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Magnetic Fields and Left Hand Rule

A force can be exerted on an electron beam by a magnetic field.

When an electron (or any charged particle) moves through a magnetic field a force is exerted on it. This force deflects the particle in a direction 90 degrees to the field and its velocity. This is easier to understand using the left hand rule for negative particles in a magnetic field.

Left hand rule for negative particles in a magnetic field demonstrates the direction of force is outward from the palm when the field is in the direction of the fingers and the particle's velocity is in the direction of the thumb.

When you open your left hand. Your fingers represent the direction of the magnetic field (B). Your thumb represents the direction of the velocity of the electron (v), and your palm represents the direction of the deflecting force (Fm).


This principle is used in a CRT (cathode ray tube or CT). A beam of electrons passes through 2 different magnetic fields: one that controls its vertical position and one that controls its horizontal position. These uniform magnetic fields are created by two sets of coils.

The horizontal steering coils are located above and below the CRT and move the beam horizontally. However the magnetic field they create acts vertically. The Vertical Steering coils are located on the sides on the CRT and control the vertical position of the beam. The magnetic field they create is horizontal.


The horizontal steering coils of a CRT are creating a magnet field downwards. The Vertical steering coils are creating a field to the Right (as seen from the front of the screen). What direction is the electron beam of a CRT deflected as seen from the front?

Electrons are guided in a CR tube by magnetic coils in accordance with the left hand rule.

SOLUTION: The beam is deflect downward and to the left.

Step1: Find out which way the beam is deflected horizontally by using the left hand rule. Your fingers should be pointed downward, and your thumb towards you. This should make your palm point to the left.
Step2: Find out which way the beam is deflected vertically. On your left hand your fingers should be facing the right and your thumb should be toward you. This should make your palm face downward.

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By Harvey Noel

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