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<h3> Statement </h3>
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$10.2.8$
A plane capacitor is placed in a homogeneous magnetic field od induction $B$ parallel to the plates. From point A, electrons enter perpendicularly to magnetic field direction. The tension (voltage) applied to the plates is $V$. What is the condition for which electrons pass through the capacitor?
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For problem 10.2.8
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<h3>Solution</h3>
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Electrons pass through capacitor if they don't collide with plates. So, supposing a capacitor of infinite length, electron's velocity must be parallel to the plates (or its horizontal range over a plate greater than its enter's height). This is possible if electric and magnetic forces are in equilibrium (equal absolute values and opposed directions).
$$evB = eE$$
$$v = \frac{E}{B}$$
As $E=\frac{V}{h}$,
$$\boxed{v = \frac{V}{Bh}}$$
This device is known as speed's selector.
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<p style="text-align: right; font-style: italic; font-size: 14;">
BSc. Luis Daniel Fernández Quintana<br>
Physics Department (FCNE)<br>
Universidad de Oriente, Cuba<br>
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