Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Current Detector

Introduction

If a magnet is kept near a current carrying wire, it tries to align itself in the direction of the magnetic field produced by the current. This fact is used to make a device which can detect current. In this demonstration we make one such current detector.

Apparatus

A coil made of about 30 turns of enameled copper wire fixed on a plastic stand using M-seal, a magnetized needle, connecting wires and a battery.

Procedure

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  1. Suspend the magnetized needle in the middle of the coil with a thread.
  2. See that, in equilibrium position, the needle points in the north-south direction. Why?
  3. Now connect the two ends of the coil to the battery.
  4. The needle swings and comes to rest in a direction perpendicular to the plane of the coil. Why?

Discussion

When current passes through a coil it establishes a strong magnetic field which points in a direction perpendicular to the plane of the coil at the center of the coil. So the needle swings in the direction of the magnetic field.

Point of discussion: In the absence of the current in the coil, the needle is visibly affected by the earth's magnetic field but when current flows in the coil, the magnetic field of the coil is so strong that the effect of earth's magnetic field on the needle is not visible.