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An electron beam travels through a region where uniform electric and magnetic fields act mutually perpendicular to each other, and both are also perpendicular to the direction of motion of the electrons. If the electric field strength is 20 V m−1 and the magnetic field strength is 0.5 T, the speed of the electrons must be

1. 8 m/s
2. 20 m/s
3. 40 m/s
4. 1/40 m/s

Correct answer: 40 m/s

Solution

For undeflected motion the electric and magnetic forces balance: qE = qvB, so v = E/B = 20/0.5 = 40 m/s.

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