Download Interactions Between Electric and Magnetic Fields.

Ch20 Magnetism
Metal or Magnet. What is the difference?
•
•
•
•
Which of the metal bars below is a magnet?
Now, let us take a closer, microscopic look.
Each metal bar has microscopic magnetic domains.
When these domains are randomly aligned, the bar is nonmagnetic.
• When these domains are aligned, the bar is a magnetic.
Magnetic Poles
• We say that a magnet has poles (ends).
• One pole is named the North pole.
• The other pole is named the South pole.
Fundamental Law of Magnetism
• Like poles repel, and unlike poles attract.
• This law is “fundamental” in understanding the operation of
many devices that use magnets or electromagnets.
• Some of these devices include speakers, microphones,
transformers, electromagnets, motors, generators, and
solenoids.
Metal or Magnet. What is the difference?
What happens if a magnet breaks?
Magnetic Field Lines
• Around every magnet there are many invisible lines known as
magnetic field lines.
• These lines influence other magnets and charged particles that
may be in the vicinity of the magnet.
• Watch what happens to the compass and the blue charged
particle as they get near to the magnet.
Magnetic Field Lines – Electric Charges
• We observed the path of a moving charged
•
•
•
•
particle is deflected when it comes close to
a magnet.
This deflection occurs because a charged
particle in motion also has a magnetic
field.
A charged particle behaves as a tiny
magnet with a north and south pole.
As a result, a charged particle obeys the
fundamental law of magnetism.
Like poles repel, and unlike poles attract.
Right Hand Rule
• The right hand rule is used in order to determine the direction in which the
vector resulting from the cross product would act.
• To find the direction of vector C, you would start by pointing the fingers of
your right hand in the direction of the first vector (A) with your palm open in
the direction in which vector B points.
• Next, curl your fingers towards the second vector (B).
• Finally, extend your thumb. The direction of C is in the same direction in
which your thumb points.
A
C  A B
B
C
Right Hand Rule
• In what direction would vector Z point?
• Start by pointing the fingers of your right
hand in the direction of the first vector (X)
with your palm open in the direction in which
vector Y points.
• Curl your fingers towards the second vector
(Y).
• Finally, extend your thumb. The direction of
Z is in the same direction in which your
thumb points.
• In which direction would vector W point?
z  x y
w  y x
z
x
y
Forces on a Charged Particle in a Magnetic Field
• You use the cross product in order to determine which direction a
charged particle would move in a magnetic field.
• The equation used to determine the magnitude and direction of this
force is
F  qv  B
• In this equation, q is the magnitude of the charge, v is the speed of
the charge, and B is the magnitude of the magnetic field.
• The cross product is used to determine the direction of the force on a
moving charge in the a magnetic field.
• This direction is found using the Right Hand Rule as discussed
earlier.
Forces on a Charged Particle in a Magnetic Field
• When a charged particle moves through a magnetic field, its magnetic field
interacts with the uniform magnetic field and is deflected.
• Use the right hand rule to determine the direction of the force (acceleration)
acting on the particle.
• Observe the path of a positive charge moving through a magnetic field.
v
F  qv  B
B
F
Forces on a Charged Particle in a Magnetic Field
• Lets now exam the path for a negatively charged particle moving through the
same uniform magnetic field.
• The right hand rule shows us the direction of the cross product is also down.
• However, the negative sign included with the negative charge would reverse
the direction of the force applied to the particle.
• As a result, the particle would move up and out of the magnetic field.
F
F  qv  B
v
B
Interactions Between Electric and Magnetic Fields.
• Right
A wire
that Rule
carries a current (flowing electrons) will bend when it passes through
Hand
•
•
•
•
•
•
a magnetic field.
Point fingers in the direction of the particles velocity
This image is a picture of a magnetic field traveling into the board.
Curl fingers to point in the direction of the magnetic field
Right now the current in the wire is off.
Thumb
points
in the direction
theonforce
Lets
turn off
the magnetic
field andofturn
the current.
Now turn the magnetic field back on and watch what happens.
The wire bent. Why?
The magnetic field produced by moving electric charges interfered with the large
magnetic field causing the wire to bend.
Motional Electromotive Force
• In the picture below, the “x” represent a uniform magnetic field.
• The gold rods are wires. The horizontal ones are electrically connected to
a LED (light emitting diode).
• Watch what happens as the vertical wire rod rolls through the magnetic
field.
• The LED lit up. Why do you think it did?
• The magnetic field exerted a force on the electrons (which are “tiny
magnets”) in the conductors causing them to move through the wires
thereby lighting the LED.
Interactions Between Electric and Magnetic Fields.
• The fact that moving electrons (or other charges) generate their
own magnetic fields can be observed by viewing the device
below.
• Note that all the compasses are pointed in the same direction
(towards the North).
• When electrons flow
through the wire, they
create a magnetic field.
• This magnetic field
interacts with the compass
needles (small magnets)
causing them to change
directions.
Interactions Between Electric and Magnetic Fields.
Right Hand Rule
•Wrap fingers around the wire with thumb pointing in the direction
of current
•Fingers point in the direction of the magnetic field

