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# Electron beams

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A2 Physics, electron beams, f =bil, hall probes

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### Electron beams

1. 1. Electron beams
2. 2. Your Learning Write an explanation of how a motor works Including Magnetic fields (of the wire and magnet) Force on a wire F = BIL (what affect the speed of the motor) Flemings left hand rule (how does this help you know which way it is going to go)
3. 3. How much force does a wire feel? Force = B-field x Current x length of wire F = BIl The Force is greatest when perpendicular to the magnetic field and zero when parallel F = BIL sin θ θ = 90o (sin θ = 1), θ = 0o (sin θ = 0), B = F/IL Force (Newtons) B- field (Teslas) Current (Amps) Length (metres)
4. 4. What we are going to do today • What happens to charged particles in a magnetic field? • Why does a force act on a wire in a magnetic field when a current flows along the wire? • What equation can we use to find the force on a moving charge?
5. 5. The Deflection tube…a basic CRO
6. 6. Thermionic emission When energy is given to a wire it heats up but….. …it can also give off electrons
7. 7. This is the basis of the electron gun... • Electrons ‘boil off’ here They are accelerated here There is a current here…. And a vacuum here And a large accelerating +Voltage V here
8. 8. Applying a magnetic field circle if v perpendicular to B spiral in a dissipative medium helix if component of v parallel to B F v B
9. 9. Researching deflecting electron beams 20th Century Technology Exhibition Nowadays most new televisions and computer monitors use LCD displays, but not so long ago they all used picture tubes. In fact, picture tubes were the main use of electron beams. Vanita and Dave are planning a museum exhibition of technology from the last century. They will provide a straight forward explanation to help people to understand how each exhibit worked. The electron beams section of the exhibition has: a television picture tube an oscilloscope an x-ray tube. Your task is to write a straight forward, concise explanation of each section, and include pictures that will develop peoples understanding of the uses of deflecting electron beams.
10. 10. Hall Probes • As electrons move through a piece of n-type semiconductor through which a magnetic field passes, then the electrons experience a force (Fleming's left hand rule) which moves them to one side of the semiconductor slab. An electric field builds up giving a force in the opposite direction. (and creating a measurable potential difference). Electron Electron + + + + + + B- field