Resistance (Ohm)
Definition
Resistance (symbol Ω, ohm) is the opposition to the flow of electric current in a circuit, measured in ohms. Materials with low resistance (copper, silver, gold) are conductors; materials with high resistance (rubber, glass, air) are insulators. Ohm's Law (V = I × R) defines the relationship between voltage, current, and resistance. In practical wiring, resistance increases with wire length and decreases with larger wire diameter (lower AWG gauge). Excessive resistance in electrical connections causes voltage drop, heat generation, and energy loss — which is why proper wire sizing is critical for safety and efficiency.
Why is Resistance (Ohm) Important?
In electrical engineering and everyday applications, Resistance (Ohm) is a fundamental concept for understanding how electrical systems work. Whether you are an engineer designing circuits, an electrician sizing wires, or a homeowner estimating energy costs, this metric is essential for safety, efficiency, and accurate calculations.
Our electrical conversion calculators help you quickly convert between units and verify calculations, reducing errors and saving time in both professional and DIY electrical work.
What is Electrical Resistance?
Resistance (symbol Ω, ohm) is the opposition to the flow of electric current in a circuit. It determines how much current flows for a given voltage — higher resistance means less current. Resistance converts electrical energy into heat, which is the operating principle behind heaters, toasters, incandescent bulbs, and electric stoves.
Ohm's Law
The fundamental relationship between voltage (V), current (I), and resistance (R):
| Find | Formula | Example |
|---|---|---|
| Voltage | V = I × R | 10A × 12Ω = 120V |
| Current | I = V ÷ R | 120V ÷ 12Ω = 10A |
| Resistance | R = V ÷ I | 120V ÷ 10A = 12Ω |
| Power | P = V × I = I²R = V²/R | 120V × 10A = 1,200W |
Resistance of Common Materials
| Material | Resistivity (Ω·m) | Classification |
|---|---|---|
| Silver | 1.59 × 10⁻⁸ | Best conductor |
| Copper | 1.68 × 10⁻⁸ | Standard wire material |
| Gold | 2.44 × 10⁻⁸ | Corrosion-resistant contacts |
| Aluminum | 2.65 × 10⁻⁸ | Power transmission lines |
| Nichrome | 1.10 × 10⁻⁶ | Heating elements |
| Rubber | ~10¹³ | Insulator |
| Glass | ~10¹⁰ | Insulator |
Wire Resistance by Gauge
| AWG | Resistance (Ω per 1,000 ft of copper) |
|---|---|
| 14 AWG | 2.525 Ω |
| 12 AWG | 1.588 Ω |
| 10 AWG | 0.999 Ω |
| 8 AWG | 0.628 Ω |
| 6 AWG | 0.395 Ω |
Longer wires and thinner gauges have more resistance, causing voltage drop. This is why long runs require upsized wire.