TAP 413-1: Deflecting electron beams in a magnetic field

Transcription

1 TAP 413-1: Deflecting electron bems in mgnetic field Circulr control Mgnetic fields re often used to steer bems of chrged prticles, in situtions from teleision tube to lrge-scle prticle ccelertor. The pths re lwys helices, circles or spirls, s the force cting on the chrged prticles within the bem is lwys t right ngles to their elocity circle if perpendiculr to B F B spirl in dissiptie medium helix if component of prllel to B Here you look t circulr pths, keeping it simple. You will need: electron deflection tube stnd for tube EHT power supply, 0 5 kv power supply, 5 V dc rheostt Helmholtz pir of coils 4 mm leds

2 Sfety Wire crefully, no bre conductor boe 40 V Although the output form the EHT power supply will not gie ftl shock, it cn still mke the user jump nd is quite unplesnt. Teltron tubes often use mle 4mm connectors so the leds must terminte in femle (socket) connectors if bre conductors re to be oided. The protectie resistor in the EHT cn not be used here. Deflecting ech electron current-crrying coil, producing mgnetic field deflection ccelertion the electron gun glss enelope 5 kv phosphor screen tht glows when struck by electrons, showing their pth 5 V The deflection tube uses n electron gun to inject electrons into region of nerly uniform mgnetic field perpendiculr to the bem. In this region mgnetic field cts on the chrge, producing uniform ccelertion in the direction perpendiculr to the mgnetic field nd to the elocity of the electrons. Ech electron experiences n ccelertion tht is lwys t right ngles to its elocity. You should recognise this s recipe for circulr motion by now.

3 = qb m constnt = r 2 constnt nd lwys perpendiculr to Think crefully bout the effect on the circle of the following chnges: incresing the ccelerting pd in the electron gun. incresing the current in the coils producing the mgnetic field. You might like to go further nd predict how the cure will chnge when you double one quntity, then the other, perhps building computer model to check your predictions, if tht helps. Below is n imge s reminder of wht you he seen. As further reinforcement of the nlysis suggested you might like to: extrct enough dt to show tht the pth followed is circle.

4 You he 1. Seen electrons in circulr motion. 2. Thought bout the principles behind this motion. 3. Seen how n ccelerting pd cn be used to mke n electron gun

5 Prcticl dice You will need to set this demonstrtion up in well drkened room for mximum effect. You will lso wnt to think crefully bout how to disply both the tube, in order to explin this ction, nd the resulting circle, to the size of group tht you will be showing it to. It is often good ide to coer the bck of the tube with blck cloth If new to wiring up electron deflection tubes you my well lso wish to prctice beforehnd, nd he smll bench top lmp to hnd, well shded so s not to destroy the drk dpttion of the group s eyes. You will need to explin nd show the ctions of the electron gun nd the coils, probbly using gimblled br mgnet to show the direction of the field. You will probbly get spred of circles, perhps due to the field being non-uniform ( Helmholtz pir only produce n pproximtely uniform field in the centrl third of the olume they enclose), due to the electrons proided by the electron gun hing rnge of elocities, nd due to rndom therml motion superimposed on the elocity cquired from the ction of the ccelerting pd. It my lso help to connect both electric deflection pltes to the node in the electron gun nd to erth, forcing the filment (cthode) to lrge negtie oltge. NB: mny low oltge power supplies must not he the output oltge djusted while deliering high currents. Sfety Wire crefully, no bre conductor boe 40 V Although the output form the EHT power supply will not gie ftl shock, it cn still mke the user jump nd is quite unplesnt. Teltron tubes often use mle 4mm connectors so the leds must terminte in femle (socket) connectors if bre conductors re to be oided. The protectie resistor in the EHT cn not be used here. Externl references This ctiity is tken from Adncing Physics chpter 16, 120D