Chapter 12 · Question 8

State Fleming's left-hand rule. An electron enters a magnetic field at right angles to it. The magnetic field is directed horizontally to the right and the electron moves vertically downwards. What is the direction of the force acting on the electron?

Answer Revealed

Direct Answer:

Fleming's left-hand rule states: Stretch the thumb, forefinger, and middle finger of your left hand such that they are mutually perpendicular to each other. The forefinger points in the direction of the magnetic field, the middle finger points in the direction of the electric current, and the thumb then points in the direction of motion or the force acting on the conductor. For the electron: conventional current is opposite to the electron's motion, so current is vertically upwards. Applying Fleming's left-hand rule — forefinger to the right (field), middle finger upwards (current) — the thumb points into the page. Therefore, the force on the electron is directed into the page.

Simple Explanation

Fleming's left-hand rule is a three-finger trick for finding which way your electric motor will spin. Stick out your left hand's thumb, forefinger, and middle finger so they all make right angles. Forefinger = magnetic field direction. Middle finger = current direction. Thumb = force (motion) direction. For the electron problem: remember that electrons have negative charge, so conventional current flows opposite to electron motion. The electron travels down, so current goes up. Field points right. Your thumb ends up pointing into the page — that is where the force pushes the electron.

Exam-Ready Structure

Fleming's left-hand rule is the standard method to determine the direction of force on a current-carrying conductor placed in a magnetic field, providing the operational basis for electric motors. 1. Statement: Stretch the thumb, forefinger (first finger), and middle finger (second finger) of your left hand such that they are mutually perpendicular to each other (Fig. 12.13). (a) Forefinger (first finger): points in the direction of the magnetic field. (b) Middle finger (second finger): points in the direction of the electric current. (c) Thumb: points in the direction of motion or the force acting on the conductor. 2. Solved example (Example 12.2): (a) Given: an electron moves vertically downwards; the magnetic field is directed horizontally to the right. (b) Key concept: The direction of conventional current is taken opposite to the direction of motion of electrons. Since the electron moves downward, the conventional current is directed vertically upwards. (c) Applying Fleming's left-hand rule: Align the forefinger to the right (direction of magnetic field). Align the middle finger upwards (direction of conventional current). The thumb then points into the page (perpendicular to both). (d) Answer: The force on the electron is directed into the page. 3. The rule is valid when the current direction, the magnetic field direction, and the force are all mutually perpendicular to each other. 4. Devices that operate on this principle include the electric motor and measuring instruments.

Key Points

Fleming's left-hand rule: thumb (force/motion), forefinger (magnetic field), middle finger (current) — mutually perpendicular
Forefinger → magnetic field; middle finger → current; thumb → force/motion
For electrons: direction of conventional current is opposite to direction of electron motion
Example 12.2: electron moving down, field right → current up → force into the page
Used to determine direction of force on a current-carrying conductor in a magnetic field

Common Mistakes

Confusing Fleming's left-hand rule (for motor effect / force on conductor) with the right-hand thumb rule (for magnetic field direction around a conductor)
Forgetting that electron motion direction is opposite to the direction of conventional current

State Fleming's left-hand rule. An electron enters a magnetic field at right angles to it. The magnetic field is directed horizontally to the right and the electron moves vertically downwards. What is the direction of the force acting on the electron?