Skin effect and proximity effect - how they change resistance of wires

Summary

This video explores the impact of skin and proximity effects on the resistance of wires. It demonstrates how the resistance of a wire can change without altering its physical properties by varying the frequency of alternating current. The skin effect causes current to concentrate on the wire's surface at higher frequencies, reducing the effective cross-sectional area and increasing resistance. The proximity effect further increases resistance when wires are placed close together. The video explains these effects through experiments and discusses their implications in practical applications like transformers and motors, where high-frequency currents can lead to power losses.

Takeaways

• 😀 The resistance of a wire is not constant and can be changed without altering its length or cross-sectional area.
• 😀 Skin effect occurs when alternating current (AC) induces a magnetic field, causing the current to concentrate on the wire's surface, increasing resistance.
• 😀 The skin depth, which determines how deep the current flows into the wire, is inversely proportional to the frequency of the applied current.
• 😀 At low frequencies, the resistance of a wire is constant as the entire wire can carry current, but at high frequencies, the current only flows on the surface, increasing resistance.
• 😀 At 50 Hz, the skin depth of a copper wire is approximately 1 cm, but it decreases significantly as the frequency increases.
• 😀 A graph of resistance versus frequency shows a log-log relationship, with a slope of 0.5 at high frequencies, which is consistent with the theory of skin effect.
• 😀 The proximity effect occurs when wires are close to each other, causing magnetic fields to influence neighboring wires and reduce the effective cross-sectional area for current conduction.
• 😀 The proximity effect increases the resistance of the wire, and it becomes more significant as the wires are brought closer together.
• 😀 When wires are far apart, only the skin effect is observed, but when wires are close together, both skin and proximity effects contribute to increased resistance.
• 😀 Proximity effect depends on the geometry and arrangement of the wires, and it seems to have a greater impact on resistance than the skin effect in the experiments conducted.

Q & A

What is the skin effect, and how does it impact wire resistance?

The skin effect occurs when alternating current (AC) flows through a wire, causing the current to concentrate near the outer surface of the wire. This happens because the AC generates a magnetic field that induces eddy currents within the wire, effectively reducing the cross-sectional area available for current flow. As a result, the resistance increases, particularly at higher frequencies.

How does the frequency of the applied current affect the skin effect?

The skin effect is frequency-dependent. As the frequency of the applied current increases, the skin depth (the effective thickness of the wire through which most of the current flows) decreases. This means less of the wire is used for current conduction, leading to higher resistance at higher frequencies.

What role does the spring play in the demonstration of skin effect?

The spring in the experiment is used as a conductor to demonstrate the skin effect. When an alternating current is applied through the spring, its resistance changes based on the frequency of the current. The resistance increases as the frequency increases, showcasing the impact of the skin effect.

What happens to the resistance of the spring when it collapses?

When the spring collapses, the resistance increases due to the proximity effect, which occurs when the wires (or coils) are brought closer together. This causes a reduction in the effective cross-sectional area available for current conduction, further increasing resistance.

What is the proximity effect, and how does it differ from the skin effect?

The proximity effect happens when two wires are close together, and the magnetic field generated by the AC current in one wire induces eddy currents in the neighboring wire. This causes the current to be concentrated in areas of the wire closer to each other, further reducing the effective cross-sectional area for current conduction, which increases resistance. Unlike the skin effect, which is due to the current distribution within a single wire, the proximity effect involves interactions between nearby wires.

How do the skin and proximity effects contribute to the overall resistance in a wire?

Both the skin effect and proximity effect contribute to increased resistance in a wire. The skin effect reduces the effective area through which current flows as the frequency increases, while the proximity effect further reduces this area when wires are close together, both resulting in higher resistance.

Why does the resistance of the wire remain constant at low frequencies?

At low frequencies, the skin depth is large enough that the entire cross-section of the wire is used for current conduction. As a result, the resistance remains constant because there is no significant reduction in the effective area available for the current to flow through.

How does the LCR meter demonstrate the change in resistance with frequency?

The LCR meter is used to measure the resistance of the spring as the frequency of the applied alternating current is varied. The meter shows that at low frequencies, the resistance is constant, but as the frequency increases, the resistance increases due to the skin effect, which reduces the effective area for current conduction.

Why is the resistance at high frequencies higher than at low frequencies?

At high frequencies, the skin effect causes the current to be concentrated near the outer surface of the wire, effectively reducing the cross-sectional area available for current flow. This increased resistance is due to the reduced effective area through which the current can pass, leading to higher resistance at higher frequencies.

How does the proximity effect influence resistance in real-world applications like transformers or motors?

In applications such as transformers and motors, the proximity effect increases resistance and leads to power losses, especially when high-frequency signals or harmonics are present. The closer the wires are to each other, the more pronounced the proximity effect becomes, resulting in higher energy losses.