F  Il B sin 
F
Force
I
Current
l length

B
Magnetic Field

Angle between current and magnetic field
Example Problem WS 11 #7
• Two long, parallel wires separated by a known distance carry current as shown.
• What is the magnetic field (magnitude and direction) at point P? B1
B2
• Begin by using the equations for the long straight wires in order to calculate the
P
magnitudes of the magnetic fields.
• We have already determined the directions
of the magnetic fields at point P.
R2
R1
x
oi
B1 
2 R1
oi
B2 
2 R2
Force Between two Wires
• Two wires carry currents as shown below.
• Calculate the force of attraction or repulsion that exists between these two wires.
• First determine the direction of the magnetic field around these two wires by using
•
•
•
•
•
our right hand rule.
Consider the force on wire 2 due to wire 1.
The magnetic field generated by wire 1 causes the force experienced by wire 2.
Note the direction of this magnetic field acting on wire 2.
Applying the cross product, we observe that the force acting on wire 1 is directed
towards wire 2.
Likewise, the force on wire 2 is directed towards wire 1. B2
i1
F2
i2 F2
F  iL  B
B1
Force Between two Wires
• What do you suppose would happen if the current in one of the wires changes
direction?
• Applying the cross product below and the right hand rule, we find that the force
generated by the magnetic fields cause the wires to move away from each other.
• In summary, if the current in both wires moves in the same direction, then the wires
will attract each other.
• If the current in the wires moves opposite direction, then the wires will repel each
other.
i1 F1
i2
F  iL  B
F2
B2
B1
Centripetal & Electric forces
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
x x x x x x x x x x x x
v
Interactions Between Electric and Magnetic
Fields.
A wire carrying a 30A current has a length of 12cm between the pole faces of a
magnet at an angle of 60o. The field strength is 0.9T. What is the magnitude and
direction of the force.

F  Il B sin 
F  30 A(.12m)(.9T ) sin 60
F  2.8N
13-7
Magnetic Induction
• A coil of wire is wrapped around a nail as shown in the top picture below.
• This coil of wire is attached to a power supply in which the current direction may
•
•
•
•
•
•
be reversed.
The bottom nail shows the alignment of the magnetic domains in the top nail.
Watch what happens as the electrons flow through the coil of wire around the nail.
The domains slowly realigned until they were all pointed in the same direction.
The nail/wire coil combination has become and electromagnet (see next slide).
What will happen when we reverse the current in the coil of wire?
In this animation, we saw that moving electrons induced the magnetic dipoles to
align.
Magnetic Induction
• Realigning magnetic dipoles can also cause electrons to move.
• Flowing electrons from a power supply cause the domains in the vicinity of the
•
•
•
•
first coil to align.
These domains in turn induce the other domains in the nail to realign.
Watch what happens to the electrons in the second coil and watch the light bulb as
the magnetic domains in the vicinity of the second coil realign.
As long as these domains were moving, electrons in the second coil moved
causing the light to go on.
Once the domains stop moving, the electrons stop moving, and the light goes out
even though the electrons are still moving in the first coil.
13-17
Electromagnetic Induction
• Electromagnetic Induction: The process of
generating a current through a circuit by
relative motion of charged objects through
a magnetic field.
• Current can be generated by:
– moving a conductor through a magnetic field
– a magnetic field can move past a conductor