Ohm's Law: Voltage, Current, and Resistance

Definition

Ohm's law relates voltage across a resistor to the current through it:

• V — voltage (volts, V)
• I — current (amperes, A)
• R — resistance (ohms, Ω)

Rearrangements: $I = V/R$ and $R = V/I$. Ohm's law applies to ohmic devices over the range where $R$ is constant.

Circuit Intuition

A useful hydraulic analogy (qualitative only):

• Voltage ≈ pressure difference driving flow
• Current ≈ flow rate
• Resistance ≈ restriction to flow

Higher voltage increases current for fixed $R$; higher $R$ reduces current for fixed $V$.

Electric Power

Power delivered to a resistor:

Combined with Ohm's law:

1. $P = I^2 R$ (useful for $I^2R$ heating)
2. $P = V^2 / R$

Worked Example

A 12 V supply drives a 100 Ω resistor.

• $I = 12/100 = 0.12$ A
• $P = (0.12)^2 × 100 = 1.44$ W

Calculator

Enter any two of $V$, $I$, $R$ to find the third and power:

Ohm's Law Calculator · Electrical Formulas Cheatsheet