Diapositiva 1

1
ISTITUTO SUPERIORE STATALE "ALFANO I" Via dei
Mille - SALERNO
Progetto PON 1.4 L Lingue comunitarie e
tecnologie per la formazione dei docenti di
discipline scientifiche
2
Scheda di programmazione  Diario di bordo 
Test dingresso  Lezioni   Test di
valutazione finale
3
Fields, field lines and field strength Fields and
field lines
When you pick up an object such as a pen, there
is direct contact between you and the pen. This
direct contact exerts a force on the pen, causing
it to move in the way that it does. However, the
pen also has a weight due to its presence in the
Earths gravitational field. How is this force
exerted, even when there is no direct contact
between the Earth and the pen? A force is exerted
on the pen from the Earth because the pen is in
the Earths gravitational field. We can define
the field due to a body as the region of space
surrounding it where other bodies will feel a
force due to it.
The picture is on http//fleursdumall.blogspot.com
/2006/11/digging-for-serendipity.html
Isaac Newton's famed apple falling from a tree
led to his musings about the nature of
gravitation
4
The gravitational force is infinite in range,
although it becomes very weak at large distances
as it is an inverse square law. The gravitational
field due to a body is thus also infinite. We
cannot see or touch this field, but we can try to
model it using field lines or lines of force. In
a field line diagram, the direction of the field
line at a point gives the direction of the force
of attraction that would be felt by a small mass
placed there. The relative density of field lines
on the diagram is an indication of the strength
of the field.
Field lines produced by a mass M m is the
explorer and M is the source
Field lines between two masses
5
Thus for a spherical mass, like the Earth, we
would have the following diagram
The field lines are directed radially inwards,
because at any point in the Earths field, a body
will feel a force directed toward the centre of
the Earth. The field lines become more spread out
as the distance from the Earth increases,
indicating the diminishing strength of the field.
Close to the surface of the Earth, the field
lines look like
They are directed downwards and they are parallel
and equidistant indicating that the field is
constant, or uniform.
6

• A couple of important points to note
• Field lines do not start or stop in empty space
  (even though on diagrams they have to stop
  somewhere!). They end on a mass and extend back
  all the way to infinity.
• Field lines never cross. (If they did, then an
  object placed at the point where they crossed
  would feel forces in more than one direction.
  These forces could be resolved into one direction
  the true direction of the field line there.)

7

• Gravitational field strength, g
• We define field strength at a point in a bodys
  field as the gravitational force exerted on an
  object placed at that point, per kg of the
  objects mass. In other words, it is just the
  number on newtons of attractive force acting per
  kg of the objects mass. Since the attractive
  force is simply what we call weight, we can write
  this as
• g W/m
• where W weight in newtons. Thus g has units
  N/kg.
• We can use this definition to get an equation for
  g using Newtons Law of Universal Gravitation.
  The attractive force of a mass M (causing the
  field) on a mass m a distance r away is simply
  GMm/r2. Thus the attractive force per kg of mass
  of the object (mass m) is (GMm/r2)/m.
• Thus,
• g GM/r2
• This gives an expression for the field strength
  at a point distance r from a (point or spherical)
  mass M.
• The gravitational field strength at a point in a
  field is independent of the mass placed there
  it is a property of the field. Thus, two objects
  of different mass placed at the same point in the
  field will experience the same field strength,
  but will feel different gravitational forces.

