01d basic concepts.ppt

8/10/2019 01D Basic Concepts.ppt

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Electric circuit theory and electromagnetic theory arethe two fundamental theories upon which all branches ofelectrical engineering are built.

Many branches of electrical engineering, such as power,electric machines, control, electronics, communications, andinstrumentation, are based on electric circuit theory.

Therefore, the basic electric circuit theory course is the mostimportant course for an electrical engineering student, andalways an excellent starting point for a beginning student

in electrical engineering education.


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1.1 System of Units (1)

Quantity Basic unit Symbol

Length meter m

Mass kilogram Kg Time second s

Electric current ampere A

Thermodynamictemperature

kelvin K

Luminous intensity candela cd

Six basic units


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1.2 Electric Charges

Charge is an electrical property of the atomic

particles of which matter consists, measured incoulombs (C) **.

The charge e on one electron is negative andequal in magnitude to 1.602 10 -19 C which iscalled as electronic charge. The charges thatoccur in nature are integral multiples of theelectronic charge.


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1.3 Current (1)

Electric current i = dq/dt . The unit ofampere can be derived as 1 A = 1C/s .

A direct current (dc) is a current thatremains constant with time.

An alternating current (ac) is a currentthat varies sinusoidally with time.(reverse direction)

The quantity symbol is I for a constantcurrent and i for a time-varying current.


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1.3 Current (2)

The direction of current flow

Positive ions Negative ions


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When a conducting wire (consisting of several atoms) is connectedto a battery (a source of electromotive force), the charges arecompelled to move; positive charges move in one direction whilenegative charges move in the opposite direction. This motion of chargescreates electric current. It is conventional to take the current flow asthe movement of positive charges, that is, opposite to the flow ofnegative charges. This convention was introduced by Benjamin Franklin(1706 1790), the American scientist and inventor. Although we nowknow that current in metallic conductors is due to negatively chargedelectrons, we will follow the universally accepted convention thatcurrent is the net flow of positive charges.


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1.3 Current (3)

Example 1

A conductor has a constant current of5 A.

How many electrons pass a fixed pointon the conductor in one minute?


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1.3 Current (4)Solution

Total no. of charges pass in 1 min is given by

5 A = (5 C/s)(60 s/min) = 300 C/min

Total no. of electronics pass in 1 min is given

minelectrons/1087.1C/electron10602.1

C/min 300 2119 x x


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Find the charge in coulombs

of ( a ) 5.31 x 1020

electrons,and ( b) 2.9 x 10 22 protons.


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Find the charge in coulombs

of ( a ) 5.31 x 1020

electrons,and ( b) 2.9 x 10 22 protons.


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How many protons have a combinedcharge of 6.8 pC?

= 42.447x10 6 protons


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Will a 10-A fuse blow for a steady rate of charge

flow through it of 45 000 C/h?


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Will a 10-A fuse blow for a steady rate of charge

flow through it of 45 000 C/h?


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1.4 Voltage (1) Voltage (or potential difference) is the energy

required to move a unit charge through anelement, measured in volts (V).

Mathematically, (volt)

w is energy in joules (J) and q is charge in coulomb (C).

Electric voltage, v ab, is always across the circuit

element or between two points in a circuit. v ab > 0 means the potential of a is higher than potential

of b.

v ab < 0 means the potential of a is lower than potential

of b.

dqdwvab /


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1.5 Power and Energy (1) Power is the time rate of expending or absorbing

energy, measured in watts (W).

Mathematical expression: vi

dt

dq

dq

dw

dt

dw p

i

+

v

i

+

v

Passive sign convention P = +vi p = vi

absorbing power supplying power


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1.6 Circuit Elements (1)Active Elements Passive Elements

Independent

sources

Dependant

sources

A dependent source is an activeelement in which the source quantityis controlled by another voltage orcurrent.

They have four different types: VCVS,CCVS, VCCS, CCCS. Keep in minds thesigns of dependent sources.


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1.6 Circuit Elements (2)

Example 2

Obtain the voltage v in the branch shown in Figure 2.1.1P for i 2 = 1A.

Figure 2.1.1P


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1.6 Circuit Elements (3)

Solution

Voltage v is the sum of the current-independent10-V source and the current-dependent voltagesource v x .

