1. Magnetic Fields Magnetic Forces
Chapter 21 Magnetism
Magnetic Fields
Magnetic Forces

2. Properties of Magnets:
magnets have polarity
north pole & south pole
opposites attract & likes repel
cannot isolate the north or the south pole
(there is no magnetic monopole)

3. Domains: in ferromagnetic materials…
atoms act in groups called domains
each domain aligns to be a microscopic bar magnet
north and south pole
aligns with the magnetic field
anything that randomizes the alignment destroys the magnetic properties
dropping a magnet or heating it
Fig 21-9

4. Making Magnets: Some metals can be turned into temporary magnets.
bring them close to a magnet
domains align within the magnetic field
magnetism is induced
A few metals can be turned into permanent magnets.
Iron
“soft” magnet
easy to make
hard to keep
Nickel & Cobalt
“hard” magnet
difficult to make
more permanent

5. Uses of Magnets: A compass is a suspended magnet.
The magnet’s north pole is attracted to Earth’s magnetic south pole.
The earth’s magnetic south pole is within 200 miles of the earth’s geographic north pole.

6. At a Molecular Level: A spinning electron is a tiny magnet.
Electrons spin about their axis like a top.
Thus, the electron is a moving charge.
Moving charges create a magnetic field.
A pair of electrons spinning in the same direction creates a stronger magnetic field.
A pair of electrons spinning in opposite directions creates a weaker magnetic field.

7. Magnetic Field: symbol is B SI unit is the Tesla or T vector quantity
direction is N to S
the environment around a magnet
where the magnetic force(s) act
lines drawn to show this

8. Electricity Magnetism + and - charges N and S poles like charges repel
like poles repel
unlike charges attract
unlike poles attract
electric monopole exists
no magnetic monopole
electric field lines flow from + to -
magnetic field lines flow from N to S
density of lines equals strength of E
density of lines equals strength of B
SI unit: ampere, 1 A = 1 C/s
SI unit: Tesla, 1 T = 1 N/Am
E exerts force on a charge, or E = F/q
Field exerts force on a moving charge, or B = F/(qvsin)

9. Current-Carrying Wire:
Oersted (1820)
Oersted found that an electric current in a wire produces a magnetic field around it;
a stationary charge does not create a magnetic field.
To predict the direction of the magnetic field produced by a current
Right-Hand Rule
conventional current flow

10. Current-Carrying Wire:
RHR #1 - Straight Wire
Curl the fingers of the right hand into the shape of a circle.
Point the thumb in the direction of the positive current
The fingers will curl in the direction of the magnetic field.
(Fig 21-6)

11. Current-Carrying Wire:
RHR #2 - Solenoid a coil of wire
Curl the fingers of the right hand in the direction of the current.
Your thumb is the north pole of the electromagnet.
Solenoids are used to make electromagnets.
field strength is proportional to the number of coils
field inside is strong and uniform
field outside is much weaker

12. Moving Charges in a “B” field:
a charged particle moving through an electric field experiences a force
produced by a magnetic field
force is strongest when particle moves perpendicular to the field
decreases to 0 N when movement is parallel
q is the charge (C)
v is the velocity of the charge (m/s)
B is the strength of the magnetic field (T)
 is the angle between v and B

13. Moving Charges in a “B” field:
RHR #3 - Magnetic Force
Don’t curl fingers.
The first finger should point in the direction of the current.
The middle finger should point in the direction of the magnetic field.
The thumb then points in the direction of the magnetic force.
(Fig 21-10)

14. Problem:
A proton moving experiences a force of 8.810-19N upward due to the earth’s magnetic field. At this location, the field has a magnitude of 5.510-5T to the north. Find the speed and direction of the particle.

15. Ampere
Ampere found that a force is exerted on a current-carrying wire in a magnetic field.
only the component of B exerts a force on the wire
if current is to B field
=900
sin(900) = 1
F = IlB
see figure 21-13
I is the current
l is the length of wire (m)
B is the magnetic field in Teslas (T)
 is the angle between I and B

16. Problem:
A wire 36m long carries a current of 22A from east to west. If the maximum magnetic force on the wire at this point is downward (towards Earth) and has a magnitude of 4.010-2N, find the magnitude and direction of the magnetic field at this location.

17. Applications:
Loudspeakers use magnetic force to produce sound. One speaker may consist of: a coil of wire a flexible paper cone attached to the coil (acts as the speaker), and a permanent magnet. A sound signal is converted to varying electric signals and is sent to the coil. See Fig
A galvanometer is a device used in the construction of both voltmeters and ammeters. In a galvanometer, when current enters the coil, which is in a magnetic field the magnetic force causes the coil to twist. See Fig