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Unit 4 Day 4 – Electron Properties & Hall Effect

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

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Unit 4 Day 4 – Electron Properties & Hall Effect

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  1. 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

  2. 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

  3. 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.

  4. 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

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

  6. 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 vd = drift velocity • The electron will tend to move to the bottom of the conductor (D)

  7. 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 EH. • This produces an electric force –eEH on the moving electrons (which is upward, equal and opposite to the magnetic force)

  8. The Hall Electric Field & EMF • The EH 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

  9. 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

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