Note that the factor 15 multiplying the control

current carries the units .

Therefore, v = 10 + v x = 10 + 15(1) = 25 V


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EFFICIENCY


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What is the operating efficiency of a dc electric motor thatdelivers 1 hp while drawing 7.45 A from a 115-Vline?

Ans.

Find the current drawn by a 100-V dc electric motor thatoperates at 85 percent efficiency while delivering0.5 hp.

Ans.

What horsepower must an electric motor develop to operate apump that pumps water at a rate of 24 000 liters per hour (L/h)up a vertical distance of 50 m if the efficiency of the pump is 90percent? The gravitational force on 1 L of water is 9.78 N.

Ans.


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What is the operating efficiency of a dc electric motor thatdelivers 1 hp while drawing 7.45 A from a 115-Vline?

Ans. 87 percent

Find the current drawn by a 100-V dc electric motor thatoperates at 85 percent efficiency while delivering0.5 hp.

Ans. 4.39 A

What horsepower must an electric motor develop to operate apump that pumps water at a rate of 24 000 liters per hour (L/h)up a vertical distance of 50 m if the efficiency of the pump is 90percent? The gravitational force on 1 L of water is 9.78 N.% Efficiency = Pout/Pin24000 L/hr * 9.78N/1L*50m

Ans. 4.86 hp


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RESISTANCE OFCONDUCTORS


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RESISTANCE OF CONDUCTORS

Conductors are materials which permit the flow of charge.However, conductors do not all behave the same way. Rather, wefind that the resistance of a material is dependent upon severalfactors: Type of material

Length of the conductor Cross -sectional area Temperature

RESISTANCE OF CONDUCTORS

Conductors are materials which permit the flow of charge.However, conductors do not all behave the same way. Rather, wefind that the resistance of a material is dependent upon severalfactors: Type of material

Length of the conductor Cross -sectional area Temperature


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RESISTIVITYThe resistance of a conductor of uniform cross section isdirectly proportional to the length of the conductor andinversely proportional to the cross-sectional area.Resistance is also a function of the temperature of theconductor. At a fixed temperature the resistance of aconductor is


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A copper wire of unknown length has a resistance of 0.80. By successivepassses through drawing dies, the length of the wire is increased to 2.5 timesits original value. Assuming that resistivity remains unchanged during the

drawing process, determine the new value of its resistance .


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The fourth band indicates the tolerance andis either gold- or silver-colored, or ismissing. Gold corresponds to a tolerance of5 percent, silver to 10 percent, and amissing band to 20 percent.


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The American Wire Gauge system for specifying wirediameters was developed using a unit called the circularmil (CM), which is defined as the area contained within acircle having a diameter of 1 mil (1 mil = 0.001 inch). Asquare mil is defined as the area contained in a squarehaving side dimensionsof 1 mil.

Wire Sizing


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Wire Sizing


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EXAMPLE Determine the cross-sectional area in circularmils of awire having the following diameters:a. 0.0159 inch (AWG 26 wire)b. 0.500 inch

Solution

a. d 0.0159 inch(0.0159 inch)(1000 mils/inch)15.9 mils


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EXAMPLE Determine the cross-sectional area in circularmils of awire having the following diameters:a. 0.0159 inch (AWG 26 wire)b. 0.500 inch

Solution

a. d 0.0159 inch(0.0159 inch)(1000 mils/inch)15.9 mils


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Temperature Effects

The resistances of most good conducting materials increase

almost linearly with temperature over the range of normaloperating temperatures, as shown by the solid line in followingfigure. However, some materials, and common semiconductors inparticular, have resistances that decrease with temperatureincreases.

If the straight-line portion in Figure is extended to the left, itcrosses the temperature axis at a temperature To at which theresistance appears to be zero. This temperature To is theinferred zero resistance temperature. (The actual zeroresistance temperature is -273 'C.) If To is known and if theresistance R , at another temperature T, is known, then the

resistance R , at another temperature T2 is, from straight-linegeometry,


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Tungsten wire is used as filaments in incandescent lightbulbs. Current in the wire causes the wire to reachextremely high temperatures. Determine the temperature ofthe filament of a 100-W light bulb if the resistance at roomtemperature is measured to be 11.7 and when the light ison, the resistance is determined to be 144 .


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01d basic concepts.ppt

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