Force on a Current-Carrying Conductor | Class 10 Science

Quick answer: A current-carrying conductor placed in a magnetic field experiences a force. Its direction is given by Fleming's left-hand rule: forefinger = field, middle finger = current, thumb = force. The force is greatest when the conductor is perpendicular to the field and zero when it is parallel to the field. Reversing either the current or the field reverses the force.

The idea behind it

Oersted showed in 1820 that a current produces a magnetic field that can deflect a compass needle. André-Marie Ampère suggested that if a current exerts a force on a magnet, the magnet must also exert an equal and opposite force on the current-carrying conductor.

The NCERT activity (aluminium rod)

Set-up:

1. A small aluminium rod AB is hung horizontally from a stand by two connecting wires.
2. A strong horseshoe magnet is placed so that the rod lies between its poles, with the north pole below and the south pole above the rod, so the field is directed upwards.
3. The rod is connected in series with a battery, a key and a rheostat.

     stand
     |    |
     A====B   aluminium rod
   S above, N below
   field B points up

Observations:

Conclusion: The direction of the force on the conductor depends on both the direction of the current and the direction of the magnetic field.

Fleming's left-hand rule

Rule: Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, the thumb points in the direction of the force (motion) on the conductor.

A simple memory aid: Field–Forefinger, Current–Centre (middle) finger, thumb–thrust (force).

When is the force largest or zero?

• The force is largest when the current is at right angles to the magnetic field.
• The force is zero when the current is parallel (or antiparallel) to the magnetic field.
• The force is larger for a stronger magnetic field and a larger current.

Beyond Class 10: In Class 12 this is written as $F = BIL\sin\theta$, where $\theta$ is the angle between the conductor and the field. When $\theta = 90^\circ$, $\sin\theta = 1$ and the force is $BIL$ (largest). When $\theta = 0^\circ$, $\sin\theta = 0$ and the force is zero.

Moving charges and current direction

The direction of conventional current is the direction of flow of positive charge, which is opposite to the direction of electron flow. So, if a beam of electrons moves towards the east, the current is towards the west. Use the current direction (west) for your middle finger.

Where this force is used

Devices such as electric motors, loudspeakers and electric fans work because a current-carrying conductor in a magnetic field experiences a force.

Syllabus note: The detailed working of the electric motor, electromagnetic induction, the electric generator and Fleming's right-hand rule were removed from the rationalised NCERT Class 10 book. The force on a conductor and Fleming's left-hand rule are still in the syllabus.

Summary table

Exam tips

• Learn the exact wording of Fleming's left-hand rule; it is often asked for 1–2 marks.
• In electron-beam questions, first change the electron direction into the current direction (opposite).
• When asked "when is the force maximum?", answer when current and field are perpendicular.
• Write the conclusion of the activity in one line: force depends on both current direction and field direction.

Common mistakes

• Using the right hand instead of the left hand.
• Mixing up the fingers: the forefinger is the field, the middle finger is the current.
• Using electron direction as current direction.
• Saying a conductor in a magnetic field always feels a force. If it is parallel to the field, the force is zero.

Frequently Asked Questions

What is Fleming's left-hand rule?

Hold the thumb, forefinger and middle finger of the left hand at right angles to each other. If the forefinger shows the magnetic field and the middle finger the current, the thumb shows the direction of the force on the conductor.

When is the force on a current-carrying conductor maximum?

The force is maximum when the conductor, and so the current, is perpendicular to the magnetic field.

When is the force on a current-carrying conductor zero?

The force is zero when the conductor is parallel to the magnetic field lines.

What happens if the direction of current is reversed?

The direction of the force on the conductor also reverses, so the rod moves the opposite way.

Is the electric motor in the Class 10 syllabus?

The detailed electric motor section was removed from the rationalised NCERT Class 10 book. You should still know that motors use the force on a current-carrying conductor in a magnetic field.