magnetism

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2. A powerful magnetic field surrounds the earth, as if the planet has an enormous bar magnet embedded within its interior. The S and N on the magnet indicate the orientation of the earth's magnetic field. Because the opposite ends of magnets attract, the northern end of magnets on the earth are attracted to the southern end of the earth's magnetic field, which is called magnetic north. Scientists believe that convection currents of charged, molten metal circulating in the earth's core are the source of the planet's magnetic field.

The orientation of the earth's magnetic field at any location can be observed by suspending a bar magnet from a string so that the bar magnet is free to rotate. The magnet will align itself with the direction of the earth's magnetic field. The angle that the magnetic field makes with the horizon is called the magnetic dip. For this reason, the magnetic pole is sometimes called the dip pole or the magnetic dip pole.

The earth's magnetic poles can be located using a compass. A compass is a small bar magnet, called a needle, suspended such that it is free to rotate in the horizontal plane but not in the vertical plane. The north magnetic pole is located by following the north-pointing end of the compass needle, the south magnetic pole by following the south-pointing end of the compass needle. At a magnetic pole, the magnetic field is vertical and the compass needle does not indicate any particular direction long the ground. From any point on earth, a compass can be followed to the north and south magnetic poles.

3. One of the first things we note in examining a bar magnet is that it has two region of magnetic concentration: poles at the ends of the magnet. We call these two poles north seeking N, and south seeking, S, since a compass needle was one of the earliest applications of magnetism. The N pole of a magnet, like a compass, points north, and the S pole points south.

Like poles repel each other.

Unlike electric charges, the magnetic poles can not be isolated. If a magnet is broken into two pieces, each piece will still have two poles.

4. There is a magnetic field around a magnet which looks somewhat like the electric field around a "dipole", a pair of positive and negative electric charges.
A magnetic field is a set of imaginary lines that indicates the direction a small compass needle would point if it were placed at a specific spot. If you are to look at a magnetic field, the arrows in the field lines indicate the direction in which a compass N pole would point. The closer together the field lines, the stronger the magnetic force is acting on the imaginary compass.

5. Magnetic fields and electric fields are both due to electrical charges. Historically, they were treated as distinct phenomena. They are, however, not different physical things. The electric fields were due to stationary charges. Magnetic fields are due to currents (moving charges).
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The interaction of electrical and magnetic effect is known as electromagnetism. We can summarize the two basic principles of Electromagnetism as follows:
1.Moving electric charge creat magnetic fields.
2.A magnetic field may deflect a moving charge.
One of the most significant application of this principle is called an electromagnet. An electromagnet consists of a current-carrying coil of insulated wire wrapped about a piece of soft iron. At what time the current is turned on, a magnet field is created inside the coil. The soft iron is magnetized by this field and makes the magnetic field nearly 2000 times stronger. When the current is turned off, the iron loses nearly all of its magnetism.

6. Not only can a magnetic field be produced by a magnetic body, but it can also be produced by a current. The result of moving charges (as current) in a wire, for instance, is a magnetic field around it. The magnetic field produced is perpendicular to the wire, and therefore the current (as the current necessarily moves along the wire), and can be found by simply using your right hand.

Make a "thumb’s up" sign with your right hand. Point your thumb in the direction of the (conventional) current. The direction outwards that the rest of your fingers coil is the direction of the magnetic flux lines perpendicular to the wire. Because, when viewed as a cross-section, a wire is simply a point, the flux lines are in the form of concentric circles. As in this diagram:

A dot indicates current flowing towards you (out of the cross-section of the wire), while a cross indicated current flowing away from you (into the cross-section). Use your right hand method to match the magnetic flux lines shown in the diagram.

Because a current creates a two-dimensional magnetic field around a wire, we can also use a wire to make a magnet. A solenoid is a coil of wire, which turns into a magnet when a current flows through it. The field patterns of a solenoid are similar to those around a normal bar magnet, except that flux lines also flow through the center of the coil. As with a bar magnet, a solenoid has poles, which can be identified using a simple method. Looking in at either end of a solenoid, determine which way (clockwise or counter clockwise) the current flows. A clockwise current indicates a south pole, and a counter clockwise current is a north pole. The powers of a solenoid can be extended to create other magnets. If a metal core is placed inside the coils of a solenoid, it turns into an electromagnet when the current is switched on. If the core is of a magnetic material such as iron or steel, the dense magnetic flux lines that flow through the center of a solenoid will magnetize the piece of metal. The effects of this will often last even after the current is switched off!

