Rob asked in Science & MathematicsPhysics · 10 years ago

# At one instant an electron is moving in the positions x direction along the x axis in a region where there is?

a uniform magnetic field in the position z direction. When viewed from a point on the positive z axis, it subsequent motion is:

B) counterclockwise around a circle in the xt plane

C) clockwise around a circle in the xy plane

D) in the positive z direction

E) in the negative z direction

Relevance
• 10 years ago

B) counterclockwise around a circle in the xy plane

Because it is an electron, it has negative charge. The "current" (actually the q*v product) of this electron points opposite the direction of the electron's motion.

Right hand rule:

http://img43.imageshack.us/img43/8656/rhrulemontag...

Keep in mind the bottom-left picture.

Point your index finger in the negative direction. You can remember index finger = current, because we use the letter I, as in uppercase i, for current.

Point your Middle finger in the direction of the Magnetic field.

Your thumb will point in the direction of the force acting on the electron. You can remember this, because if you haven't learned to say the letters "th" by putting your tongue on your teeth, you say "fum" when attempting to say "thumb". Your "Fum" will point in the direction of the Force.

If the x-axis points right, y points up, and z points at us...

(to construct this coordinate system, look at the RH rule montage and see the middle-left picture)

Index finger points left

Middle finger points toward us

And you see that your thumb points upward

When an object travels rightward and experiences a net force upward, it will make a "left turn", as in circulate counter clockwise as we observe it looking at the origin and electron from a location on the +z-axis.