage and inversely proportional to resistance.
Formulas
- E = I × R — voltage
- I = E / R — current
- R = E / I — resistance
- P = E × I — power (Watt's Law)
- P = I² × R — power from current
- P = E² / R — power from voltage
Applications
Used daily for wire sizing, voltage drop, overcurrent protection, and troubleshooting.
Voltage drop: VD = I × (2 × R × L / 1000). NEC recommends 3% branch, 5% total.
Series: Rt = R1+R2+R3, same current throughout. Parallel: 1/Rt = 1/R1+1/R2+1/R3, same voltage across branches.
Why It Matters
The foundation of all electrical calculations tested on IBEW, JE, and ME exams.
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