Lecture 1ELECTRICAL ENGINEERING BASICS

OUTLINE

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• Electrical Charge

• Voltage

• Current

• Resistance

• Energy & Power

ELECTRICAL CHARGE

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An electron is the smallest particle that exhibits negative electricalcharge. When an excess of electrons exists in a material, there is a netnegative electrical charge. When a deficiency of electrons exists,there is a net positive electrical charge.

The charge of an electron and that of a proton are equal inmagnitude.

Electrical charge, an electrical property of matter that exists becauseof an excess or deficiency of electrons, is symbolized by Q.

Static electricity is the presence of a net positive or negative charge ina material. Everyone has experienced the effects of static electricityfrom time to time. for example, when attempting to touch a metalsurface or another person or when the clothes in a dryer clingtogether.

ELECTRICAL CHARGE

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Materials with charges of opposite polarity are attracted to eachother, and materials with charges of the same polarity are repelled,as indicated in Figure 2-5. A force acts between charges, asevidenced by the attraction or repulsion. This force, called an electricfield, consists of invisible lines of force, An electron is the smallestparticle that exhibits negative electrical charge. When an excess ofelectrons exists in a material, there is a net negative electrical charge.When a deficiency of electrons exists, there is a net positive electricalcharge.

ELECTRICAL CHARGE

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Electrical field of lines between two oppositely charge surfaces.

COULOMB: THE UNIT OF CHARGE

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COULOMB: THE UNIT OF CHARGE

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Solution:

VOLTAGE

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As you have seen, a force of attraction exists between a positiveand a negative charge. A certain amount of energy must beexerted, in the form of work, to overcome the force and move thecharges a given distance apart. All opposite charges possess acertain potential energy because of the separation between them.The difference in potential energy per charge is the potentialdifference or voltage. Voltage is the driving force in electric circuitsand is what establishes current.

As an analogy. consider a water tank that is supported severalfeet above the ground. A given amount of energy must be exerted inthe form of work to pump water up to fill the tank. Once the water isstored in the tank, it has a certain potential energy which, if released.can be used to perform work.

Voltage, symbolized by V, is defined as energy or work per unitcharge.

VOLTAGE

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VOLTAGE

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If 50 J of energy are available for every 10 C of charge, what isthe voltage.

Solution:

CURRENT

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Voltage provides energy to electrons, allowing them to movethrough a circuit. This movement of electrons is the current. whichresults in work being done in an electrical circuit

free electrons are available in all conductive and semi conductivematerials. These electrons drift randomly in au directions, fromatom to atom, within the structure of the material, as indicated infigure.

Random motion of free electrons in a motion

CURRENT

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If a voltage is placed across a conductive or semi conductivematerial, one end becomes positive and the other negative, asindicated in Figure. The repulsive force produced by the negativevoltage at the left end causes the free electrons (negative charges)to move toward the right. The attractive force produced by thepositive voltage at the right end pulls the free electrons to the right.The result is a net movement of the free electrons from the negativeend of the material to the positive end, as shown in Figure

CURRENT

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The movement of these free electrons from the negative end of thematerial to the positive end is the electrical current, symbolized by I.

Electrical current is the rate of flow of charge.

Current in a conductive material is determined by the number ofelectrons (amount of charge) that flow past a point in a unit of time.

CURRENT

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CURRENT

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Ten coulombs of charge flow past a given point in a wire in 2 s. Whatis the current in amperes.

Solution:

RESISTANCE

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When there is current through a material, the free electrons movethrough the material and occasionally collide with atoms. Thesecollisions cause the electrons to lose some of their energy, thusrestricting their movement. The more collisions, the more the flow ofelectrons is restricted. This restriction varies and is determined by thetype of material.

The property of a material to restrict or oppose the flow of electronsis called resistance, R.

Resistance is the opposition to current.

Resistance is expressed in ohms, symbolized by the Greek letteromega (Ω).

RESISTANCE

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One ohm (1 Ω) of resistance exists if there is one ampere (1 A) ofcurrent in a material when one volt (1V) is applied across thematerial. The schematic symbol for resistance is shown in Figure

ENERGY & POWER

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When there is current through a resistance, electrical energy isconverted to heat or other form of energy, such as light. A commonexample of this is a light bulb that becomes too hot to touch. Thecurrent through the filament that produces light also producesunwanted heat because the filament has resistance. Electricalcomponents must be able to dissipate a certain amount of energy in agiven period of time.

Energy is the ability to do work, and power is the rate at whichenergy is used.

Power (P) is a certain amount of energy (W) used in a certainlength of time (t), expressed as follows:

ENERGY & POWER

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ENERGY & POWER

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An amount of energy equal to 100 J is used in 5 s. What is the powerin watts.

Solution:

THE KILOWATT-HOUR (KWH) UNIT OF ENERGY

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The Joule has been defined as a unit of energy. However, there isanother way to express energy. Since power is expressed in watts andtime in seconds, units of energy called the watt-second (Ws), watt-hour(Wh), and kilowatt-hour (kWh) can be used.

When you pay your electric bill, you are charged on the basis of theamount of energy you use, not the power. Because power companiesdeal in huge amounts of energy, the most practical unit is the kilowatt-hour. You use a kilowatt-hour of energy when you use one thousandwatts of power for one hour. For example, a 100 W light bulb burningfor 10 h uses 1 kWh of energy.

THE KILOWATT-HOUR (KWH) UNIT OF ENERGY

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Determine the number of kilowatt-hours (kWh) for each of thefollowing energy consumptions:

(a) 1400 W for l h, (b) 2500 W for 2 b, (c) 100,000 W for 5 h

Solution:

POWER IN ELECTRIC CIRCUITS

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POWER IN ELECTRIC CIRCUITS

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POWER IN ELECTRIC CIRCUITS

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Calculate the power in each of the three circuits.

POWER IN ELECTRIC CIRCUITS

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Solution:

POWER IN ELECTRIC CIRCUITS

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A 100 Watt light bulb operates on 120V. How much current does itrequire.

Solution: