Lesson 3
MAGNETIC FORCE ON
MOVING CHARGE
Announcements
Magnetism HW #2 due today
Modern Physics HW #6 due today
HW #3 due tomorrow
Physics Labs will meet this week
– Diffraction lesson
Physics Bowl – Next Thursday, 6th period
– Bring a #2 pencil only.
– Calculator will be provided
AP Physics B Learning Objectives
III.D.I. 1. Forces on moving charges in magnetic fields
Students should understand the force experienced by a
charged particle in a magnetic field, so they can:
a) Calculate the magnitude and direction of the force in terms of q, v,
and, B, and explain why the magnetic force can perform no work.
b) Deduce the direction of a magnetic field from information about the
forces experienced by charged particles moving through that field.
Student Learning Objectives
Students will be able to
1. Calculate the magnitude and direction of the
magnetic force on a moving charge.
Magnetic Force on Particles
Magnetic fields cause the existence of
magnetic forces.
A magnetic force is exerted on a particle
within a magnetic field only if
– the particle has a charge.
– the charged particle is moving with at least
a portion of its velocity perpendicular to the
magnetic field.
Magnetic Force on a Charged
Particle
magnitude: F
= qvBsin
– q: charge in Coulombs
– v: speed in meters/second
– B: magnetic field in Tesla
– : angle between v and B
direction: Right Hand Rule
FB = q v x B
(This is a “vector cross product”)
The Right Hand rule to Determine a
Vector Cross Product
1.
2.
3.
Align your hand along the first vector.
Orient your wrist so that you can
“cross” your hand into the second
vector.
Your thumb gives you the direction of
the third vector (which is the result).
Sample Problem 3.1: Calculate the magnitude force exerted
on a 3.0 C charge moving north at 300,000 m/s in a magnetic
field of 200 mT if the field is directed
a) North.
b) South.
c) East.
d) West.
• Sample Problem: Calculate the magnitude
and direction of the magnetic force.
v = 300,000 m/s
34o
q = 3.0C
B = 200 mT