What happens when you increase frequency through an inductor?

What happens when you increase frequency through an inductor?

• A. The current increases without limit
• B. The resistance of the coil doubles
• C. The inductance increases
• D. The magnetic field collapses

Answer: (C)

When current flows through an inductor, what changes with frequency is the opposition to that changing current, not the coil’s inherent inductance. The inductance L stays the same for a given coil, but the inductive reactance X_L grows with frequency: X_L = ωL = 2πfL. So as frequency increases, the coil presents a larger impedance to AC, which makes the current for a fixed voltage smaller (and the current lags the voltage by 90 degrees in a pure inductor). The magnetic field inside the coil is proportional to the current, so a higher frequency—which reduces the current amplitude—also leads to a smaller peak magnetic field (the field reverses direction each half cycle). In short, increasing frequency does not increase inductance; it increases inductive reactance and diminishes current and the accompanying magnetic field.

When current flows through an inductor, what changes with frequency is the opposition to that changing current, not the coil’s inherent inductance. The inductance L stays the same for a given coil, but the inductive reactance X_L grows with frequency: X_L = ωL = 2πfL. So as frequency increases, the coil presents a larger impedance to AC, which makes the current for a fixed voltage smaller (and the current lags the voltage by 90 degrees in a pure inductor).

The magnetic field inside the coil is proportional to the current, so a higher frequency—which reduces the current amplitude—also leads to a smaller peak magnetic field (the field reverses direction each half cycle). In short, increasing frequency does not increase inductance; it increases inductive reactance and diminishes current and the accompanying magnetic field.