An electron beam passes undeflected through mutually perpendicular electric and magnetic fields. If the electric field is switched off while the magnetic field is maintained, how will the electrons move?
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Question

An electron beam passes undeflected through mutually perpendicular electric and magnetic fields. If the electric field is switched off while the magnetic field is maintained, how will the electrons move?

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G1 Physics · Accepted Answer

Step 1: In crossed electric and magnetic fields, initially undeflected electrons satisfy qE⃗+q(v⃗×B⃗)=0. Step 2: Switching off E⃗ leaves only the force q(v⃗×B⃗), perpendicular to velocity. Step 3: This produces a circular path of radius r=mv/(eB). Final A. Final answer: (A) in a circular orbit.