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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.

In one line: Field, current, force: left hand tells the push.

Practice This Concept

Understand it

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.

Example

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

Formula and rule

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

See it worked out

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

Worked example

  1. 1If 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.
  2. 2This is the basic idea behind motor action.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
The thumb indicates the direction of force or motion acting on the current-carrying conductor placed in a magnetic field.

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.

Don't mix this up

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

Exam tip

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

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 force direction, or connect the rule to motor working.
  1. 1

    Enough for 1 mark

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

  2. 2

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    Stretch the thumb, forefinger, and middle finger of the left hand so that they are mutually perpendicular.

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