Example Problem 1.

 

A negatively charged particle is travelling right in a magnetic field that is directed downward.

  1. Is the magnetic force "out of" or "into" the plane of the page?

  2. If the particle where positively charged, in what direction would the magnetic force point?


Solution

 

  1. According to the left-hand rule, the force is directed "out of" the plane of the page.

  2. For a positive charge, using your right hand, the force would be "into" the plane of the page.

 

Example Problem 2.

 

A positively charged particle is travelling left in a magnetic field that is directed downward.

  1. Is the magnetic force "out of" or "into" the plane of the page?

  2. If the particle where negatively charged, in what direction would the magnetic force point?


Solution

 

  1. According to the right-hand rule, the force is directed "out of" the plane of the page.

  2. For a negative charge, using your left hand, the force would be "into" the plane of the page.


A combination of both equation and the appropriate hand rule can be used to determine the magnitude and direction of the magnetic force acting on a moving charged particle.  You will need to refer to your physics data sheet for the charge of an electron, proton, and alpha particle.  Note the definition of the following terms that apply to the practice problems that follow:

  • "upward" means "out of" the plane of the page ( ) (not to be confused with the term "north")

  • "downward" means "into" the plane of the page ( ) (not to be confused with the term "south")

 

Example Problem 3.

 

Calculate the magnitude and direction of the magnetic force acting on a proton travelling north at a speed of 2.75 × 10 3 m/s through magnetic field of 3.50 × 10 -1 T that is directed west.

 

Magnitude

  Fm=qvB

F m = (1.60 × 10  - 19 C)(2.75 × 10 3 m/s)(3.50 × 10 -1 T)

F m = 1.54 × 10 -16 N

 

Direction

 

 According to the right-hand rule for positive charges, the force is upward.

  

Self-Check

Answer the following self-check (SC) question then click the "Check your work" bar to assess your responses.

 

SC 5.  

Based on the following diagram, predict the magnitude and direction of the magnetic force acting on each proton. You can only verify the direction using the simulation.

 

a.
b.
c.


 

   Self-Check Answer

SC 5.

 

a. 

 

direction: south

 

F m  =  qvB

F m  = (1.60 × 10 -19  C)(8.50 × 10 4  m/s)(2.50 × 10 -1  T)

F m  = 3.40 × 10 -15  N

 

b. 

 

direction: north

 

F m  =  qvB

F m  = (1.60 × 10 -19  C)(7.65 × 10 5  m/s)(5.00 × 10 -1  T)

F m  = 6.12 × 10 -14  N

 

c. 

 

Direction: none

 

magnitude: none − the motion and field direction are parallel.