You shot a beam of electrons into a region with electric or magnetic fields, but

Question

You shot a beam of electrons into a region with electric or magnetic fields, but the wiring is confusing, and you don’t remember what knob controls the electric field strength and what controls the magnetic field strength. You play with each knob and find the following:

When you turn on Knob A but keep Knob B off, you find that the electron beam traces a perfectly circular arc on your phosphorescent screen.

When you turn on Knob B but keep Knob A off, you find that the electron beam traces a parabola on your phosphorescent screen.

Given this information, identify which knob controls the electric field and which controls the magnetic field strength. Explain your reasoning.

Explanation / Answer

Knob a must be magnetism. Magnetic fields will always cause charges to move in circles because they put a force perpendicular to the charges motion.

Knob B must be the electric field, as an electric field will affect a charge like how gravity effects a thrown stone, forming a parabola.

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