Ohmsches Gesetz

What is Ohm's Law?

Ohm's Law establishes the fundamental relationship between the three basic electrical quantities: voltage (volts), current (amps), and resistance (ohms). Formulated by Georg Simon Ohm in 1827, it's the foundation of all electrical and electronic calculations. With it you can size cables, calculate power consumption, design circuits, and diagnose electrical faults.

Ohm's Law formulas

The relationship is: V = I × R

The three rearranged forms:

• Voltage: V = I × R (volts = amps × ohms)
• Current: I = V / R (amps = volts / ohms)
• Resistance: R = V / I (ohms = volts / amps)

And the related electrical power formula: P = V × I = I² × R = V² / R

Step-by-step example

A 60 W bulb is connected to 230 V. What current does it draw and what is its resistance?

1. Current: I = P / V = 60 / 230 = 0.261 A
2. Resistance: R = V / I = 230 / 0.261 = 881 Ω
3. Verification: P = I² × R = 0.261² × 881 ≈ 60 W ✓

Practical applications of Ohm's Law

• Cable sizing: Calculate the minimum cable cross-section so it doesn't overheat at maximum expected current.
• Circuit design: Choose correct resistors to control current through LEDs and other components.
• Electrical protection: Verify that fuses or circuit breakers are correctly rated for the circuit.
• Fault diagnosis: Measure voltage and current with a multimeter and calculate whether resistance is as expected.
• Solar PV installations: Calculate wiring losses between panels and batteries.

Limits of Ohm's Law

Ohm's Law is valid for ohmic components: resistors, cables, heaters. It doesn't apply directly to:

• Diodes and LEDs: Their resistance varies with voltage.
• Transistors: They control current in a non-linear way.
• Capacitors and inductors: Their behavior depends on frequency (impedance).