Lesson 4
PATH OF CHARGED
PARTICLES IN MAGNETIC
FIELD
Announcements
Magnetism HW #3 due today
HW #4 due tomorrow
Physics Labs will meet this week
Physics Bowl –
Thursday, 4/11 6th & 7th periods
Mock AP on Monday, April 15th
– 6:30 PM amphitheater
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:
c) Describe the paths of charged particles moving in uniform
magnetic fields.
d) Derive and apply the formula for the radius of the circular path of
a charge that moves perpendicular to a uniform magnetic field.
e) Describe under what conditions particles will move with constant
velocity through crossed electric and magnetic fields.
Student Learning Objectives
Students will be able to
1. Describe the path of charges in a magnetic field
2. Arrange combinations of magnetic and electric fields
to select charges with specific velocities.
F
Magnetic Forces are…
B
v
at right angles to the plane established
by the velocity and magnetic field
vectors.
centripetal.
– Remember that centripetal acceleration is v2/r.
– Remember centripetal force is therefore mv2/r.
Magnetic forces can…
accelerate charged particles by
changing their direction.
cause charged particles to move in
circular or helical paths.
Magnetic forces cannot...
change the speed or kinetic energy of
charged particles.
do work on charged particles.
Sample Problem 4.1: Do a Newton’s 2nd Law analysis of the
motion of this positively charged particle in a magnetic field.
V
B
Sample Problem 4.2:
An electric field of 2000 N/C is directed to the south. A proton is
traveling at 300,000 m/s to the west. What is the magnitude and
direction of the force on the proton? Describe the path of the
proton? Ignore gravitational effects.
Sample Problem 4.3:
A magnetic field of 2000 mT is directed to the south. A proton is
traveling at 300,000 m/s to the west. What is the magnitude and
direction of the force on the proton? Describe the path of the
proton? Ignore gravitational effects.
Sample Problem 4.4:
Calculate the force and describe the path of this electron.
E = 2000 N/C
300,000 m/s
e
Sample problem 4.5:
Calculate the force and describe the path of this electron.
e
300,000 m/s
B = 2000 mT
Sample Problem 4.6:
How would you arrange a magnetic field and an electric field so
that a charged particle of velocity v would pass straight through
without deflection?
Sample Problem 4.7: It is found that protons traveling at 20,000
m/s pass undeflected through the velocity filter below. The
distance between the two plates is 1 cm. What is the magnitude
and direction of the magnetic field between the plates?
+e
20,000 m/s
400 V