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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.

In one line: Thumb for current, fingers for field.

Practice This Concept

Understand it

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.

Example

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

Formula and rule

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

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1If current in a wire is reversed from upward to downward, the thumb changes direction.
2As a result, the curled fingers also reverse, showing that the magnetic field around the wire reverses too.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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.

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.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

1
Enough for 1 mark
The right hand thumb rule is a rule used to find the direction of the magnetic field around a straight current-carrying conductor.
2
Add for 2 marks
Example: If current flows upward in a straight wire, the curled fingers show the magnetic field direction around the wire in circles.
3
Add for 3 marks
If current in a wire is reversed from upward to downward, the thumb changes direction.

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