Variable Inductor

Chapter 4 - AC Circuits

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PARTS AND MATERIALS

Paper tube, from a toilet-paper roll
Bar of iron or steel, large enough to almost fill diameter of paper tube
28 gauge “magnet” wire
Low-voltage AC power supply
Incandescent lamp, rated for power supply voltage

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 1, chapter 14: “Magnetism and Electromagnetism”

Lessons In Electric Circuits, Volume 1, chapter 15: “Inductors”

Lessons In Electric Circuits, Volume 2, chapter 3: “Reactance and Impedance—Inductive”

LEARNING OBJECTIVES

To determine the effects of magnetic permeability on inductance.
To determine how inductive reactance can control current in an AC circuit.

SCHEMATIC DIAGRAM

variable inductor schematic diagram

ILLUSTRATION

variable inductor illustration

INSTRUCTIONS

Wrap hundreds of turns of magnet wire around the paper tube. Connect this home-made inductor in series with an AC power supply and lamp to form a circuit. When the tube is empty, the lamp should glow brightly. When the steel bar is inserted in the tube, the lamp dims from increased inductance (L) and consequently increased inductive reactance (X_L).

Try using bars of different materials, such as copper and stainless steel, if available. Not all metals have the same effect, due to differences in magnetic permeability.

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