Multiple Choice
A 2 kg, −3 C charge is accelerated through a potential difference of 4 V. The velocity selector has an electric field of magnitude 5 N/C. How far from the velocity selector will the charge collide against the spectrometer "wall"?
28. Magnetic Fields and Forces
Mass Spectrometer
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A negative charge in a spectrometer is accelerated in the negative x-axis. It is later deflected and collides some distance ABOVE velocity selector. What are the orientations of the electric and magnetic fields, respectively, inside the selector?
A
up and out of the page
B
up and into the page
C
down and out of the page
D
down and into the page
Verified step by step guidance1
Understand the role of the velocity selector: A velocity selector uses perpendicular electric and magnetic fields to allow particles with a specific velocity to pass through without deflection.
Recall the force directions: For a negative charge, the electric force direction is opposite to the electric field direction, and the magnetic force direction is given by the right-hand rule, but reversed due to the negative charge.
Analyze the deflection: Since the charge is deflected above the velocity selector, the net force must be upward. This means the electric force and magnetic force must balance each other in such a way that the net force is upward.
Determine the electric field orientation: To have an upward net force, the electric field must be directed downward, causing the electric force on the negative charge to be upward.
Determine the magnetic field orientation: Using the right-hand rule for a negative charge moving in the negative x-axis, the magnetic field must be directed into the page to produce an upward magnetic force that balances the downward electric force.
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