As the two wires of the previous diagram move together, their flux lines overlap: This effectively strengthens the magnetic field in between the two wires. Remember though, the diagrams do not have all the possible flux lines drawn in. In a scale diagram, the concentric circles would extend much farther from the center. In fact, on the far sides of the pari of wires, flux lines from both would meet once again. These lines would be in opposite directions, and therefore cancel each other out. The result of having an increased magnetic field between them, and a decreased magnetic field on their outsides pushes the two current-carrying wires back out to a more stable position.

This is one of the few cases in electromagnetism where opposites do not attract! When two wires with current in the same direction are moved together, the opposite happens. Flux lines in the middle are in opposite directions, and so cancel out, whereas lines on the outside are increased. The result is that the two wires move even closer.

When a current-carrying wire is placed in a magnetic field, as in the diagram, it too is forced to move.

As the diagram shows, magnetic flux line are increased on the right side of the wire, but on the left side, they are canceled out. The result is that the wire gets pushed out of the field, towards the left. We do not consider any other part of the circular flux lines of the wire except those in the same line of action as the magnet’s field lines. This is because the wire’s field lines become more and more perpendicular to the magnet’s field lines, until they have no effect on each other. If you were to reverse either the direction of the current in the wire, or the poles of the magnet, the wire would move out to the right instead.

7. The movement of a current-carrying wire from a magnetic field is called the motor effect. You can determine which way the wire will move by using your left hand:

Spread your first three fingers (including the thumb) apart so that they are at right angles to each other. Keeping them in position, point your first finger in the direction of the field, and your second finger in the direction of the electron current. Your thumb will then tell you the direction the wire will move. This rule is called Fleming’s Right Hand Rule. The actual force on a current-carrying wire in a field is dictated by a simple equation, the variables of which are the amount of current in the wire(I), the length of wire perpendicular to the field lines, and the magnetic field strength (B):
The direction in which the magnetic force pushes is perpendicular to the direction in which the particle moves and to the direction of the magnetic field. Each charge is acted on and hence the resultant superimposes.

Force is measured in Newtons, current in Amps, magnetic field strength in Teslas (T), and length in meters. q is the angle the wire is from the magnetic field’s line of action.

9. The amount of current in a coil directly relates to magnetic field strength.
Moving charges create magnetic fields. Each electron moving in a conductor creates its own magnetic field. As electrons move through the coil of wire, the magnetic field of one electron adds to the field of any others moving in the same direction. The faster a charge moves, the stronger the magnetic field it creates. For this reason alone, a higher current implies an electron is moving faster, and as a result, it would create a stronger magnetic field.

The number of turns of wire directly relate to the strength of the magnetic field.
Since the magnetic field is created by moving electrons we could argue that the more electrons are moving, the stronger the magnetic field would be. A given length of wire contains a certain number of electrons. Twice that length will contain twice as many electrons. If a solenoid is made with more "turns" or "wraps" of wire, then it must create a stronger magnetic field.

The higher the magnetic permeability, the stronger the magnetic field.
Some materials are more susceptible to magnetic fields than others. We say they are more "permeable" to the magnetic field. .

10.

In this movie you see two charges of equal mass moving with equal velocities through a magnetic field. The magnetic field (B-field) is pointing into the screen. The charge on the upper mass is five times greater than the charge on the lower mass. The greater charge causes the force on the charge to be greater which forces the mass into a smaller, tighter circular path. The greater the force on the charge, the greater the centripetal acceleration experienced by the charge.

11. The Factors affecting the force on a charged particle moving through a magnetic field arise from the formula F = Bvq.
To increase the force on the particles either, increase the magnetic field strength through the use of a solenoid, increase the velocity of the particle (greater potential difference in electron gun) or increase the charge of the particle by using a di or tri-charged particles instead of a mono-charged one.

