Electricity and Magnetism

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18 Οκτ 2013 (πριν από 3 χρόνια και 5 μήνες)

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Science 2201 laboratory

Experiment #
5


Electricity

and Magnetism

Magnetic Field
s
:

Bar Magnets:

Go to the PhET link, click on simulations, then click on “Electricity, Magnets & Circuits” and click on the “Faraday’s
Electromagnetic Lab”
.

The first tab shows a magnet and a compass like shown in th
e Figure. The magnet results on a
magnetic field. This is represented by the small compasses shown throughout. The stronger the magnetic field, the brighter ar
e
the small compasses. The larger compass can be used to visualize the effect of the magnetic fie
ld on a small magnet (the
magnet of the compass).

Describe what
you observe?

The

north pole of the bar magnet attracts the south pole and attracts the north
pole of the compass. The strength of the magnetic field diminishes as you move away from the magnet
.


Now go the simulations at KSU and try the four simulations labeled as Electrical Fields. Which of these simulations provide a

field similar to the one observed with a magnet?

The

simulation “Electric Field 3” shows a similar field patter as the pattern

obtained by
the bar magnet. When you press the “Field vectors” button, like the small compasses, arrows representing the electrical field

point away from
the positive charge and
point
towards the negative charge.


Use the provided magnet and iron fillings

to observe the effects and shape of the magnetic field. Describe what you observe.

The

iron fillings
align in the same direction as the magnetic field. Each of the iron bits acts as a small compass.


Magnetic Domains:

An experiment a little exp
e
nsive to p
erform would consist of breaking up magnets and showing that whenever we break a
magnet with a north and
a
south pole, we obtain two magnets each having a north and a south pole.
So far, we cannot observe
a north pole or a south pole by itself.

That is par
tly because each magnet we observe is actually made up of tiny magnets, the
smallest of which is the electron. In a larger scale, the bar
-
magnets like the ones we are using are made up of materials that are
called ferromagnetic. Iron is an example of a fer
romagnetic material. The simulation titled “Magnetic Domains” illustrates
this concept. It shows a piece of ferromagnetic material that we can subject to an outside magnetic field. Initially, the
magnetic field is zero, the magnetic domains are oriented at

random.

Check it out; how many of the domains are oriented
towards the right and how many are oriented to the left?

About an equal number are oriented in each direction. The difference, when it
exists
, is tiny.


Use the slider to impose a positive magneti
c field and repeat the count. How many of the domains are oriented towards the
right and how many are oriented to the left?

More domains are oriented towards the right.


Use the slider to impose a negative magnetic field and repeat the count. How many of t
he domains are oriented towards the
right and how many are oriented to the left?

More domains are oriented towards the left.


What do you conclude?

Domains are originally pointed in random directions. When you subject them to a magnetic field, they orient
in the
same direction as the field.

................................
................................
................................
................................
..............................


Electromagnets
:

Go back to the PhET “Faraday’s Electromagnetic Lab” simulations. Click on the “Electromagnet” tab. You should observe
something similar to what is in the Figure. Describe what you observe and compare it

to what you have observed with a bar
magnet?

The magnetic field pattern is the same as the magnetic field pattern due to a bar magnet. In this case (the default) the righ
t hand
side of the coil acts as a north pole.


Use the slider on the battery to chang
e the type of current flowing through the coil. (1) What happens when the current is
reduced? (2) What happens when the current is zero? (3) What happens when the current is reversed?

(1) The strength of the
magnetic field diminishes, (2) the coil stops fr
om acting as a magnet, (3) the poles are reversed, the right hand side of the coil acts now as a
south pole.


Inducing Electricity
:

Click on the “Pickup Coil” tab. You should observe something similar to what is in the Figure.
You can move the coil or the
magnet; you can also change the strength of the magnet and flip its polarity. You can change the loop area and the number of
loops. Make all of these changes, one at a time
.
Which of these changes result in the light bulb giving off light?

All of these
cha
nges result in the light bulb giving off light.

ELECTRICITY AND MAGN
ETISM

2



For how long does the bulb give off light?

The light bulb gives off light only when the change is occurring.

................................
........

What do you conclude (seek help from the instructor in discussing magnetic flux.)

The light bulb
gives off light whenever a change
of magnetic flux occurs. The amount of light produced depends on the amount of flux change and the rate at which this change
occurs. The
faster the change, the more light is giving off.

Transformer
:

Now c
lick on the “
Trans
former
” tab. You should observe something similar to what is in the
left
Figure.
This simulation
illustrates the concept of a transformer.
You can move
either of
the coil
s
; you can also change the
current through one of the
coils

and flip its polarity. You

can change the loop area and the number of loops

for each of the coils
.
Focus on changing the
current through the first coil and describe what you observe? Does the light bulb give off light when you are not in the proc
ess
of changing the current?

The lig
ht bulb in the second coil gives off light whenever the current in the first is being changed. No light is
giving
-
off when the change stops.


Change the battery now into an AC Power supply. Use the slider in the bottom of the supply unit to change its fre
quency and
the slider to the right, to change the voltage. Describe what you observe. Can a transformer be used with circuits powered wi
th

batteries?

The AC power source results in the light bulb giving off light the whole time. However, as the AC current
alternates, the light in the
light bulb flickers. A transformer can be used only with AC currents. Due to their alternating nature, AC currents result in
a continuously
changing magnetic flux. That is why they result in the light bulb giving off light. Bat
tery powered circuits do not result in a change in the
magnetic flux. Thus, a transformer does not work for DC (battery powered) currents.

Electrical Generator
:

Now click on the “Gen
e
rator” tab. You should observe something similar to what is in the Figure
. This simulation illustrates
the concept of an
electric generator. You can change the strength of the magnet,

the loop area and the number of loops for
the
coil
.
You can also regulate the amount of water flow. Manipulate, one at a time, each of the parame
ters, and describe in your
own word the recipe for producing electricity.

Can you think of another method for producing electricity not relying on the
flow of water?

All what is needed to produce electricity is to change the magnetic flux through the coil.

The factors that contribute to higher
power output are: (1) strength of bar magnet, (2) number of coils, (3) water flow level.
Instead of water, any mechanism capable of rotating to
bar magnet will work.


Electrical Motor:

Use the provided kit to build an
d run the electrical motor. Try to explain how it works in terms of magnets attracting and
repelling
.

Ask your instructor for help.

Because of the way the wire is stripped at the end, the coil acts as an electromagnet during half
of the turn and as a non
-
m
agnet during the half. During the half turn it is magnetic, the coil gets attracted to the pole of the magnet

(the one
sitting on top of the battery)

gaining rotational speed in the process. As it turns, it stops from being attracted
to the magnet but keep
s on
rotating because of inertia. Once it makes another half turn, it is powered again. This means that it acts again as an electr
omagnet and is
attracted again to the magnet.