Earth's Magnetic Poles Are Not the Same As It Geographic Poles

25 Electromagnetic Induction book

·  Earth's magnetic poles are not the same as it geographic poles.

·  The North magnetic pole on Earth is a magnetic S pole and the South magnetic pole is a magnetic N pole.

·  The poles on magnets have been named for the poles the end seeks. So the N poles of a magnet seeks the North magnetic pole on Earth because it is a magnetic S pole and the S poles of a magnet seeks the South magnetic pole on Earth because it is a magnetic N pole.

Summary of Terms
Electromagnetic induction / The induction of voltage when a magnetic field changes with time. If the magnetic field within a closed loop changes in any way, a voltage is induced in the loop:
This is a statement of Faraday’s law. The induction of voltage is the result of the induction of an electric field.
Faraday’s law / An electric field is created in any region of space in which a magnetic field is changing with time.
The magnitude of the induced electric field is proportional to the rate at which the magnetic field changes.
The direction of the induced field is at right angles to the changing magnetic field.
Generator / An electromagnetic induction device that produces electric current by rotating a coil within a stationary magnetic field. A generator converts mechanical energy to electrical energy.
Transformer / A device for transferring electric power from one coil of wire to another, by means of electromagnetic induction, for the purpose of transforming one value of voltage to another.
Maxwell’s counterpart to Faraday’s law / A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric field.

·  Early 1800s, the only current-producing devices were voltaic cells- currents dissolved metals in acids- batteries.

·  In 1820, Oersted found that magnetism was produced by current-carrying wires.

·  in 1831 by two physicists, Michael Faraday in England and Joseph Henry in the U S —changed the world by making electricity commonplace— discovered that electric current (voltage) can be induced and charges are set in motion in a wire simply by moving a magnet in or out of a coiled part of the wire – which is now powering industries and lighting cities at night.

Electromagnetic

·  Voltage is induced in the wire loop either when the magnetic field moves past the wire or the wire moves through the magnetic field.

·  When a magnet is plunged into a coil of twice as many loops as another, twice as much voltage is induced. If the magnet is plunged into a coil with three times as many loops, then three times as much voltage is induced.

·  It is more difficult to push the magnet into a coil made up of a greater number of loops because the induced voltage makes a current, which makes an electromagnet, which repels the magnet in our hand.

·  The amount of voltage induced depends on how fast the magnetic field lines are entering or leaving the coil. Very slow motion produces hardly any voltage at all. This phenomenon of inducing voltage by changing the magnetic field in a coil of wire is called electromagnetic induction.

Faraday’s Law
The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops.

The amount of current produced by electromagnetic induction depends

·  on the induced voltage

·  the resistance of the coil

·  The circuit to which it is connected.

·  For example, we can plunge a magnet in and out of a closed loop of rubber and in and out of a closed loop of copper. The voltage induced in each is the same, provided that the loops are the same size and the magnet moves with the same speed. But the current in each is quite different. Electrons in the rubber sense the same electric field as those in the copper, but their bonding to the fixed atoms prevents the movement of charge that so freely occurs in copper.

·  Changing a magnetic field in a closed loop induces voltage. If the loop is in an electrical conductor, then current is induced.

Voltage can be induced in a loop of wire:

·  by moving the loop near a magnet

·  by moving a magnet near the loop

·  By changing a current in a nearby loop.

·  All 3 cases possess the same essential ingredient—a changing magnetic field in the loop.

Electromagnetic induction is all around us in shake flashlights, Hybrid cars , the electromagnetic waves we call light.

Generators and Alternating Current
When one end of a magnet is plunged into and back out of a coil of wire, the direction of the induced voltage alternates. As the magnetic field strength inside the coil is increased (as the magnet enters the coil), the induced voltage in the coil is directed one way. When the magnetic field strength diminishes (as the magnet leaves the coil), the voltage is induced in the opposite direction.

·  The frequency of the alternating voltage that is induced equals the frequency of the changing magnetic field within the loop.

Generator. rotating the coil in a stationary magnetic field

·  As the loop rotates, the induced voltage (and current) changes in magnitude and direction. One complete rotation of the loop produces one complete cycle in voltage (and in current). Because the voltage induced by the generator alternates, the current produced is ac: current in our homes is 60 hertz.

·  The construction of generators & motors is the same, but the roles of input and output are reversed. Both devices simply transform energy from one form to another.

·  in a generator, mechanical energy is the input and electric energy is the output.

·  (a) Motor effect: When a current moves along the wire, there is a perpendicular upward force on the electrons. Since there is no conducting path upward, the wire is tugged upward along with the electrons. (b) Generator effect: When a wire with no initial current is moved downward, the electrons in the wire experience a deflecting force perpendicular to their motion. There is a conducting path in this direction that the electrons follow, thereby constituting a current.

