Unit 4 Day 4 – Electron Properties & Hall Effect Cathode Rays and Cathode Ray Tubes Electron Beam in the Presence on an Electric & magnetic Field The Velocity Selector The Hall Effect & Hall EMF

Cathode Rays In the 1890’s, devices were built called discharge tubes What was emitted & observed as a “glow” was called cathode rays. It was later determined that these were ionized electrons. Evacuated but back filled with rarified gas

Cathode Ray Tube Cathode Ray Tube (CRT) starts with a beam of electrons which are passed through a set of parallel plates, and a set of coils, 90° to the plates. When the E-Field is applied, the electrons curve up. When the B- Field is applied, the electrons curve down.

Electron Properties Remember, in previous experiments performed by J. J. Thompson, if the Electric and Magnetic forces are balanced: The electron velocity becomes: E, B, & r, were all measurable quantities

Electron Properties Note: In later experiments by Millikan (Oil-drop Experiment), the charge of the electron was established. Knowing e and e/m e, then m e was calculated to be:

The Hall Effect If a current carrying conductor is held fixed in a magnetic field, the magnetic force on the electrons in the conductor is: where v d = drift velocity The electron will tend to move to the bottom of the conductor (D)

The Hall Effect The movement of the electron will develop a ΔV between the top (C) and the bottom (D) which will set-up an electric field E H. This produces an electric force –eE H on the moving electrons (which is upward, equal and opposite to the magnetic force)

The Hall Electric Field & EMF The E H is called the Hall Field, after E. H. Hall, who discovered this effect in 1879 The EMF produced by the Hall Field is then: where d is the width of the conductor The magnitude of the Hall EMF is proportional to the strength of the magnetic field

Hall Effect Applications A Hall Effect Probe can be constructed to measure the strength of a magnetic field A Hall Effect device can also be used to measure the drift velocity, given a known magnetic field