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At a place an electric field and a magnetic field are in the downward direction. There an electron moves in the downward direction. Hence this electron ______.

Asked in GSEB Board July 2017 · Crossed electric and magnetic fields

• (1) will bend towards left
• (2) will bend towards right
• (3) will gain velocity
• (4) will lose velocity

Step-by-step solution

Why the other options are wrong

• (1) Bending needs a sideways force. The magnetic force vanishes here because ⃗v is parallel to ⃗B, so ⃗v×⃗B=0, and the only remaining force lies along the line of motion.
• (2) Same reason: with the velocity along the field there is no magnetic force at all, and the electric force acts along the line of motion. Nothing pushes the electron to either side.
• (3) The electric force on an electron is -e⃗E, which points opposite to ⃗E — upward — while the electron travels downward. A force against the motion cannot speed it up; it would speed up a positive charge.

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