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Right Hand Thumb Rule

The right hand thumb rule is a rule used to find the direction of the magnetic field around a straight current-carrying conductor.

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Student-friendly explanation

Hold the conductor in your right hand with the thumb pointing in the direction of current. The curled fingers then show the direction of the magnetic field lines around the wire. This rule is very useful because the magnetic field around a straight wire is circular, and the direction changes with current direction. Students should use the right hand, not the left hand, for this rule.

How to write this in exams

  1. 1

    Start with the exact idea

    The right hand thumb rule is a rule used to find the direction of the magnetic field around a straight current-carrying conductor.

  2. 2

    Then show how to use it

    1. Hold the wire in your right hand. 2. Point the thumb in the direction of current. 3. Observe the curl of the fingers. 4. The curled fingers show field direction. 5. Reverse current to reverse field direction.

  3. 3

    Add one concrete example

    If current flows upward in a straight wire, the curled fingers show the magnetic field direction around the wire in circles.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to use the left hand or to say the thumb gives field direction. That reverses the rule and leads to wrong answers.

Definition

The right hand thumb rule is a rule used to find the direction of the magnetic field around a straight current-carrying conductor.

Example

If current flows upward in a straight wire, the curled fingers show the magnetic field direction around the wire in circles.

Rule to remember

Thumb points in current direction; curled fingers give the direction of circular magnetic field lines around a straight conductor.

Memory hook

Thumb for current, fingers for field.

Examples and method

Worked example

If current in a wire is reversed from upward to downward, the thumb changes direction. As a result, the curled fingers also reverse, showing that the magnetic field around the wire reverses too.

Method to apply

1. Hold the wire in your right hand. 2. Point the thumb in the direction of current. 3. Observe the curl of the fingers. 4. The curled fingers show field direction. 5. Reverse current to reverse field direction.

Diagram support

Show a right hand gripping a straight wire with thumb upward and fingers wrapped around the wire in circles.

How CBSE asks it

State the rule, identify the current direction, or tell the direction of magnetic field around a straight conductor.

Avoid common mistakes

Common confusion

A common mistake is using the left hand or confusing the thumb direction with the magnetic field direction.

Common wrong answer

A common wrong answer is to use the left hand or to say the thumb gives field direction. That reverses the rule and leads to wrong answers.

Exam tip

In a short-answer question, mention the thumb for current and the curled fingers for magnetic field direction.

Quick check

How do the thumb and curled fingers help in the right hand thumb rule?

The thumb shows the direction of current in the straight conductor, and the curled fingers show the direction of the magnetic field lines around it.

Answer writing and exam use

1-mark answer

The right hand thumb rule is a rule used to find the direction of the magnetic field around a straight current-carrying conductor.

2-mark answer

The right hand thumb rule is a rule used to find the direction of the magnetic field around a straight current-carrying conductor. Thumb points in current direction; curled fingers give the direction of circular magnetic field lines around a straight conductor. If current flows upward in a straight wire, the curled fingers show the magnetic field direction around the wire in circles.

3-mark answer

Hold the conductor in your right hand with the thumb pointing in the direction of current. The curled fingers then show the direction of the magnetic field lines around the wire. This rule is very useful because the magnetic field around a straight wire is circular, and the direction changes with current direction. Students should use the right hand, not the left hand, for this rule. Thumb points in current direction; curled fingers give the direction of circular magnetic field lines around a straight conductor. If current in a wire is reversed from upward to downward, the thumb changes direction. As a result, the curled fingers also reverse, showing that the magnetic field around the wire reverses too. State the rule, identify the current direction, or tell the direction of magnetic field around a straight conductor. A common wrong answer is to use the left hand or to say the thumb gives field direction. That reverses the rule and leads to wrong answers.
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