Chapter 36, Problem 39A

By present-day custom, the direction of electric current is the direction in which positive charge flows. The direction of a magnetic field surrounding a current- carrying wire can be shown by what is called the 'right-hand rule.' Wrap your right-hand fingers around a current-carrying wire in such a way that your thumb points in the direction of the electric current; then your fingers point in the direction of the magnetic field. Suppose instead that the current direction were defined as the direction in which electrons move in the wire. Why then would a 'left-hand rule' be used for identifying magnetic field direction?

To determine

The reason for which “left − hand rule” is used for identifying the magnetic field direction when the direction of current is defined in the direction of electron’s motion.

Explanation of Solution

Introduction: In the “Right - hand rule”, if the thumb of right hand is represent the direction of electric current then the direction of curled finger represent the direction of magnetic field. The electric current is known as the rate of change of charge.

The electric current is flow in the opposite direction of electron’s motion because the electrons have negative charge and current is always move from positive to negative. According to the given condition, when the direction of current is in the direction of motion of electron then the direction of magnetic field is measure by the left − hand rule instead of right − hand rule, because the right − hand rule is applicable direction of current is opposite to electron’s motion.

Conclusion:

Hence, the left − hand rule is used because the direction of current is reversed.