In such a situation the magnetic force serves to move the particles in a circular path. According to the "right hand rule" the magnetic force acting on the particle always remains perpendicular to its velocity. The magnitude of the magnetic force is F = q V B , where q is the magnitude of the charge of the particle, V its velocity, and B is the magnetic field. This force can be also considered as the centripetal force Fc = m V2 / R , where m is the particle's mass and R is the radius of the circular tragectory.Making two above expressions equal to each other and solving the resulting equation for R we can easily find that the radius R of the circular path is proportional to the velocity of the particle. R = m V / q B.

12. A charge of 1.5 x 10 - 10 C moves at 1.0 km/s through a magnetic field in such a direction that the magnetic force on it is a maximum. What is the magnetic field strength if the maximum force on the charge is 8 x 10 - 8 N?
                                                        F = q v B sin
                                                         Fmax = q v B
                                                         B = Fmax/[q v]
                                            B = 8 x 10 - 8 N / [(1.5 x 10 - 10 C)(1 000 m/s)]
                                                          B = 0.533 T

An electron moves in a circular path, perpendicular to a magnetic field of B = 1.25 T, with a a speed of 1.2 x 104 m/s. What is the radius of the electron's circular path?
                                                     Fmag = q v B sin
                                                     Since = 90, sin = 1
                                                     Fmag = q v B
                                                     This is also the centripetal force,
                                                        Fc = m v2 / r
                                                        q v B = m v2 / r
                                                         r = m v / [q B]
                                       r = (9.11x10-31 kg) (1.2x104 m/s) / [(1.6x10-19 C) (1.25 T)]
                                                        r = 5.47 x 10-8 m

14. A motor is simply a coil (usually, several coils) of wire placed in a magnetic field. Current flows in and out of the circuit by a special device called a split-ring commutator that keeps the coil of wire from ever getting twisted or tangled. The continuous movement created is used for many things in everyday life.
A simplified diagram of a motor:

The two blue half-circles make up the split-ring commutator, and the two green parts are the carbon brushes, not attached, only touching the commutators, through which the current enters the circuit. In the circuit shown, conventional current enters from the right, goes through the left side of the commutator, and enters the ring of wire (shown in red). The current is now inside a field, so the wire carrying it must move, due to the motor effect. Using Fleming’s Left Hand Rule, we find that the wire goes down. Now let’s take a look at the other side of the loop. The field is still going in the same direction, but this time the current is going in the opposite direction. This means that this part of the wire moves up to escape the field. The wire has now completed a one-quarter turn. But since the commutators are attached to the wire loop, they have also completed quarter turn. At this point, it is the two slits, separating the two sides of the commutators, that are lined up with the carbon brushes. This prevents any current from entering the loop, but momentum keeps the loop rotating clockwise. Now the side of the commutator that was on the right is on the left, and vice versa. But current still enters through the right carbon brush (and leaves through the left). The new right side goes down, and the new left side of the loop goes up, and the turning continues.

It is important that the current entering the motor is D.C. because this means that the positive side of the power source is always positive. Were A.C. being used in the motor, the wire loop would get a quickly changing current input, which wouldn’t allow the loop to move very far in either direction. In reality, the motor would have many, many more coils in the wire, and the magnet would be much closer-fitted to the coil.. There would also be an axle attached throught the center of the coils of wire, so that the turning of the motor could be used, for instance, to turn a drill, or an electric fan.

15.

In this movie you see an electron moving and then entering a magnetic field (B-field). The direction of the B-field is into the screen. Notice that the force created by the magnetic field ends up always pointing to the center of the curve. This force is considered to be a centripetal force because it causes the electron to travel in a circular path.
If the charge on the electron were greater, the force would be greater.
If the speed of the electron were greater, the force would be greater.
If the magnetic field strength were greater, the force on the electron would be greater.
If the mass of the electron were greater, it would have no effect on the force, but the circular path would be larger.

The screen of your computer (unless it is an LCD) and your television screen both use magnetic fields to deflect electrons fired from an "electron gun". The moving electrons are directed toward different areas of the screen by magnetic fields created by electromagnets. Wherever the electrons strike the screen, they cause phosphorus to give off light. If millions of electrons are directed to the screen in the right places, the little bursts of light leave an impression our eyes which forms an
image. An instant later (1/24th of a second) a million more electrons strike the screen and form a new image. Our mind pieces them together to create the impression of smooth motion.