The article above is on http//www.iop.org/activi
ty/education/Teaching_Resources/Teaching20Advance
d20Physics/Fields/Gravitational20Fields/page_479
1.html
8
A little more about the concept of field
The previous slides showed that the word field
refers to a modified space. When we put an
explorer mass in the field, the mass is subjected
to a force and the space is called gravitational
field. What do we use to prove that an area of
the space is a magnetic field? The immediate
answer is a needle compass in the area that we
will explore. If the compass orientates itself a
preferential direction, in that place there is a
magnetic field.
The fields concept can be understood using an
elastic deformable membrane the small ball goes
towards the big metallic ball.
9
A brief introduction to magnetism
Magnetism is a force of nature, like gravity. But
it is quite different from gravity in many
respects.
Imagine yourself far out to sea, no land in
sight, sailing in a small ship. During the day,
you navigate by the sun and at night by the
stars. Then it becomes overcast for several long
days. I'll bet you wish you had a compass...
The interesting magnetic properties of
lodestone, a mineral known as magnetite to
geologists, have been known since the time of the
ancient Greeks. It wasn't until centuries later
when mariners in China (and, by the 12th century,
mariners in Europe) noticed that a piece of
lodestone, when floated on a stick in a bowl of
water, aligned itself to point in the direction
of the north star. This was a discovery which
revolutionized the world since it allowed for
improved seafaring navigation and exploration.
This simple discovery has been developed, over
time, into the modern compass.
10

• Compasses work because the earth acts like a
  giant bar magnet. Magnetic lines of force connect
  the earth's north and south magnetic poles as
  show below

Compasses work because a magnetized compass
needle will align itself with the earth's
magnetic lines of force and point approximately
north. I said approximately because you'll note
in the figure above that the north and south
magnetic poles don't exactly align with the
earth's axis of rotation which defines the north
and south geographic poles.
The article above is on http//earthsci.org/educa
tion/fieldsk/compass/compass.html
11

• Poem
• Compass Guide
• How do we know Which way to go?Look at the
  magnetand it will show.North, south, east or
  west,For finding directions it is the best.How
  does it work?Its as simple as can be.The
  planets biggest magnet is itself, you see.The
  biggest, and strongest magnet of allCompared to
  it, all others are quite small.Because of its
  size, its pull is so strongthat all other
  magnets are pulled along.Try as they might, for
  all that theyre worth,Magnets cant help but
  point toward north.So the next time youre
  lostwithout a clue,Let a magnet find your
  wayto rescue you.
• Gareth Wicker

