Chapter 12 · Question 4

State the right-hand thumb rule. A current through a horizontal power line flows from east to west. What is the direction of the magnetic field at a point directly below the wire and at a point directly above it?

Answer Revealed

Direct Answer:

The right-hand thumb rule states: Imagine holding a current-carrying straight conductor in your right hand such that the thumb points in the direction of the electric current. Then, the fingers wrapping around the conductor indicate the direction of the magnetic field lines. For the power line with current flowing east to west: applying the right-hand thumb rule, the magnetic field turns clockwise in a plane perpendicular to the wire when viewed from the east end, and anti-clockwise when viewed from the west end. Therefore, at a point directly below the wire, the magnetic field is directed from north to south, and at a point directly above the wire, it is from south to north.

Simple Explanation

The right-hand thumb rule is a hand trick to find which way a magnetic field curls around a current-carrying wire. Grip the wire with your right hand, thumb pointing in the direction of the current. Your fingers now curl in the direction of the magnetic field lines. For a power line running east to west (current east to west): your thumb points east. If you stand at the east end and look along the wire, your fingers curl clockwise — so below the wire the field points from north to south, and above the wire it points south to north. This rule is also called Maxwell's corkscrew rule: driving a corkscrew in the direction of the current gives the direction of the field rotation.

Exam-Ready Structure

The right-hand thumb rule provides a convenient method to determine the direction of the magnetic field around a current-carrying conductor. 1. Statement (also known as Maxwell's corkscrew rule): Imagine holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field. Alternatively, if we consider driving a corkscrew in the direction of the current, the direction of rotation of the corkscrew is the direction of the magnetic field. 2. Application to the power line problem (Example 12.1): (a) Given: Current flows horizontally from east to west. (b) Applying the right-hand thumb rule, the thumb points east (direction of current). (c) The magnetic field at any point below or above the wire turns clockwise in a plane perpendicular to the wire when viewed from the east end, and anti-clockwise when viewed from the west end. (d) Therefore: at a point directly below the wire, the magnetic field is directed from north to south; at a point directly above the wire, the magnetic field is directed from south to north. 3. The rule is fundamental for determining the direction of magnetic field produced by a current and is used throughout the chapter for straight conductors, circular loops, and solenoids.

Key Points

Right-hand thumb rule: grip conductor with right hand, thumb points in direction of current, fingers curl in direction of magnetic field
Alternative name: Maxwell's corkscrew rule — corkscrew direction of rotation matches magnetic field direction
For horizontal east-to-west power line: field is clockwise when viewed from east, anti-clockwise from west
Point directly below the wire: magnetic field directed north to south
Point directly above the wire: magnetic field directed south to north

Common Mistakes

Using the left hand instead of the right hand to determine the magnetic field direction
Confusing the right-hand thumb rule (for magnetic field around a conductor) with Fleming's left-hand rule (for force on a conductor)

State the right-hand thumb rule. A current through a horizontal power line flows from east to west. What is the direction of the magnetic field at a point directly below the wire and at a point directly above it?