16. Magnetic flux (FB and SI unit is the Weber, Wb) the number of magnetic field lines that pass through a surface of area A. A changing magnetic flux produces an
electric field. This is true not only of wires and conductors, but also applies to any region in space. We can better understand or describe a "moving magnet" or a "changing magnetic field" in terms of magnetic flux. We define the magnetic flux as

You might think of the "air flux" as air blows through a window. The size of the window (A), the speed of the air (B) and the direction (theta) all determine how much air comes through the window.

17. There are two ways that electricity and magnetism are related: an electric current produces a magnetic field and a magnetic field exerts a force on an electric current or moving charged particle. Henry and Faraday independently found found that a current could be induced in a wire by moving it in a magnetic field. An electric current is generated in a wire when the wire cuts across magnetic field lines. Faraday found that a steady magnetic field does not produce any current, only a changing magnetic field produces an electric current.

Faraday's Law : The potential difference induced across the ends of a coil of wire is equal to the time rate of change of the magnetic flux through that coil of wire. This potential difference is known as an induced "electromotive force" or as an induced emf.

The flux through a loop of wire can change due to many different situations.

Anything that causes a change the the flux through a loop of wire produces a potential difference across the ends of the loop. If there are N loops, the potential
differences across each of the loops will be added and the total potential difference will be N times that across the ends of a single loop.

Stated differently, Faraday's Law of Induction Faraday found that the amount of emf induced in a coil of wire depended upon how rapidly the magnetic field changes in the coil of wire. The faster the magnetic field changes, the greater the induced emf. If the flux through a coil of N loops of wire chagnes by an amount FB during a time Dt. e = - N (FB/Dt)
The negative sign indicates the direction in which the induced emf acts. For our purposes, we will use Faraday's law to calculate the magnitude of the induced
emf and apply right hand rules for Lenz's Law to determine the direction of the induced emf.
An emf can be induced three ways:
   1.By a changing magnetic field
   2.By changing the area of the loop in the field
   3.By changing the loop's orientation with respect to the field

18. An electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it. These opposing fields occupying the same space at the same time result in a pair of forces.   These forces are felt when you turn a generator and generate electricity. The more current you generate, the greater the force opposing you.
To predict the direction of the induced current using Lenz's Law:
        1.Determine whether the magnetic field strength is increasing or decreasing.
        2.Determine the direction in which the original field enters the coil.
        3.Determine the direction of the induced magnetic field so that it opposes the change in the magnetic flux.
        4.Use RHR to predict the direction of the current knowing the direction of the induced magnetic field.

The direction of an induced current is such that the magnetic field it produces (the induced magnetic field) will oppose (or try to compensate for) the change in the magnetic field which causes the induced current and induced magnetic field. That is the meaning of the "minus sign".

Lenz’s Law helps to explain what happens when a magnet is passed through a solenoid. As the magnet is pushed into the solenoid, the current induced in it creates
an electromagnet out of the solenoid; an electromagnet that repels the magnet inside of it already. This means you have to do work against the repulsive force in
order to get the magnet through. However, once the magnet leaves the solenoid, you might expect that the repulsive force would continue. Unfortunately, this is not
the case. When the magnet has left the solenoid, the current induced in the coils is such that an attracting electromagnet is created. This means that work has to be
done against an attractive force, in order to move the magnet away from the solenoid.

19. A current can be induced in a coil by rotating it in a magnetic field. To help concentrate the magnetic flux, the coil is wound on a soft iron core. As the coil rotates, its individual conductors cut across the lines of force and an EMF is induced in them. The induced EMF's in each turn of the armature add up and hence set-up a potential difference between the ends of the coil. The slip rings make contact with two carbon or copper brushes that connect them with an external circuit, supplying it with electrical energy. A generator that produces an alternating current produces an alternating EMF. The armature cuts the flux lines fastest when approaching its horizontal position. The first half turn of the armature rises form zero to a maximum point and then drops back to zero.

