Magnetic Fields
What happens when a charged particle interacts with a magnetic field?
What do you know about the Earth’s magnetic field?
Can you think of any similarities between electric fields and magnetic fields? If so, what?
Can you think of any differences between electric fields and magnetic fields? If so, what?
We have a simple equation, F = qE, that gives the force experienced by a charged object in an electric
field. Let’s try to find the equivalent expression for the force on a charged object in a magnetic field from
observation. Describe the force on the object (or what is happening to the force) in various situations. For
simplicity let’s assume the field is uniform in all cases.
Go to http://physics.bu.edu/~duffy/NS543.html > click Session 9 > Click observing charges
and test the following conditions.
Situation
Charged Particle in an
Electric field
Charged particle in a
Magnetic Field
The charged object is initially at
rest.
The charged object is initially
moving parallel to the field.
The charged object is initially
moving perpendicular to the field.
The magnitude of the field is
doubled.
The charge increases.
The speed of the object increases.
AP Physics
Chapter 21: Magnetism
Page 1/2
Magnetic Fields
What happens when a charged particle interacts with a magnetic field?
Is there a simple equation that satisfies all these observations about the force on a charged object in a
magnetic field?
One of the trickiest parts about the force experienced by a charged particle in a magnetic field is finding
the direction of the force. We generally use the right hand rule to do this. Using your right hand, point
your fingers in the direction of the magnetic field. Then put your thumb in the direction of the velocity of
the charged particle. The palm of your hand then faces in the direction of the force on the charged
particle. If the charge is negative, your right hand lies to you and the force is opposite, or you can just use
your left hand.
Practice with Right Hand Rule.
Go to Go to http://physics.bu.edu/~duffy/NS543.html > click Session 9 > Click Practice RHR
and test the following conditions.
1. In what direction is the force on a positive charge with a velocity to the left in a uniform magnetic
field directed down and to the left?
2. In what direction is the force on a negative charge with a velocity down in a uniform magnetic
field directed out of the paper/screen?
3. In what direction is the force on a positive charge that is initially stationary in a uniform magnetic
field directed into the paper/screen?
4. In what direction is the force on a negative charge with a velocity to the left in a uniform electric
field directed out of the paper/screen?
5. In what direction is the velocity of a positive charge if it feels a force directed into the paper/screen
in a magnetic field directed to the right?
AP Physics
Chapter 21: Magnetism
Page 2/2