12
A compass tells you what direction is 'North',
but have you ever wondered how it can do that?
The answer has to do with something called
magnetism. Every magnet produces an invisible
area of influence around itself. When things made
of metal or other magnets come close to this
region of space, they feel a pull or a push from
the magnet. Scientists call these invisible
influences FIELDS. You can make magnetic fields
visible to the eye by using iron chips sprinkled
on a piece of paper with a magnet underneith.
13
Magnetic field lines are imaginary lines used to
map magnetic fields (just as lines of latitude
and longitude are imaginary lines mapping the
face of the Earth). They follow the direction
of a compass needle freely suspended in 3
dimensions. Michael Faraday originally named
them "Lines of Force." They may have convinced
him that space around a magnet was somehow
modified, leading to the concept of fields,
regions of modified space.
The fact that the North Pole of a compass needle
turns towards the North of the earth shows that
the Earth itself behaves like a magnet whose
North and South Poles are respectively in
proximity of the geographical South and the
geographical North.
14
ELECTRICITY AND MAGNETISM
Before studying what is the effect of magnetism
on electrical current we want to linger over the
meaning of electric current.
Flow of charge
An electric discharge, such as a lightning bolt,
can release a huge amount of energy in an
instant. However, electric lights, refrigerators,
TVs, and stereos need a steady source of electric
energy that can be controlled. This source of
electric energy comes from an electric current,
which is the flow of electric charge. In solids,
the flowing charges are electrons. In liquids,
the flowing charges are ions, which can be
positively or negatively charged. Electric
current is measured in units of amperes (A) . A
model for electric current is flowing water.
Water flows downhill because a gravitational
force acts on it. Similarly, electrons flow
because an electric force acts on them .
15
A model for a Simple Circuit
How does a flow of water provide energy? If the
water is separated from Earth by using a pump,
the higher water now has gravitational potential
energy, as shown in figure. As the water falls
and does work on the waterwheel, the water loses
potential energy and the waterwheel gains kinetic
energy. For the water to flow continuously, it
must flow through a closed loop. Electric charges
will flow continuously only through a closed
conducting loop called a circuit.
16
Look at Physics 231 Lecture Notes - YF Chapter
25.pdf
17
Electric Circuits
The simplest electric circuit contains a source
of electrical energy, such as a battery, and an
electric conductor, such as a wire, connected to
the battery. For the simple circuit shown in
figure, a closed path is formed by wires
connected to a lightbulb and to a battery.
Electric current flows in the circuit as long as
none of the wires, including the glowing filament
wire in the lightbulb, is disconnected or broken.
18
Voltage
In a water circuit, a pump increases the
gravitational potential energy of the water by
raising the water from a lower level to a higher
level. In an electric circuit, a battery
increases the electric potential energy of
electrons. This electric potential energy can be
transformed into other forms of energy. The
voltage of a battery is a measure of how much
electric potential energy each electron can gain.
As voltage increases, more electric potential
energy is available to be transformed into other
forms of energy. Voltage is measured in volts
(V).
The article above is on electricity.pdf
(http//www.science.glencoe.com -
www.pittcentralcatholic.org/faculty/lhorner/Chapte
r2022/2220Chapter.ppt )
The waters flow is given by the difference of
pressure (Tevins law). The electrons flow is
given by an analogous reason called potential
difference (d.d.p.)
19
How a current flows
You may think that when an electric current flows
in a circuit ,electrons travel completely around
the circuit. Actually individual electrons move
slowly through a wire in an electric circuit.
When the ends of the wire are connected to a
battery, electrons in the wire begin to move
toward the positive battery terminal. As an
electron moves it collides with other electric
charges in the wire, and is deflected in a
different direction. After each collision, the
electron again starts moving toward the positive
terminal. A single electron may undergo more than
ten trillion collisions each second . As a
result, it may take several minutes for an
electron in the wire to travel one centimetre.
http//www.ac.wwu.edu/vawter/PhysicsNet/Topics/DC
-Current/WaterFlowAnalog.html
Simple Circuit
20
Now we come back to the magnetism to see again
something that we will deepen click on the
following website!
http//www.ndt-ed.org/EducationResources/HighSchoo
l/Magnetism/magnetismintro.htm
21
Check Your Understanding Click on the following
icon

• See answers on
• http//www.glenbrook.k12.il.us/gbssci/Phys/Class/
  circuits/u9l2c.html
• http//www.ndt-ed.org/EducationResources/HighScho
  ol/Magnetism/magnetismintro.htm

22
Lets consider two concept maps made with Cmap
Tools click on the following icons
23
Some experiments
The needle follows the magnet because its
attracted by its magnetic field.
The electric current in the wire generates a
magnetic field which attracts the needle in a
preferential direction. Changing power lines
position, magnetic field due to the current
changes its polarity and attracts the needle in
another direction.
24
The iron chips sprinkled, attracted by the magnet
underneath the paper, place making the field
lines visible. Look at the following video
A thing made of metal, like a pivot, crossed by
electric current becomes a magnet. Indeed it
attracts needle and other metallic things.
25
(No Transcript)
26
In the oil the iron chips sprinkled line up
magnetic field lines of the field due to the
magnet. Look at the following video
Click on the photo
27
and now lets read some page of
In particular the Undulating aluminum strip
28
Undulating aluminum strip
Explanation An electric conductor, in this case
the aluminium strip, perpendicular to a magnetic
field which is caused by the horseshoe magnets,
feels a force perpendicular to the current and
the direction of the magnetic field - called
Lorentz force. Depending on the polarity of the
horseshoe magnets, the aluminium strip is lifted
or pressed down. With a direct current, several
hills (depending on how many horseshoe magnets
are used) can be observed. In the case of an
alternating current, the Lorentz force impacting
on the aluminium strip changes in direction and
strength, which results in a slowly varying
wave. The distance of the horseshoe magnets
influences the shape of the observed wave.
29

