In general, AC resistance is higher than DC resistance.
Understanding Resistance in AC vs. DC Circuits
Resistance is a fundamental property of materials that opposes the flow of electric current. While the inherent ohmic resistance of a conductor (its DC resistance) remains constant regardless of the current type, alternating current (AC) experiences additional resistive effects that are not present with direct current (DC). This makes the effective resistance encountered by AC higher than that by DC for the same conductor.
The Dominant Factor: Skin Effect
The primary reason for higher AC resistance is the skin effect.
- What is Skin Effect? The skin effect describes the tendency of an alternating electric current to distribute itself within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths. In simpler terms, AC tends to flow through the outer layer or "skin" of the wire, rather than uniformly across its entire cross-section.
- Why it Happens in AC: The skin effect is a direct consequence of the changing magnetic fields produced by alternating current. As the AC current fluctuates, it creates a continually changing magnetic field that, in turn, induces eddy currents within the conductor. These induced eddy currents oppose the main current flow, especially towards the center of the conductor, pushing the current towards the surface. The higher the frequency of the AC, the more pronounced this effect becomes.
- Absence in DC: Direct current (DC) has a frequency of zero. Because there is no changing magnetic field, no significant eddy currents are induced within the conductor. Therefore, in DC circuits, the current distributes uniformly across the entire cross-sectional area of the wire, and there is no skin effect.
- Impact on Resistance: Since AC is effectively confined to a smaller cross-sectional area (the "skin" of the conductor) compared to DC which uses the full area, the effective resistance to AC current flow increases. A smaller effective area means higher resistance for a given conductor length and material.
Other Contributing Factors to AC Resistance
While the skin effect is the most significant, other phenomena can also contribute to increased AC resistance, especially at higher frequencies or in specific configurations:
- Proximity Effect: When multiple conductors carrying AC are close to each other, their magnetic fields interact, further distorting the current distribution within each conductor and increasing their effective resistance.
- Eddy Current Losses (in nearby materials): Changing magnetic fields from AC can induce eddy currents in surrounding metallic structures (like conduit or transformer cores), leading to energy dissipation and an apparent increase in the circuit's overall resistance.
- Dielectric Losses: For conductors with insulation (dielectric material), the alternating electric field can cause energy dissipation within the dielectric itself, especially at very high frequencies, contributing to effective resistance.
Comparing AC and DC Resistance
Here's a summary of the key differences:
| Feature | DC Resistance | AC Resistance (Effective Resistance) |
|---|---|---|
| Current Flow | Uniformly distributed across conductor's area. | Concentrated near the surface (skin effect). |
| Frequency | Zero (constant current). | Non-zero (current direction and magnitude change periodically). |
| Primary Cause | Ohmic resistance of the material (resistivity). | Ohmic resistance + Skin effect + Proximity effect + other losses. |
| Value | Generally lower (base ohmic resistance). | Generally higher than DC resistance for the same conductor. |
| Frequency Dep. | Independent of frequency. | Increases with increasing frequency. |
Practical Implications and Solutions
The difference between AC and DC resistance has important practical implications:
- Transmission Lines: At high voltages and frequencies, power transmission lines experience significant resistance increases due to the skin effect. Large conductors are often made as hollow tubes or bundles of thinner strands (like Litz wire) to maximize surface area and mitigate the skin effect.
- High-Frequency Circuits: In radio frequency (RF) and microwave circuits, designers must carefully consider the skin effect. Conductors are sometimes silver-plated to improve conductivity on the surface where the current flows, or specialized conductor geometries are used.
- Measurement: When measuring the resistance of a component, it's crucial to specify whether it's AC or DC resistance. A standard multimeter typically measures DC resistance.
In conclusion, while the material's inherent ohmic resistance is constant, the presence of frequency in alternating current introduces phenomena like the skin effect, which restrict current flow to a smaller effective area, thereby increasing the overall effective resistance experienced by AC compared to DC.
