1_Foundation of DC Concepts, Ohm's Law, Power and Energy

01JUNE 18 2011

Different type of Electric CircuitsCircuit Elements

TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY1Foundation of DC Concepts, Ohm's Law, Power and Energy 3rd / 2013-2014 L. Valcos

01JUNE 18 2011

Electric Circuit An electric circuit consist of a voltage source, a load,a path for current between the source and the load.

A load is a device on which work is done by thecurrent through it.

Electric Circuit Types:1. Closed2. Open3. Short


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Voltage (V) It is the potential energy in an electrical circuit that makes things happen. It is sometimes called Electromotive Force or EMF. The basic unit (measurement) of EMF is volt.

Electric Potential. The amount of work required in moving a unit charge.Potential Difference. The rise and fall (gain or loss) of potential energy in

moving a charge from one point to another.Absolute Potential. Potential developed in moving a charge from infinity to a

point.Relative Potential. Potential developed in moving a charge from one point to

another point.

V = WQwhere W = energy in Joules (J)

Q = charge in Coulomb (C)


01JUNE 18 2011

Current (I) It is the flow of electrons in a conductor (wires) from the negative end to the positive end. It is measured in amperes (amps)

I = Qtwhere Q = charge in Coulomb (C)

t = time in sec (s)


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Resistance (R) It is the opposition to the flow of current. It is measured in ohms ().

R = lAwhere l = length in meter (m)

A = cross-sectional area in m2R = resistance in ohms () = resistivity or specific resistance

in ( m)= resistance per unit length andcross-sectional area

Notes:Resistivity for Annealed Cu at 20C

20C = 1.72 x 10-8 m (for annealed Cu)= 10.37 -cmil per ftCross sectional area in terms of square mils

1 cmil = sq. mils4


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Conductance (G) It is the ability for electricity to flow in a certain path It is measured in sigma ()

G = = A A l lwhere = conductivity or specific conductance in siemens per meter (S/m)

Notes:Annealed Cu posses 100% conductivityLess than 100% conductivity mean a poor conductorGreater than 100% conductivity means a better conductor

% conductivity = x 100% = x 100% material annealed Cu1/ material1/ Cu

% conductivity = x 100% Cu

material


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Thank You forlistening!

Lets discuss the Circuit Elements..


01JUNE 18 2011

Different type of Electric CircuitsCircuit Elements


01JUNE 18 2011

An electrical network is an interconnection of circuit element as follows:1. Voltage Source2. Resistors3. Inductors4. Capacitors5. Transmission Lines6. Switches

3. Inductor Inductor is a passive electrical component formed by a coil of wire thatexhibits the property of inductance.

Inductance is a measure of a coils ability to establish an induced voltageas a result of a change in its current, and that induced voltage is in adirection to oppose that change in current. Unit is Henry.

Is can store energy in a magnetic field created by the electriccurrent passing through it.

An Inductive load lags the applied voltage. Example of Inductive loads are hair dryers, fans, blenders, vacuumcleaners, and many other motorized devices.


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4. Capacitor Capacitor is a passive electrical component that stores electrical charge and aproperty of capacitance.

It is device made of two metal conductors separated by an insulator called dielectric. Capacitance is the amount of charge that a capacitor can store per unit voltage acrossits plate. Units is farads.

A Capacitive load leads the voltage. Example of capacitive loads are TV picture tubes, longextension cords, and components used in electronic devices.


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5. Transmission Line Transmission Line transfers bulkelectrical energy, from generatingpower plants to substationslocated near population centers.


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6. Switches Switches are commonly used for controllingthe opening or closing of a circuits,interrupting current or diverting it fromone conductor to another.

Type of Switch1. Mechanical Switches


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SPST --- Single-Pole-Single-Throw DPST --- Double-Pole-Single-Throw DPDT --- Double-Pole-Double-Throw Push Button --- PB Rotary


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2. Electrical Switches (Relay)It is an electromagnetic switch that is used to open or close electrical contactsrather than to provide mechanical movement.


01JUNE 18, 2011


Lets discuss factors affecting Resistance of the Conductors..


01JUNE 21 2011

Resistance of the ConductorsFactors affecting the Resistance


01JUNE 21 2011

Resistance of the ConductorRecall R = lA

where l = length in meter (m)A = cross-sectional area in m2R = resistance in ohms () = resistivity or specific resistance

in ( m)= resistance per unit length andcross-sectional area

Notes:Resistivity for Annealed Cu at 20C

20C = 1.72 x 10-8 m (for annealed Cu)= 10.37 -cmil per ftCross sectional area in terms of square mils

1 cmil = sq. mils4


01JUNE 21 2011

Factors affecting the Resistance

1. Type of materials (conductor)2. Length of wire3. Cross-sectional area of wire4. Temperature


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01JUNE 21, 2011


Lets discuss Temperature effect on the Resistance...

TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY#Foundation of DC Concepts, Ohm's Law, Power and Energy 3rd / 2013-2014 L. Valcos

01JUNE 21 2011

Temperature Effect on the Resistance


Effect of Temperature on Resistance1. As temperature increases, Resistance increases (+ temp. coeff.)

eg. Cu, Al, and all metallic conductor2. As temperature increases, Resistance decreases (- temp. coeff.)

eg. Insulator and semiconductor3. As temperature increases or decreases, Resistance remains fairly constant

eg. Alloy, Mg

T

R

0TX

R1

t1

R2

t2

TX= inferred zero resistance temperature= inferred absolute zero temperature= - 234.5C (for Cu)

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TR

0TX

R1

t1

R2

t2

By the law of Geometry,

=R2R1|Tx | + t2|Tx | + t1

R2 R1 |Tx | + t2|Tx | + t1=

Let s get R T curve as shownR2 - R1

R1t2 t1= |Tx | + t1

R2 - R1 R1 t2 t1= |Tx | + t1R2 = R1 t2 t1|Tx | + t1R1 +

R2 1 + (t2 t1)|Tx | + t1R1=1

But,

1 = 1|Tx | + t1= temp. coefficient of resistance at t1 per unit

resistance change per degree (C) change intemp. referred to a point on the R-T curve.

= RT1-R1 in per C

therefore,R2 = R1 1 + 1 (t2 t1)

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01JUNE 21, 2011

Resistivity VS Temperature

T

0

1

t1

2

t2

Let m = slope of the linear region ofthe T curve

1t2 t12m = --

m (+2 = 1 t2- t1 )m12 1= 1 + (t2 - t1 )

but m1 = temp. coeff. of resistivity = 1

therefore,2 = 1 + t1 )1 (t2 -1


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01JUNE 21, 2011


Lets discuss on the next Chapter: Ohms Law, Energy, Work, and Power...


01JUNE 21 2011

Electrical Relevance of Ohms Lawto DC CircuitWork, Electric Power andElectrical Energy


01JUNE 21 2011

Ohms LawStates that the current through a conductor between two points isdirectly proportional to the potential difference across the twopoints, and inversely proportional to the resistance between them.+

V

I

-

element

V Ibut there is always a constant figure that opposes tocurrent I, say this is kV = k x I=VI k

therefore, V = I x R

where R is the resistance for k whoopposes the current.


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Lets discuss Work, Electrical Energy, and Electric Power


01JUNE 21 2011

Electrical Relevance of Ohms Lawto DC CircuitWork, Electric Power andElectrical Energy


01JUNE 21 2011

WorkWork is the accomplishment of motion againsta force that tends to oppose that motion.W = F x d

where, F = force in newtond = distance inmeterW = work in joules

Note: F = maweight = mg

Whenever Work is done, there is alwaysan accompanying energy conversion.

MMECHANICAL

ENERGY

GELECTRICALENERGY

HEAT ENERGY


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Power Current itself does not do any real work. Voltage and Current together can produce real work. Electric Power is the product ofvoltage times current.

Power is used to producereal work.

Power is the rate of doingwork or energy conversion.

Measurement in watt.

Energy is the ability to do work.Power is the rate at which Energy is used

Energy Electrical Energy is theproduct of electrical powerand time.

It is the amount of time aload is on (i.e., current isflowing) time the amountof power used by the load(i.e. watts).

Measurement is watt-hour(Wh).

Electric Power and Electrical Energy


01JUNE 21 2011


TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY#Ohms Law, Power, and Energy 1st / 2011-2012 L. Valcos

tP =W

where,W = work done or energy converted in joulest = time in secondsP = power in joules/second or watts

01JUNE 21 2011

from,V = WQ and I =

Qt

tP =WP = V x I then

By Ohms Law, V = I x R

P = V x IP = I2 x R

P = V2R

Kilowatt-Hour (kWh)1 Joule = 1 watt x second or 1 J = 1 W x S

1 J = 1 W x S 1 kW1000 W1 h

3600 S

1 J = 13.6 x 106 kWh

1 kWh = 3.6 MJ

Horsepower (unit or mechanical output power)1 Hp = 746 Watts = 0.746 kW


01JUNE 21, 2011

x timePercent Power Efficiency, y

y = WoutWin =PoutPin x time

Pin% y =Pout x 100%

where,

Pout = heat energyPin = mechanical energy

Overall Efficiency = y1 x y2 x y3 which is always less than thesmallest efficiency.


01JUNE 21, 2011


Lets discuss some sample problems relating to Ohms law, Electric Power andElectrical Energy


1_Foundation of DC Concepts, Ohm's Law, Power and Energy

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Transcript of 1_Foundation of DC Concepts, Ohm's Law, Power and Energy