Download Chapter 20-21

Chapter
20&21
Like Poles of a magnet repel;
unlike poles attract.
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Electricity and Magnetism – how
are they related?
When an electric current passes through a wire a
magnetic field is formed.
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Right hand rule in a
current carrying wire
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See image on page 770
Magnetic Field of a
current loop
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See images on page 771
Solenoids – continuous loops of
wire, they form strong magnets on
the inside.
How can you tell North / South?
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Deflection of a compass needle
near a current carrying wire,
showing the presence and direction
of the magnetic field
Right Hand Rule
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B Magnetic Field
4 fingers
V velocity of moving charge thumb
F force on a positive charge
palm
of hand A negative charge would have
the opposite direction of force.
Arrows into a page are drawn as X’s,
arrows out of the page are drawn as
points.
Study Image on page 774
Answer questions on page 783 #35
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B, to the right
V, up the page
What would be the direction of force on an electron? A proton?
X
Force is into the
page, Magnetic field
is down the page.
What is the direction
of velocity for a
positive charge?
What is the direction
of velocity for a
negative charge?
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2 parallel wires carrying current, they
attract if the current is in the same
direction
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2 current carrying wires repel if their
currents run in opposite directions.
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Loudspeakers work by
this idea
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F = ILB sinθ
I = current (amps)
 L= Length of wire(m)
 B = magnetic field (T)
Tesla, (G) Gauss
 F = Force on electric
current in magnetic field
(N)
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F = q vB sinθ
q= particle charge,
coulombs (C)
 V = velocity of
charge (m/s)
 B = magnetic field
(T)
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B = µo I/ 2πr
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Magnetic field due to a
straight wire
µo = permeability of free
space 4p x10–7 Tm/A
 I = current (amps)
 r = perpendicular distance to
the wire
 B = magnetic field (T) Teslas
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(a) An unmagnitized piece of
iron is made up of domains
that are randomly arranged.
Each domain is like a tiny
magnet; the arrow represent
the magnetization direction,
with the arrowhead being the
N pole. (b) In a magnet, the
domains are preferentially
aligned in one direction and
may be altered in size by the
magnetization process
Φm = BA Cosθ
Mag Flux
 Φm = magnetic flux
(Tm2) Tesla meter2
Weber(1wb=1Tm2)
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B = magnetic field
 A = area of loop
EAVE = ΔΦm /Δt
Faradays Law of
Induction
 EAVE = EMF (Tm2/ sec)
 Φm = magnetic flux
(wb)
 Δt = time (sec)
E = BLV
Emf Induced in a moving
Conductor
 B = magnetic Field
 V = Velocity
 L = length (m)
 True as long B,V,L are
mutually perpendicular
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Transformers
A device that changes one ac
potential difference to a different ac
potential difference.
Power companies increase voltages
for long distance transmission,
then they must decrease voltages
before going into your home.
We say the voltage has been
stepped up or it has been stepped
down.
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Transformers
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They have 2 sides – primary and
secondary
Primary is the side closest (wired)
to the generator
Secondary is the side wired to the
resistor or the consumer.
A soft Iron core connects both
sides.
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Transformer Equation
V 2 N1 = N 2 V 1
V = voltage
N = number of turns or coils of wire
1 = Primary
2 = Secondary
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Vs/ Vp = Ns/Np
 V= Voltage
 N = # of Turns
Ns > Np step up
transformer, increase
voltage
Ns < Np step down
transformer, decrease
voltage
Power stays the same,
(almost)
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What is a galvanometer?
A galvanometer is an electromagnet that interacts with a
permanent magnet. The stronger the electric current
passing through the electromagnet, the more is interacts
with the permanent magnet.
Galvanometers are
used as gauges in
cars and many other
applications.
The greater the current passing through the wires, the stronger
the galvanometer interacts with the permanent magnet.
Galvanometer
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A simple instrument designed to
detect electric current.
When calibrated to measure
current, it is an ammeter.
When calibrated to measure
voltage, it is a voltmeter.
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Electromagnetic
Induction
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The production of an emf (electromotive force – kinda like voltage) in
a conducting circuit by a change in
the strength, position, or
orientation of an external magnetic
field.
Faradays Law
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The induced emf (electromotive
force) in any closed circuit is equal
to the time rate of change of the
magnetic flux through the circuit.
It is the operating principle of
transformers, inductors, many
types of motors and generators.
(A) A current induced when a magnet is
moved toward a coil. (B) The induced
current is opposite when the magnet is
moved away from the coil. Note that the
galvanometer zero is at the center of
the scale and the needle deflects to the
left or right, depending on the direction
of the current. In (C) no current is
induced if the magnet does not move
relative to the coil.
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Lenz Law
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An induced current is
always in such a direction
as to oppose the motion or
change causing it
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Induction
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Wires spinning in magnetic fields is
what underlies all electric motors
and electric generators.
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Most of the rest of the chapter
deals with applications of this.
Electric motors, electric
generators, and transformers will
be as far as we go down this road.
What are electric motors?
An electric motor is a device which changes electrical
energy into mechanical energy.
How does an electric motor work?
Go to the next slide 
Simple as that!!
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We have seen how electricity can produce a magnetic
field, but a magnetic field can also produce electricity!
How?
What is electromagnetic induction?
Moving a loop of wire through a magnetic field produces
an electric current. This is electromagnetic induction.
A generator is used to convert
mechanical energy into electrical energy by
electromagnetic induction.
Carefully study the next diagrams:
Direct current versus alternating current –
AC vs DC : What’s the difference?
Direct current is electrical current which comes from a
battery which supplies a constant flow of electricity in
one direction.
Alternating current is electrical current which comes
from a generator. As the electromagnet is rotated in the
permanent magnet the direction of the current alternates
once for every revolution.
Go to this website and click the button for DC then for
AC to visually see the difference between the two.
You can see that the DC source is a battery – current
flows in one direction. The AC source is the generator
and the current alternates once for each revolution.
Explanation of Fig. 21-17
(a) Schematic (simplified) diagram of an
alternator. The input electromagnet
current to the rotor is connected through
continuous slip rings. Sometimes the
rotor is made to turn by a belt from the
engine. The current in the wire coil of the
rotor produces a magnetic field inside it
on its axis that points horizontally from
left to right, thus making north and south
poles of the plates at either end. These
end plates are made with triangular
fingers that are between them as shown
by the blue lines. As the rotor turns,
these field lines pass through the fixed
stator coils (shown on the right for clarity,
but in operation the rotor rotates within
the stator) inducing a current in them,
which is the output
The emf is induced in the
segments ab and cd, with
velocity components
perpendicular to the field B
are v sin θ
Determining the flux
through a flat loop of wire.
This loop is square, of side
l and area A=l2
A current can be induced by changing
the area of the coil. In both this case
and that of Fig. 21-6, the flux through
the coil is reduced. Here the brief
induced current acts in the direction
shown so as to try to maintain the
original flux (Φ = BA) by producing its
own magnetic field into the page. That
is. as the area A decreases, the
current acts to increase B in the
original (inward) direction
http://www.youtube.com/watch?v=QPd963cCeec