ohmic resistance
That portion of the apparent resistance or impedance of a circuit which depends only upon the dimensions of the conductors, their temperature, and the materials of which they are composed.
ohmic resistance: the pure electrical friction in a conductor
Ohmic resistance is the opposition to current flow that arises from the material properties and geometry of a conductor itself, independent of frequency or any external fields. It obeys Ohm's law directly: the voltage drop across a resistor equals the current multiplied by its resistance value, measured in ohms. This is the baseline resistance you encounter in DC circuits and the resistive component that appears in AC circuits at all frequencies.
The resistance of any conductor depends on three factors: resistivity (a material property, typically given in ohm-meters), the length of the conductor, and its cross-sectional area. Resistance equals resistivity times length divided by area. Copper at 20 degrees Celsius has a resistivity around 1.7 times 10 to the negative eighth ohm-meters; aluminium around 2.8 times 10 to the negative eighth. Temperature changes resistivity: resistance increases by roughly 0.3 to 0.6 percent per degree Celsius for common metals, which is why large cables and high-power resistors must account for thermal effects.
In AC circuits, ohmic resistance differs from other forms of opposition. Inductive reactance and capacitive reactance also impede current, but they store and release energy; ohmic resistance dissipates it as heat. The total impedance of a circuit combines these, but only the ohmic resistance contributes to real power loss. This distinction matters when calculating power factor, efficiency, and thermal loading in motors, transformers, and transmission lines.
Where ohmic resistance matters most
Cable sizing in electrical distribution is the most direct application. An undersized conductor carrying high current will have excessive ohmic resistance, causing voltage drop and dangerous heating. A 50 millimetre squared copper cable at 20 degrees Celsius has roughly 0.00036 ohms per metre; reducing it to 10 millimetre squared increases that to 0.0018 ohms per metre. In long runs, that difference computes to significant losses. Similarly, contact resistance at terminals and joints can contribute unexpectedly large ohmic losses if corrosion or poor surface contact occurs.
The term ohmic exists specifically to distinguish this pure resistive component from impedance in general, which includes reactive elements. In older literature, you may encounter true resistance used the same way. Understanding ohmic resistance alone is necessary but not sufficient for AC circuit analysis; you must also account for skin effect (current crowding at high frequencies, which increases effective resistance) and parasitic inductance, neither of which is ohmic resistance but both of which affect the real impedance seen in practice.