Transcript Diapositiva 1 - Chiara D'Amico

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
Scheda di programmazione
Diario di bordo
Test d’ingresso
Lezioni
Test di valutazione finale
Fields, field lines and field strength
Fields and field lines
The picture is on http://fleursdumall.blogspot.com/2006/11/digging-forserendipity.html
Isaac Newton's famed apple
falling from a tree led to his
musings about the nature of
gravitation …
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 Earth’s 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 Earth’s 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.
Field lines produced by a mass M:
m is the explorer and M is the source
Field lines between two masses
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.
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 Earth’s 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.
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.)
Gravitational field strength, g
We define field strength at a point in a body’s field as the gravitational force
exerted on an object placed at that point, per kg of the object’s mass. In
other words, it is just the number on newtons of attractive force acting per
kg of the object’s 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 Newton’s 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/activity/education/Teaching_Resources/Teaching%20Advanced%20Physics/Fields/Grav
itational%20Fields/page_4791.html
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 field’s concept can be understood using an elastic deformable
membrane: the small ball goes towards the big metallic ball.
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.
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/education/fieldsk/compass/compass.html
Poem:
Compass Guide
How do we know
Which way to go?
Look at the magnet
and it will show.
North, south, east or west,
For finding directions it is the best.
How does it work?
It’s as simple as can be.
The planet’s biggest magnet is itself, you see.
The biggest, and strongest magnet of all
Compared to it, all others are quite small.
Because of its size, its pull is so strong
that all other magnets are pulled along.
Try as they might, for all that they’re worth,
Magnets can’t help but point toward north.
So the next time you’re lost
without a clue,
Let a magnet find your way
to rescue you.
Gareth Wicker
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.
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).
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.
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.
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 .
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.
Look at: Physics 231 Lecture Notes - YF Chapter 25.pdf
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.
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/Chapter%2022/22%20Chapter.ppt )
The water’s flow is given by the difference of pressure (Tevin’s law). The electron’s
flow is given by an analogous reason called potential difference (d.d.p.)
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.
Simple Circuit
http://www.ac.wwu.edu/~vawter/PhysicsNet/T
opics/DC-Current/WaterFlowAnalog.html
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/HighSchool/Magnetism/magnetismintro.htm
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/HighSchool/Magnetism/magnetismintro.htm
Let’s consider two concept maps made with Cmap
Tools: click on the following icons
Some experiments:
The needle
follows the
magnet because
it’s 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.
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.
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
… and now let’s read some page of
In particular the “Undulating aluminum strip”
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.
Since in laboratory there are only two horseshoe magnets, different in
size, and there isn’t 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
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
Since we don’t 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
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_lorentz.htm
Let’s see two applets that we made with
“GIF Movie Gear”
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
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
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)
Let’s 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
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.
Watch the video
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.
The copper coil
The strength of Lorentz force put in rotation the copper coil, which is free to rotate.
Video 1
Video 2
Check Your
Understanding:
Click on the
following icon
Let’s consider some concept maps made with
Cmap Tools: click on the following icons
If you want click on the following icon:
it’s a link for a glossary that we done during the lessons