Physics Chapters 24 & 25 Lecture Notes

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18 Οκτ 2013 (πριν από 4 χρόνια και 22 μέρες)

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Physics

Chapters 2
4 & 25

Lecture Notes


General properties of magnets


What did you notice about a magnet suspended from a string?








N & S poles of a magnet vs. N & S poles of the earth






















Prope
rties of magnets:


Poles




Attraction vs. repulsion





Remember hanging the nail from the magnet and then picking up metal with
the nail? What is happening…






What happens to the nail when you detach it from the magnet?




Domains


Each electron movin
g around acts like a tiny electromagnet (creates a small
magnetic field). Magnetic fields in groups of neighboring atoms add together
to create a
domain.


Domains are naturally oriented in different directions and tend to cancel
each other out. When an ex
ternal magnet is applied:


Before





After










How do you make a permanent magnet?
Start with magnetite or…




The earth’s magnetic field orientation is not permanent:





Magnetic Fields Around Permanent Magnets


Magnetic fields exist in the sp
ace around a magnet

Field lines are imaginary and show strength and direction

of the field
.


They are similar to electric fields

can attract or repel.

Concentration of field lines is greatest at the end of the magnet.




































Ma
gnetic Flux



How are poles oriented in a disk magnet?













Oersted’s Experiment


Up until 1820, everyone thought magnetism and electricity were completely
separate. Hans Oersted (Danish physicist) sent electric current through a
wire and laid one o
f his wires across a compass needle

he noticed it was
deflected.



Orientation:





Reverse current:






First Right Hand Rule
(How to find the direction of a magnetic field around a wire
carrying current

check out

http://www.youtube.com/watch?v=9p3t9NOf
CtA

)


Thumb:




Curled fingers:






Magnetic Fields Near a Coil


Imagine the magnetic field around a loop of wire with current in it:













Magnetic fields are always in the same direction inside the loop and the
opposite direction outside the loop
. (Here it is
up out of the page inside the
loop and down into the page outside of the loop).


If you add more loops (make a coil of wire), you increase the strength of the
magnetic field

they add to each other.


If you put a ferromagnetic core inside the
coil of wire, you make an
electromagnet.

The core becomes magnetized by induction

the strength of
the magnetic field is proportional to the number of coils, the amount of
current in the wire and the nature of the core.











This leads to the
second ri
ght hand rule:
This is the same as the first RHR
except it tells you the direction of the field produced by a coil. Grasp the
coil with the right hand and curl your fingers around the loops in the
direction of the current flow. Your thumb points toward the

N pole of the
magnet.

Forces Caused by Magnetic Fi
elds


If you put a wire carrying current in between two magnets, it is moved (a
force acts on it). This is because the current in the wire has
created

a
magnetic field and it will interact with the other
magnetic field.


There are now three directions to consider: 1. The direction of the current
2. The direction of the magnetic field 3. The direction of the force on the
wire.

These all act at right angles to each other.


These directions are determined by
the
third right hand rule
:

(see p. 658)


Fingers:



Thumb:



Palm of the hand:



Sign convention for 3
-
D representation:







How about a formula to calculate the force? Ok. Experiments show the
magnitude of the force on the current carrying wire that is
perpendicular to
a magnetic field is proportional to the field strength (B), current (I), and
the length of the wire (L). So: F = BxIxL
sin
θ
. We can measure I, L and F, so
it is usually expressed as:


B is the strength of the
magnetic field a
nd the angle i
s
between B and I. Units are
t
eslas (T). One T is 1 N/A
-
m.


One T is very strong

lab magnets are on the order of 0.1T. The earth’s
magnetic field is approximately 5 x 10
-
5
T

Galv
a
nometer


Used to measure very small currents

and voltages
. It consists of a sm
all coil
of wire placed in a strong magnetic field. Using the third RHR you can see
that one side of the loop is forced up, and the other side of the loop is
forced down. The torque on the loop is proportional to the
current (see
picture on page 661

of the

book). The torque on the loop is opposed by a
spring and the net movement can be shown using a needle gauge.







