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Chapter 37 Electromagnetic Induction**

37.1 Electromagnetic Induction

 * Electromagnetic Induction**//- The phenomenon of inducing a voltage in a conductor by changing the magnetic field near the conductor.//

Faraday and Henry discovered that electric current could be produced in a wire by simply moving a magnet in or out of a wire coil. Voltage is induced by the relative motion between a wire and a magnetic field. The production of voltage depends only on relative motion. It is produced when the magnetic field or magnet is moved past a stationary conductor, or the conductor moves through a stationary magnetic field. Also, the greater number of loops of wire that move in a magnetic field, the greater the induced voltage and the greater current in the wire. For example, pushing a magnet into twice as many loops will induce twice as many voltage. Pushing it into five times as many loops will induce five times as much voltage. When there are more loops, you cause more current to flow in the coil, and this makes the coil act as a more powerful electromagnet. Another thing that affects the amount of voltage induced is how quickly the magnetic field changes. For example, if you move the magnet or magnetic field slowly, hardly any voltage is produced. However if it is moved quicker, a greater voltage is produced.

37.2 Faraday's Law

 * 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 not only on the induced voltage but also on the resistance of the coil and the circuit in which it is connected. If the coil is not connected to anything, it takes no work to plunge the magnet into the coil, no matter how many loops there are. There is no current flow and no transfer of energy.

37.3 Generators and Alternating Current

 * Generator-** //A machine that produces electric current by rotating a coil withi a stationary magnetic field.//

When a magnet is pushed in and out of a coil of wire, the induced voltage alternates in direction. As the magnetic field strength inside the coil is increased (the magnet entering the coil), the induced voltage in the coil is directed one way. When the magetic field strength diminishes (the magnet exiting the coil), the voltage is induced in the opposite direction.

When rotating a loop of wire in a magnetic field, there is a change in the number of magnetic field lines within the loop. The magnetic field inside the loop changes in a cyclic fashion. The voltage induced by the generator alternates, the current produced is calld alternating current (AC)- The current changes magnitude and direction periodically.

In order to operate a generator, an energy source of some kind is required. The electricity that is produced from a generator simply carries this energy to distant places. Some people think that electricity is a source of energy. However, this is wrong, it is a //form// of energy that must have a source.

37.4 Motor and Generator Comparison
Moving charges experience a force that is perpendicular to both their motion and the magnetic field they traverse.


 * The motor effect**- the current moves to the right and there is a perpendicular upward force on the electrons. There is no conductingpath upward, so the wire is pulled upward along with the electrons.


 * The generator effect**- When a wire that has no initial current is moved downward, the electrons in the wire experience a deflecting force perpendicular to their motion.

37.5 Transformers

 * Transformer**- //A device for increasing or decreasing voltage through electromagnetic induction.//

If two coils are next to each other, the magnetic field that builds up around the primary extends into the secondary coil. Changes in the magnetic field of the primary are sensed by the nearby secondary. These changes of magnetic field intensity at the secondary induce voltage in the secondary, in accord with Faraday's law.

math \frac{Primary Voltage}{Number of primary turns} = \frac{Secondary Voltage}{Number of secondary turns} math (Primary voltage over the number of primary turns equals the secondary voltage over the number of secondary turns)

The power used in the secondary is supplied by the primary. The primary gives no more power than the secondary uses.

Power into primary = power out of secondary

37.6 Power Transmission
Power is transmitted great distances at high voltages and correspondingly low currents, a process that otherwise would result in large energy losses owing to the heating of the wires. Energy, then, is transformed from one system of conducting wires to another by electromagnetic induction. The same principles account for eliminating wires and sending energy from a radio-transmitter antenna to a radio reciever many kilometers away, and for the transformation of energy of vibrating electrons in the sun to life energy on earth.

37.7 Induction of Electric and Magnetic Fields
//An electric field is created in any region of space in which a magnetic field is changing with time. The magnitude of the created electric field is proportional to the rate at which the magnetic field changes. The direction of the created electric field is at right angles to the hanging magnetic field.//

//A magnetic field is created in any region of space which an electric field is changing with time. The magnitude of the created magnetic field is propotional to the rate at which the electric field changes. The direction of the created magnetic field is at right angles to the changing electric field.//

37.8 Electromagnetic Waves
The changing electric field induces a magnetic field. The changing magnetic field acts back to induce an electric field. Maxwell's equations showed that only one speed could preserve this harmonious balance of fields.

From his equations of electromagnetic induction, Maxwell calculated the value of this critical speed and found it to be 300 000 kilometers per second.

The electric and magnetic fields of an electromagnetic wave are perpendicular to each other.



Works Cited: Conceptual Physics-Third Edition with expanded technology written by Paul G. Hewitt [] (Electromagnetic Wave Video)