Moving Charges and Magnetism | Class 12 Physics Notes

4.1 INTRODUCTION

Electricity and magnetism have been known as separate phenomena for more than 2000 years. However, it was only around 1820 that their intimate connection was discovered. Danish physicist Hans Christian Oersted, during a lecture demonstration, noticed that a current in a straight wire caused a deflection in a nearby magnetic compass needle. This deflection was tangential to an imaginary circle centered on the wire and lying in a plane perpendicular to the wire. Reversing the current reversed the needle's orientation. The deflection increased with current magnitude and proximity of the needle to the wire. Iron filings sprinkled around the wire arranged themselves in concentric circles, revealing the magnetic field pattern. Oersted concluded that moving charges or currents produce a magnetic field in the surrounding space.

Following Oersted's discovery, intense experimentation led to the unification of electricity and magnetism by James Maxwell in 1864, who also realized that light is an electromagnetic wave. Radio waves were discovered by Hertz and produced by J.C. Bose and G. Marconi by the end of the 19th century. The 20th century saw remarkable scientific and technological progress due to the understanding of electromagnetism and the invention of devices for electromagnetic wave production, amplification, transmission, and detection.

This chapter explores how magnetic fields exert forces on moving charged particles and current-carrying wires, how currents produce magnetic fields, the acceleration of particles in cyclotrons, and the detection of currents and voltages using galvanometers. The convention adopted is that a current or field emerging out of the plane of the paper is depicted by a dot (⊙), and going into the plane by a cross (⊗). Figures 4.1(a) and 4.1(b) illustrate these two situations respectively.

📊 Diagram: Figure 4.1 (a) The magnetic field due to a straight long current-carrying wire. The wire is perpendicular to the plane of the paper. A ring of compass needles surrounds the wire. The orientation of the needles is shown when the current emerges out of the plane of the paper; (b) The orientation when the current moves into the plane of the paper; (c) The arrangement of iron filings around the wire. The darkened ends of the needle represent north poles. The effect of the earth's magnetic field is neglected.

🧪 Activity: Oersted's experiment demonstrating the deflection of a magnetic needle near a current-carrying wire.

🔗 Connection: Leads into the concept of magnetic force and magnetic fields produced by currents, setting the stage for detailed study of magnetic interactions.