When a wire, connected to a sensitive ammeter, is moved through a magnetic field, an E.M.F. is induced. Since the wire is part of a closed circuit, providing a path of conduction, a current will flow in the wire (which is detected by the ammeter). However, the current only flows as long as the wire is moving in the field; once it has left it, it ceases to flow. Also, a slower-moving wire in a field produces less current. To find the direction of the current induced, you only need your hand, but this time, it is the right hand.

Once again, the thumb stands for the motion of the wire, the first finger points in the direction of the field, and the second finger will tell you which way the current will flow in the wire. Remember to hold the three fingers so they are all at right angles with each other. This rule is Fleming’s Right Hand Rule.

In this diagram, the red wire (in a properly closed circuit, of course) is moved up through the field. By using the Right Hand Rule, we find that the current induced in the wire flows towards the back of the picture. But wait! What happens when the Left Hand Rule is used, now that we know the direction of the current? We find that when the current flowing in the wire exists in the magnetic field that induced it, the new motion of the wire would be opposite to the motion it was actually moved!. Thus, the current produced is in a direction that actually opposes the change (this case, the motion) producing it. This is Lenz’s Law.

Let’s just say, for a moment, that the opposite was true. By moving a wire through a field, a current would be produced. This current, being in a field, would continue moving the wire through the field (instead of trying to move it in the opposite direction). This, in turn, would produce a current . . . In short, what would have been created is a self-perpetuating system that continues increasing in both motion and current, both of which require a lot of energy. This is where our theory fails. We forget a one of the most important laws of physics, the law of conservation of energy. What we would be doing is creating infinite energy from nothing but merely inching a wire through a field. Since energy cannot be created nor destroyed, this entire situation is impossible.

As we turn the handle in the picture, the wire loop rotates (note: this is in the opposite direction as the last use). This moves the wires through a field. Because of the split-ring commutator, the side of the loop on the left always produces current flowing out of the loop, and the side on the right always has current flowing in. This means that a D.C. current is produced. A.C. current can also be produced, using two slip rings instead of the split ring commutator. Each side of the wire loop is connected to a separate ring, which is in turn connected to a brush contact. As a side of the loop rotates, it moves first in one direction through the field, than in the opposite. This produces an A.C. output at each ring, and therefore for the entire circuit.

An E.M.F. (voltage), as well as a current, is produced by this simple generator, in a process called electromagnetic induction. This voltage, also called an induced E.M.F., is then used in the circuit to light the light bulb. The voltage is produced because there is mechanical power going into the handle of the generator.

When a conductor passes through a magnetic field, a voltage is induced across the ends of the conductor. The generator is simply a mechanical arrangement for moving the conductor and leading the current produced by the voltage to an external circuit, where it actuates devices that require electricity. In the simplest form of generator the conductor is an open coil of wire rotating between the poles of a permanent magnet. During a single rotation, one side of the coil passes through the magnetic field first in one direction and then in the other, so that the induced current is alternating current (AC), moving first in one direction, then in the other. Each end of the coil is attached to a separate metal slip ring that rotates with the coil. Brushes that rest on the slip rings are attached to the external circuit. Thus the current flows from the coil to the slip rings, then through the brushes to the external circuit. In order to obtain direct current (DC), i.e., current that flows in only one direction, a commutator is used in place of slip rings. The commutator is a single slip ring split into left and right halves that are insulated from each other and are attached to opposite ends of the coil. It allows current to leave the generator through the brushes in only one direction. This current pulsates, going from no flow to maximum flow and back again to no flow. A practical DC generator, with many coils and with many segments in the commutator, gives a steadier current. There are also several magnets in a practical generator. In any generator, the whole assembly carrying the coils is called the armature, or rotor, while the stationary parts constitute the stator. Except in the case of the magneto, which uses permanent magnets, AC and DC generators use electromagnets. Field current for the electromagnets is most often DC from an external source. The term dynamo is often used for the DC generator; the generator in automotive applications is usually a dynamo. An AC generator is called an alternator. To ease various construction problems, alternators have a stationary armature and rotating electromagnets. Most alternators produce a polyphase AC, a complex type of current that provides a smoother power flow than does simple AC. By far the greatest amount of electricity for industrial and civilian use comes from large AC generators driven by steam turbines.

This current changes direction every 180 degrees, producing alternating current (AC current).