Electric Motors


Galvanometers can rotate no more than 180 degrees (why?). For this
principle to be used as an e
lectric motor, you would need to have a loop
rotate 360 degrees.


In order for the loop to rotate 360 degrees, the current must reverse
direction just as the loop reaches the vertical position (use the third RHR to
convince your self). Thi
s is accomplished

using brushes and a split ring
commutator.







One application of electro
-
magnetism is Maglev trains. Electromagnets are
used to lift the train 1cm above the track and create a very low friction
surface. Guidance magnets keep

the train on the track.



Force on a Single Charged Particle


The force of a magnetic field on a current
-
carrying wire is the result of
forces on individual charges that make up the current. The charges do not
need to be confined to the wire. The cathode
ray tube (picture tube)

is an
example of using a magnetic field to deflect a single charge.


Electrons are generated in the cathode, pass through the anode ring and are
focused and accelerated, and then magnets are used to deflect the beam so
that it hits
different parts of the phosphorescent screen and make a
picture.


What happens if you bring a magnet too close to your TV monitor?


Remember F = BxIxL. The current is charge per unit time (I = q/t),
and the
distance the electron travels in t is d = vt or t

= d/v. In our case the electron
travels L (length of the wire), so t = L/v. Substitute this into I = q/t so

I = qv/L. Substitute this into F = BxIxL and you have F = Bx(qv/L)xL or:








F = the force on the particle






B = the strength of the magne
tic field






q = the charge of the particle






v = velocity of the particle (in m/s)

Warning
: The charge on an electron is negative, and conventional current has
a positive charge. Therefore, the direction of the force is opposite that
predicted by the

RHR
.

One nice benefit of magnetism is Aurora Borealis (Northern Lights).



Electrons trapped in the earth’s magnetic fields (called the Van Allen
radiation belts)



Solar storms send charge particle to the earth. This disrupts the
earth’s magnetic fields and
dumps electrons out of the Van Allen
belts.



Electrons excite atoms in the earth’s atmosphere and cause them to
emit light

the Aurora Borealis which is a halo around the north pole.


Check out:
http://www.geo.mtu.edu/weather/aurora/images/aurora/jan.curtis/


Creating Electric Current from Changing Magnetic Fields (CH 25)


Oersted discovered that electric current produces magnetic fields…what
about the other way around?


Faraday
tried to generate electricity with all types of combinations of field
and wire, but without success until he moved a wire in a magnetic
field
.




The process of generating a current by moving a wire in through a magnetic
field is called __________________
__________________.


Electromotive Force (EMF)


In circuits we learned it takes a charge pump to keep current flowing
(usually a battery). This provides potential difference or voltage.


We can create a voltage potential that causes current to flow in a wi
re
moving through a magnetic field

this is called EMF.


This is not a force

but
just an increase in electric potential (voltage). When
a wire is moved through a magnetic field, a force acts on the charges and
they move in the direction of the force. Work i
s done on the charges and
their potential increases. This is induced EMF and it is measured in volts.
EMF depends on B (magnetic field), L (length of wire in the field), and v
(velocity of wire in the field).





An example of a simple EMF generator is a
microphone. Sound waves vibrate
a diaphragm attached to a coil of wire that moves in and out of a magnetic
field. The movement of the coil induces EMF and voltage is generated in
relation to the frequency response of the sound.








The
electric generat
or,
invented by Faraday, converts mechanical energy
into electrical energy using electromagnetic induction.


An armature (wire wound around an iron core) rotates freely in a strong
magnetic field. As the armature turns, the wire loops cut through the
magne
tic field lines, inducing an EMF.




One example of a large scale EMF generator is Hoover Dam. The movement
of water through turbines generates EMF by rotating the wire coils. The
dam is over 700 feet tall (think of potential energy!) and has produced ov
er
4 billion kWhr of energy since i
t

was built during the depression.


Electric motors and generators are almost identical in construction, but
convert energy in opposite directions. In a generator, mechanical energy
turns an armature in a magnetic field a
nd the induced voltage causes current
to flow. In a motor, a voltage is placed across an armature coil in a magnetic
field. The voltage causes current to flow in the coil and the armature turns,
producing mechanical energy.


Alternating Current