Moving Charges and Magnetism Introduction

The branch of physics which deals with magnetism due to the electric current is called electromagnetism. For a long time it was hard to believe if there existed a relation between electricity and magnetism.

Hans Christian Oersted observation made a discovery demonstrating a close relationship between the two. Oersted observed a pivoted magnet was deflected when kept in the neighborhood of a wire carrying current. Twelve years later Michael Faraday, an English physicist found that a momentary current existed in the circuit while the current in nearby circuit was being started or stopped. Shortly afterwards it was discovered that the motion of a magnet produced the same effect. The work of Oersted thus demonstrated that magnetic effects could be produced by moving electrical charges.

It is now known that all magnetic phenomenon result from forces between electric charges in motion. Charges in motion relative to an observer set up a magnetic field as well as electric field.

Not only that electricity and magnetism are closely related, electrical and magnetic forces are not obscure but pervade all matter. Determining the structure and properties of atoms, molecules and collections of them.

Sub Topics

Oersted's Experiment
Arago's Experiment
Rule to Find Direction of Field Around a Wire
Maxwell's Screw Rule
The Right Hand Grip Rule

Oersted's Experiment

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In 1819, Oersted first discovered the link between magnetism and electric current while lecturing to his students.

The following circuit diagram depicts the rotation of a magnetic needle NS which can rotate freely about a vertical axis in a horizontal plane. On completing the circuit, the current flows from A to B. During the experiment, it was found that the needle gets deflected towards the west. If the direction of current is reversed, the needle gets deflected towards the east.

a) When current is flowing from A to B above the magnetic needle

b) When current is flowing from A to B below the magnetic needle

The above experiment clearly indicates a magnetic field exists around the wire. It is for this reason the compass needle gets deflected. The direction of the magnetic field thus generated depends on

1) direction of current

2) position of needle with respect to the wire.

Arago's Experiment

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Francois Arago demonstrated that iron filings were attracted to a current. The current behaved like an ordinary magnet. The iron filings on the cardboard arranged themselves in concentric circles when a current passed through a copper as shown in the diagram.

ii)

Rule to Find Direction of Field Around a Wire

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The following two rules are used to predict the direction of the magnetic field around the wires in different situations.

Maxwell's Screw Rule

If a right-handed screw is turned so that, it moves forwards in the same direction as an electric current, its direction of rotation gives the direction of the magnetic field due to the current.

Maxwell's Screw Rule

The Right Hand Grip Rule

If a wire carrying a current is gripped with the right hand and with the thumb pointing along the wire in the direction of the current, then the fingers point in the direction of the magnetic field around the wire.