20. Transformers use induction to change the voltage of AC electricity.
In a transformer, the voltage across a coil is proportional to the number of turns in that coil.

A transformer is an essential part of power transportation, and plays an important role in getting the right electricity into your home. A transformer also works along
the principle of electromagnetic induction. This is the general structure of a transformer:

1. A U-shaped soft iron core, which is easily magnetized, sometimes with a further bar of iron across the top of the U.
2. Two separate solenoids coiled around opposite sides of the U

When an A.C. current is passed through one of the solenoids, a magnetic field is produced, but since the current is A.C., this magnetic field is constantly changing.
As stated by Faraday’s Law, a changing magnetic field induces an E.M.F. in a conductor, this case the second solenoid. The current in this solenoid is once again
A.C. Because energy is always conserved (in a 100% efficient transformer), the input power of a transformer is always equal to the power output. It is the coils of
the two solenoids that allow both the current and the voltage of the input power to be transformed, according to this equation:

where NP is the number of coils on the primary solenoid, NS is the number on the secondary solenoid, and VP and VS are their respective voltages.
And, since PP = PS, (due to the law of conservation of energy)

A changing magnetic filed will create an EMF in a nearby coil. Transformers make use of this principle by creating a continually changing magnetic field in a primary coil which in turn creates a changing EMF in a secondary coil. If the current in the primary coil is increasing steadily, then a constant current in the opposite direction is induced in the secondary coil. The purpose of a transformer is to allow the efficient change of ac voltage, either up to a higher or down to a lower voltage. The relative number of turns in the primary and secondary coils determines the ratio of input voltage to output voltage. The use of a laminated soft iron core, onto which both primary and secondary coils are wound, increases the efficiency of a transformer. The iron core increases the flux link between the coils. This allows the flux produced by the current in the primary coil to link with the 2nd coil. The laminations reduced eddy currents and minimise energy lost in the form of heat.

21. ELECTRIC POWER THE SMALL SCALE
The amount of power used is measured in kilowatt hours by a watt hour meter. 1 kilowatthour is the amount of energy used by a 1 Kw device in one hour. eg 2kw for 5 hrs = 10kwh. Power stations usually charge approx 12-15 cents per kwh (3.6 MJ) compared to gas at .7 cents per MJ.

WA POWER GENERATION
Muja (near collie) is the largest station producing at a capacity of 1040MW. 11000 tonnes of coal per day used. Produed at 11800 - 16000V but stepped up to 330000V. Kwinana is perths main station producing at a capacity of 900MW but supplies 33 of western power's requirements. Kwinana is special as it can use any of the three main fuels gas, coal, light.

All high voltage transmissions we see have 3 conductors. The lines on the street have at least three lines for this reason however the line to the houses are only two.
Power generated in 3 phase as it is more efficient. 3 phases evens the forces on the rotor. the lines active, neutral, and earth. the earth grounds the current, the active supllies current and the neutral completes the circuit.

USING ELECTRICAL ENERGY
The amount of electrical energy needed can only be supplied by power stations near a primary source (coal or hydro). Although wind wave or solar can be adapted. the transmission of power over long distances is very importantfor australia as there are large distances between populated areas. Eg melbourne needs 5000MW at peak times. At 250V the current would need to be 2 x 10^7 which is not possible. The answer to this is to raise the voltage. as P=VI enlarge V to have high power.

SAFTEY
Over the past decade electrical appliances has increased but deaths have dropped. This is due to the use of earth leakage detectors. The appliance is connected to an active and neutral which are normally equal. However during a fault and a current could flow through you the current in the neutral and actives would not be equal the detector would realise this a shut down all power. It cant eliminate all chances but it does reduce it.

Electrical energy is transmitted over long distances using thick inexpensive wires of low resistivity at high voltage and low alternating currents since power loss is essentially due to P= I (IR). Low resistivity means low resistance hence low power loss. Thick wires have a large cross sectional area which leads to low resistance and reduced loss of power. Since P=VI Increasing voltage will mean a reduction in current hence low power loss as less electrical energy is being converted into thermal energy. Alternating current can be transformed using tranformers as the constantly changing B induces an emf and its safer. The fact it can be stepped up or down means that high voltages reduces current, hence power loss. Inexpensive wires is cost effective.