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Fleming's left hand rule

Fleming's left hand rule helps find the direction of force on a current-carrying conductor placed in a magnetic field.

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Student-friendly explanation

Stretch the thumb, forefinger, and middle finger of the left hand so that they are mutually perpendicular. The forefinger shows the magnetic field direction, the middle finger shows current direction, and the thumb shows the direction of force or motion. This rule is very important in electric motors, where current and magnetic field interact to produce motion.

How to write this in exams

  1. 1

    Start with the exact idea

    Fleming's left hand rule helps find the direction of force on a current-carrying conductor placed in a magnetic field.

  2. 2

    Then show how to use it

    1. Use the left hand. 2. Stretch thumb, forefinger, and middle finger at right angles. 3. Point forefinger along magnetic field. 4. Point middle finger along current. 5. Thumb gives force direction.

  3. 3

    Add one concrete example

    If current flows in a conductor placed between magnet poles, the conductor experiences a force that makes it move in a specific direction.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to use the right hand or to swap field and current directions. That gives the wrong force direction.

Definition

Fleming's left hand rule helps find the direction of force on a current-carrying conductor placed in a magnetic field.

Example

If current flows in a conductor placed between magnet poles, the conductor experiences a force that makes it move in a specific direction.

Rule to remember

Forefinger = magnetic field, middle finger = current, thumb = force or motion, all mutually perpendicular.

Memory hook

Field, current, force: left hand tells the push.

Examples and method

Worked example

If the magnetic field is from north to south and current is upward in a conductor, the thumb will show the direction in which the conductor moves. This is the basic idea behind motor action.

Method to apply

1. Use the left hand. 2. Stretch thumb, forefinger, and middle finger at right angles. 3. Point forefinger along magnetic field. 4. Point middle finger along current. 5. Thumb gives force direction.

Diagram support

Draw a left hand with three perpendicular fingers labelled field, current, and force.

How CBSE asks it

State the rule, identify force direction, or connect the rule to motor working.

Avoid common mistakes

Common confusion

Students often use the right hand here or mix up the directions of current and force.

Common wrong answer

A common wrong answer is to use the right hand or to swap field and current directions. That gives the wrong force direction.

Exam tip

Remember the order: field, current, force. Left hand is used for force on current in a field.

Quick check

In Fleming's left hand rule, what does the thumb indicate?

The thumb indicates the direction of force or motion acting on the current-carrying conductor placed in a magnetic field.

Answer writing and exam use

1-mark answer

Fleming's left hand rule helps find the direction of force on a current-carrying conductor placed in a magnetic field.

2-mark answer

Fleming's left hand rule helps find the direction of force on a current-carrying conductor placed in a magnetic field. Forefinger = magnetic field, middle finger = current, thumb = force or motion, all mutually perpendicular. If current flows in a conductor placed between magnet poles, the conductor experiences a force that makes it move in a specific direction.

3-mark answer

Stretch the thumb, forefinger, and middle finger of the left hand so that they are mutually perpendicular. The forefinger shows the magnetic field direction, the middle finger shows current direction, and the thumb shows the direction of force or motion. This rule is very important in electric motors, where current and magnetic field interact to produce motion. Forefinger = magnetic field, middle finger = current, thumb = force or motion, all mutually perpendicular. If the magnetic field is from north to south and current is upward in a conductor, the thumb will show the direction in which the conductor moves. This is the basic idea behind motor action. State the rule, identify force direction, or connect the rule to motor working. A common wrong answer is to use the right hand or to swap field and current directions. That gives the wrong force direction.
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