Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

What is the relationship between voltage, current, resistance and power?

The four fundamental relationships are: V = I × R (Ohm's Law), P = V × I (power), P = I² × R (power from current and resistance), and P = V² / R (power from voltage and resistance). Knowing any two values allows you to calculate the other two.

How do series and parallel resistances differ?

In series, resistances add directly: R_total = R1 + R2 + R3. In parallel, reciprocals add: 1/R_total = 1/R1 + 1/R2 + 1/R3. Series increases total resistance, while parallel decreases it. For two parallel resistors: R_total = (R1 × R2) / (R1 + R2).

What is electrical conductance?

Conductance (G) is the reciprocal of resistance: G = 1/R. It is measured in siemens (S). A higher conductance means the material conducts electricity more easily. It is useful when analyzing parallel circuits, where conductances add directly.

Does Ohm's Law apply to AC circuits?

Ohm's Law applies to AC circuits when using impedance (Z) instead of resistance (R). In AC, V = I × Z, where Z includes resistance, inductive reactance (XL = 2πfL), and capacitive reactance (XC = 1/2πfC). For purely resistive AC loads, V = IR still applies directly.

Free Engineering Tool

Ohm’s Law Calculator

Enter any two known electrical parameters — Voltage, Current, Resistance, or Power — and instantly calculate all four, plus conductance and energy per hour.

V = I × R P = V × I Ohm’s Wheel

Results

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Current

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Resistance

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Power

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Conductance (G = 1/R)

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Energy per Hour

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Ohm’s Law

Ohm’s Law defines the fundamental relationship between voltage, current, and resistance in an electrical circuit:

Power Relationships

Electrical power can be calculated from any combination of two known values:

The Ohm’s Law Wheel

All twelve formulas derived from V = IR and P = VI:

Find	From V & I	From V & R	From I & R	From P
V	—	—	V = I × R	V = P / I or √(P × R)
I	—	I = V / R	—	I = P / V or √(P / R)
R	R = V / I	—	—	R = V² / P or P / I²
P	P = V × I	P = V² / R	P = I² × R	—

Practical Example

Example — LED Resistor Calculation

Given: Supply = 12 V, LED forward voltage = 2 V, desired current = 20 mA

Voltage across resistor: 12 − 2 = 10 V

R = V / I = 10 / 0.020 = 500 Ω

P = V × I = 10 × 0.020 = 0.2 W (use ¼W resistor minimum)

Tip: Conductance (G) is the reciprocal of resistance: G = 1/R, measured in siemens (S). It’s useful in parallel circuit analysis where conductances add directly: G_total = G₁ + G₂ + G₃.