Power Production

·  Fifty years after Michael Faraday and Joseph Henry discovered electromagnetic induction; Nikola Tesla and George Westinghouse put those findings to practical use showing how electricity could be generated reliably and in sufficient quantities to light entire cities.

Turbogenerator Power; Tesla’s generators had armatures—iron cores wrapped with bundles of copper wire—that were made to spin within strong magnetic fields by means of a turbine, which, in turn, was spun by the energy of steam or falling water.

•  The rotating loops of wire in the armature cut through the magnetic field of the surrounding electromagnets, thereby inducing alternating voltage and current. The energy of this electronic sloshing is tapped at the electrode terminals of the generator.

•  MHD Power- magnetohydrodynamic

•  oppositely directed forces act on the positive and negative particles in the high-speed plasma moving through the magnetic field. The result is a voltage difference between the two electrodes. Current then flows from one electrode to the other through an external circuit. There are no moving parts; only the plasma moves. superconducting electromagnets are used.

·  Some people think that electricity is a primary source of energy. It is not. It is a carrier of energy that requires a source.

·  Generators

·  Generators don’t produce energy—they simply convert energy from some other form to electric energy. Unlike the turbogenerator, the MHD generator can operate at any temperature to which the plasma can be heated.

·  The high temperature results in a high efficiency meaning there is more power for the same amount of fuel and less waste heat. The “waste” heat can be used to change water into steam and to run a conventional steam-turbine generator.

·  Energy from a source, fossil etc… is converted to mechanical energy to drive the turbine and the attached generator converts most of this mechanical energy to electrical energy.

Transformers

·  Energy can be transferred from one device to another. Note that one coil is connected to a battery and the other is connected to a galvanometer. It is customary to refer to the coil connected to the power source as the primary (input) and to the other as the secondary (output).

·  As soon as the switch is closed in the primary and current passes through its coil, a current occurs in the secondary also—even with no material connection between the two coils. Only a brief surge of current occurs in the secondary, however. When the primary switch is opened, a surge of current again registers in the secondary, but in the opposite direction.

·  Whenever the primary switch is opened or closed, voltage is induced in the secondary circuit.

·  when the magnetic field is no longer changing—no further voltage is induced in the secondary. But, when the switch is turned off, the current in the primary drops to zero. The magnetic field about the coil collapses, thereby inducing a voltage in the secondary coil, which senses the change.

·  Voltage is induced whenever a magnetic field is changing through the coil, regardless of the reason.

step-up & step down Transformers

·  A transformer steps voltage up or down without a change in energy. The rate at which energy is transferred is called power. Transformers can in no way step up energy—a conservation of energy no-no

·  If the secondary is wound with three times as many loops, then three times as much voltage will be induced.

· 

·  If the slight power losses due to heating of the core are neglected, then

·  Power into primary = power out of secondary

·  Electric power is equal to the product of voltage and current, so

·  (Voltage × current)primary = (voltage × current)secondary

·  The ease with which voltages can be stepped up or down with a transformer is the principal reason that most electric power is ac rather than dc

Self-Induction

·  Current-carrying loops in a coil interact not only with loops of other coils but also with loops of the same coil. Each loop in a coil interacts with the magnetic field around the current in other loops of the same coil. This is self-induction.

·  What happens when a magnetic field suddenly changes in a coil, even if it is the same coil that produced it? The answer is that a voltage is induced. The rapidly collapsing magnetic field with its store of energy may induce an enormous voltage, large enough to develop a strong spark across the switch—or to you, if you are opening the switch!

·  This voltage is always in a direction opposing the changing voltage that produces it and is commonly called the “back electromotive force,” or simply “back emf

·  electromagnets are connected to a circuit that absorbs excess charge and prevents the current from dropping too suddenly, Reducing self-induced voltage.

Power Transmission

·  Transformers work only if the current is changing; this is one reason why electricity is transmitted as ac.

·  Energy, then, is transferred from one system of conducting wires to another by electromagnetic induction.

·  Almost all electric energy sold today is in the form of ac, traditionally because of the ease with which it can be transformed from one voltage to another.

·  Large currents in wires produce heat and energy losses, so power is transmitted great distances at high voltages and correspondingly low currents (power = voltage × current).

·  Power is generated at 25,000 V or less and is stepped up near the power station to as much as 750,000 V for long-distance transmission, then stepped down in stages at substations and distribution points to voltages needed in industrial applications (often 440 V or more) and for the home (240 and 120 V).

·  Energy, then, is transferred from one system of conducting wires to another by electromagnetic induction.

Field Induction
Faraday’s law states:An electric field is induced in any region of space in which a magnetic field is changing with time.

·  Maxwell’s counterpart to Faraday’s law:A magnetic field is induced in any region of space in which an electric field is changing with time.

·  1989 an alarmist published –and cited data that confirmed his accusations while ignoring data that didn’t that electromagnetic fields were hazardous to human health

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