Question
In a TV or computer monitor, a constant electric field accelerates the electrons to high speed; then a magnetic field is used to deflect the electrons to the side. Why can't a constant magnetic field be used to speed up the electrons?
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In these devices, a cathode emits electrons, which are then accelerated by a constant electric field towards a phosphor screen. This electric field provides the necessary energy to increase the speed of the electrons. Now, let's consider the role of the magnetic Show more…
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To become familiar with the order of magnitude of magnetic effects, consider the situation in a television cathode ray tube (not a flat-panel TV). In order to bend the electron trajectory so that the electron hits the top of the screen rather than going straight through to the center of the screen, you need a radius of curvature in the magnetic field of about $20 \mathrm{cm}$ (Figure 20.95 ). If the electrons are accelerated through a 15,000 $\mathrm{V}$ potential difference, they have a speed of $0.7 \times 10^{8} \mathrm{m} / \mathrm{s}$ Calculate the magnitude of the magnetic field required to make the electrons hit the top of the screen. Is the magnetic field into or out of the page?
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