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Writer's picturekeshprad

PhysicsBowl 2018 Q27

27) This question asks us to find the direction of the magnetic force acting on the proton.


When a current flows through a wire, it creates both a magnetic and an electric field. Using conventional current(flow of protons) we would say that the electric field is in the same direction as the flow of current. However, we just want to know the direction of the magnetic force on the proton, so we aren't concerned with the electric field. The direction of the magnetic field produced by a current is determined by the right-hand rule.


Since this problem gives us the flow of electrons, we can imagine it as conventional current in the opposite direction.

Where red is for protons, and v is velocity

If the conventional current is directed to the right, then using the right-hand rule, the magnetic field above the wire is directed out of the page.

Where orange is for magnetic field, red is for protons, and v is for velocity

Now that we know the direction of the magnetic field acting on the proton and the direction of its velocity, we can again use the right-hand rule to find the direction of the force produced.


After using the right-hand rule, we would find that the force points to the right of the page.


Answer: B

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