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Shvoong Home>Science>Physics>Electricity and Magnetism Summary

# Electricity and Magnetism

Book Summary   by:Shivam     Original Author: James Clerk Maxwell

Electricity and Magnetism Do you know where electricity comes from? To be able to say yes to that question, you must understand a bit about the physics of matter. Simply stating electricity is the transfer of electrons (negative charges) from one place to another and the basis of understanding how electricity works is Ohm’s law. Most students of electricity begin their study with what is known as direct current (DC), which is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. In DC circuits, the polarity of the voltage source does not change over time. When a DC source is connected in a closed electrical circuit, current will flow in a direction determined by the polarity of the source. By convention, we show DC current flow as originating at the positive terminal of the source, traveling through the circuit and returning to the negative terminal. Common DC sources include batteries, photocells, fuel cells, rectifiers and the common DC machines are motors and generators. Alternating current (AC) unlike Direct current (DC) flow first in one direction then in the opposite direction. The most common€ waveform is a sine (or sinusoidal) waveform. This electrical training course provides a basic introduction to AC theory, electrical circuits, AC generator and voltage regulation. This course will be extremely helpful to individuals who are just beginning a career in electrical work, or who require a basic knowledge of electrical principals and equipment to better their primary responsibilities. This course is also a prerequisite for the all other electrical training. It is true that in some cases AC holds no practical advantage over DC. In applications where electricity is used to dissipate energy in the form of heat, the polarity or direction of current is irrelevant, so long as there is enough voltage and current to the load to produce the desired heat (power dissipation). However, with AC it is possible to build electric generators, motors and power distribution systems that are far more efficient than DC, and so we find AC used predominately across the world in high power applications. To explain the details of why this is so, a bit of background knowledge about AC is necessary. Remember these facts 1) Voltage is the pressure or the Electromotive Force that will push or pull electrons 2) Amperage is the number of electrons moving through a wire or conductor 3) Resistance will restrict how many amps can flow through a conductor 4) Increasing the electrical pressure will cause more amps to flow 5) Increasing the resistance will cause less amps to flow 6) The same current flows through each part of a series circuit. 7) Total Resistance of a series circuit is equal to the sum of the individual resistances. 8) The total voltage across a series circuit is equal to the sum of the individual voltage drops. 9) The sum of the voltage drops around a closed loop is equal to the sum of the voltage sources of that loop (Kirchhoff’s Voltage Law) 10) The current arriving at any junction point in a circuit is equal to the current leaving that junction (Kirchhoff’s Current Law) 11) When an inductor has a DC current flowing through it, the inductor will store energy in the form of a magnetic field.
12) An inductor will oppose a change in current flow by the CEMF induced when the field collapses or expands. Inductors in series are combined like resistors in series and inductors in parallel are combined like resistors in parallel. 13) A capacitor stores energy in the form of an electric field caused by the attraction of the positively-charged particles in one plate to the negatively charged particles in the other plate. Capacitors in series are combined like resistors in parallel and capacitors in parallel are combined like resistors in series. 14) A voltaic cell is a combination of materials used to convert chemical energy into electrical energy. A battery is a group of two or more connected voltaic cells. 15) The left-hand rule states that if you point the index finger of the left hand in the direction of the magnetic field and point the thumb in the direction of motion of the conductor, the middle finger will point in the direction of current flow. 16) The right-hand rule for motors states that when the forefinger is pointed in the direction of the magnetic field lines, and the center finger is pointed in the direction of current flow, the thumb will point in the direction of motion. 17) Power factor is a measure of how far current leads or lags voltage. In pure resistive circuits power factor is unity; In inductive circuits current lags voltage; In capacitive circuits current leads voltage 18) As in the case with DC power, the instantaneous electric power in an AC circuit is given by P = E x I, but these quantities are continuously varying. Power in AC circuits where power factor is not 100 percent or unity: Power = E x I x power factor (for single phase) and Power = E x I x 1.732 X power factor (for three phase) 19) Inductance: The property of a circuit, which impedes a change in current. In electronic circuits, the usual measure of inductance is henrys (H), milihenrys (mH) or micro henrys. 20) Capacitance: The property of a circuit which impedes a change in voltage. Capacitance is measured in farads in honor of Michael Faraday.
Published: February 27, 2006
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