Conceptual Physics

The particle is moving to the

right. What are the magnitude

and direction of the force on it?

ș = 90°

F = |q|vB sin ș

F = 2.3×10í^5 N

directed down (opposite to thumb)

28.8 - Interactive checkpoint: a charge in a magnetic field

An electron is traveling in the yzplane

at an angle of 60.0° from the positive

z axis and 30.0° from the positive y

axis as shown. Its speed is a constant

365 m/s when a uniform magnetic

field of 1.22×10í^3 T is turned on,

pointing in the negative y direction.

State the acceleration of the electron

in the instant after the magnetic field

is turned on as a vector in rectangular

notation.

Answer:

a = ( , , ) m/s^2

28.9 - Interactive problem: charged particle moving in a B field

In this simulation you can change two properties of a particle: its charge and its velocity (both speed and direction). We ask you to alter the velocity of the particle in order to achieve certain forces. In the initial view of the magnetic field it is directed straight down, and the charge starts near the middle of the screen. You can use the slider in the control panel to change the viewing angle. Press GO to start the simulation, and press RESET whenever you want to change the particle’s initial velocity or charge. You can only change the direction of the initial velocity when the viewing angle is set to the far right and the magnetic field is pointing straight down. This orientation makes the angle between the velocity vector and the magnetic field vector easy to see. You change the direction of the velocity vector by dragging it with the mouse.

Your first task is to set the direction of the particle’s initial velocity so that no force is exerted on the charge by the field. No shortcuts! You could solve this by setting the velocity to 0 m/s, but we want you to solve the problem by setting the velocity vector’s direction correctly. When the field exerts no force, both the direction and speed of the particle will remain constant. Note that after you change the charge or the velocity vector (speed or direction), but before you press GO, the force meter tells you what the force on the particle will be, not what it is. The force is zero whenever the particle is not moving, but we thought you would appreciate seeing immediate feedback on your changes, without having to launch the particle every time.