• Since in laboratory there are only two horseshoe
  magnets, different in size, and there isnt an
  alternating current generator with variable
  frequency, we used the following materials
• Two horseshoe magnets
• Flexible aluminium strip, length 1 m, width
  2 cm
• Two clamps for current
• A power supply (recycled) from an old mobile
  phone (0.5 1 A)
• Two nails to connect the aluminium strip

30
In this case, acting on conductive strip the
strength of Lorenz direction upward for both
magnets.
In the event that other conductive strip acts oh
the strength of Lorenz direction upward magnet
for the right and down to the left.
Watch the video
31
Since we dont have an alternating current
generator with variable frequency, we carried out
the effect of an undulating motion by exchanging
quite fast the polarity of the alimentation.
Watch the video
32

Watch the following applets(they are on the
web) Lorentz force Electricmotor
The following contents are on the web address
www.school-for-champions.com/science/magnetism_lo
rentz.htm
33
Lets see two applets that we made with GIF
Movie Gear
34

• Magnetism and the Lorentz Force
• When an electric charge moves through a magnetic
  field, there is a force on the charge,
  perpendicular to the direction of the charge and
  perpendicular to the direction of the magnetic
  field. This force is called the Lorentz Force.
  This also applies to electric current in a wire.
  The direction of the force is demonstrated by the
  Right Hand Rule.
  Moving charged particle in magnetic field
• A moving particle with an electric charge--such
  as a proton or electron--creates a magnetic
  field. If that charge is moving through an
  external magnetic field there will be an
  attraction or repulsion force, as the magnetic
  fields interact.
• There is a relationship between the movement of
  the particle through the magnetic field, the
  strength of that magnetic field and the force on
  the particle. The following equation describes
  the force
• F qvB
• where
• F is the force in Newtons
• q is the electric charge in Coulombs
• v is the velocity of the charge in
  meters/second
• B is the strength of the magnetic field in
  Teslas
• qvB is q times v time B

35

• Current through wire
• If instead of a moving charge such as an electron
  or proton, there was electric current through a
  wire, the force would a result of the current and
  the magnetic field
• F BIL
• where
• F is the force in Newtons
• B is the strength of the magnetic field in Teslas
  
• I is the electrical current in Amperes
• L is the length of the wire through the magnetic
  field in meters
• BIL is B times I times L

Force on wire with current flowing
36
Right Hand Rule The direction of the force for a
given direction of current and magnetic field can
be remembered by the Right Hand Rule. If you took
your right hand and stuck your thumb up, your
forefinger or first finger forward, and your
second finger perpendicular to the other two,
then the directions would be as indicated in the
drawing below.
Force on moving charge through magnetic field
(Right Hand Rule)
37
Lets see some other experiments on the Lorentz
Force
The pendulum in the picture consists of a brass
bar that bound the support of wood, is free to
oscillate close to a magnet made from a hard disk
of an old computer. The ends of the bar are
connected trough appropriate copper wires that
were wrapped for some , in order to allow greater
fluctuation. The circuit is also resistance in
series to limit the current to protect the power
supply . The power supply (recycled) from an old
mobile phone. The next slide is schematized the
circuit.
The swing opendolo a pendulum
38
Closing the circuit current flowing trough the
rod generating a force perpendicular to the
direction of the magnetic field and by the
swinging bar to the right or left. To reverse the
magnet supply the strength changes towards and
the rod will swing in the opposite direction.
39
Watch the video
40
When the conductor is resting on the positive A,
since it is constructed so as to make contact in
B, it lets in a stream between the positive and
negative battery. As the current passes trough
the magnetic field generated by the magnet, the
strength of Lorentz force put in the rotation
conductor, which is free to rotate. Exchanging
the poles of the magnet inverts the direction of
rotation changing the direction of the magnetic
field changes to the strength of Lorentz force.
41
The copper coil
The strength of Lorentz force put in rotation the
copper coil, which is free to rotate.
Video 1
Video 2
42
(No Transcript)
43
Check Your Understanding Click on the following
icon
44
Lets consider some concept maps made with Cmap
Tools click on the following icons
45
If you want click on the following icon its a
link for a glossary that we done during the
lessons