Progetto PON 1.4 L

Lingue comunitarie e tecnologie per la formazione dei docenti di discipline

scientifiche

ISTITUTO SUPERIORE STATALE "ALFANO I"

Via dei Mille - SALERNO

Scheda di programmazione 

Diario di bordo 

Test d’ingresso 

Lezioni  

Test di valutazione finale

Fields, field linesFields, field lines and field strengthand field strength

Fields and field linesFields 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 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. Isaac Newton's famed

apple falling from a tree led to his musings about

the nature of gravitation …

The picture is on http://fleursdumall.blogspot.com/2006/11/digging-for-serendipity.html

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 between two masses

Field lines produced by a mass M: m is the explorer and M is the source

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: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, gGravitational 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/mg = 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/Gravitational%20Fields/page_4791.html

A little more about the concept of fieldA 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.

Magnetism is a force of nature, like gravity. But it is quite different from gravity in many respects.

A brief introduction to magnetism

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 magnetand 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 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 they’re worth,Magnets can’t help but point toward north.So the next time you’re lostwithout a clue,Let a magnet find your wayto 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).

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.

ELECTRICITY AND MAGNETISM

Before studying what is the effect of magnetism on electrical current we want to linger over the meaning of electric current.

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 .

Flow of chargeFlow of charge

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.

A model for a Simple Circuit

Look at: Physics 231 Lecture Notes - YF Chapter 25.pdf

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.

Electric Circuits

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).

Voltage

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.)

The article above is on: electricity.pdf (http://www.science.glencoe.com - www.pittcentralcatholic.org/faculty/lhorner/Chapter%2022/22%20Chapter.ppt )

Simple Circuit

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.

How a current flows

http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/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 Check Your Understanding:Understanding:

Click on the Click on the following icon 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 stripUndulating aluminum strip

Explanation: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 ForceMagnetism and the Lorentz ForceWhen 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 fieldMoving charged particle in magnetic fieldA 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 = qvBwhere:

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 wireIf 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 = BILwhere:

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 RuleRight 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.

Video 1 Video 2

The strength of Lorentz force put in rotation the copper coil, which is free to rotate.

The copper coil

Check Your Check Your Understanding:Understanding:

Click on the Click on the following icon 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