EXAMPLE
A particle of mass kg has a negative charge of –10 C. It moves in a clockwise
circular pattern of radius 2 m at a speed of m/s. What is the magnitude and direction
of the magnetic field acting upon it?We know the velocity, mass, charge, and radius of the orbit of the particle. These four
quantities are related to magnetic field strength, B, in the equation r = mv/qB. By
rearranging this equation, we can solve for B:
Now we just need to determine the direction of the magnetic field. To find the direction,
apply the right-hand rule in reverse: point your thumb in the direction of the force—
toward the center of the circle—and then stretch your fingers in the direction of the
velocity. When you curl your fingers around, they will point out of the page. However,
because the particle has a negative charge, the magnetic field has the opposite direction—
into the page.
Magnetic Fields and Electric Fields Overlapping
There’s no reason why a magnetic field and an electric field can’t operate in the same
place. Both will exert a force on a moving charge. Figuring out the total force exerted on
the charge is pretty straightforward: you simply add the force exerted by the magnetic
field to the force exerted by the electric field. Let’s look at an example.
EXAMPLE