IVE Basic Concepts of electricity. IVE SI Units In science and engineering the International System of Units (SI units) form the basis of all units used

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Basic Concepts of electricityBasic Concepts of electricity

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SI UnitsSI Units

• In science and engineering the International System of Units (SI units) form the basis of all units used.

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Six base units in SI systemSix base units in SI system

Quantity Quantity UnitUnit Unit symbolUnit symbol• Electric currentElectric current ampereampere AA• MassMass kilogramkilogram kgkg• LengthLength metremetre mm• TimeTime secondsecond ss• TemperatureTemperature kelvinkelvin KK• Luminous intensityLuminous intensity candelacandela cdcd

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Basic Electrical UnitsBasic Electrical Units

Quantity Quantity UnitUnit Unit Unit symbolsymbol

• Potential Potential voltsvolts V V• PowerPower WattWatt W W• EnergyEnergy Joule/Watt hour J/WhJoule/Watt hour J/Wh• ResistanceResistance OhmOhm Ω Ω• FrequencyFrequency HertzHertz Hz Hz

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Common PrefixesCommon PrefixesMultiplying factor Multiplying factor Prefix namePrefix name SymbolSymbol

• 10101212 teratera TT• 101099 gigagiga GG• 101066 megamega MM• 101033 kilokilo kk• 1010-3-3 millimilli mm• 1010-6-6 micromicro μμ• 1010-9-9 nanonano nn• 1010-12-12 picopico pp

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Atom modelAtom model

Nucleus

Electron

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AtomAtom

• ProtonsProtons

• NeutronsNeutrons

• ElectronsElectrons

• Each atom has the same number of protons Each atom has the same number of protons and electronsand electrons

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Proton and ElectronProton and Electron

• Protons carries positive chargeProtons carries positive charge– it is relatively large massit is relatively large mass

– does not play active part in electrical current flowdoes not play active part in electrical current flow

• Electrons carries negative chargeElectrons carries negative charge– light masslight mass

– play an important role in electrical current flowplay an important role in electrical current flow

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Unit of ChargeUnit of Charge

• Unit of Charge is called Unit of Charge is called Coulomb (C)Coulomb (C)

• An electron and a proton have exactly same An electron and a proton have exactly same amount of chargeamount of charge

• One coulomb of charge is equal to One coulomb of charge is equal to approximately 628 x 10approximately 628 x 1016 16 electron chargeelectron charge

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Free ElectronsFree Electrons

Free Electrons

Applying Heat or Light

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Electrical MaterialsElectrical Materials

• All material may be classified into three All material may be classified into three major classesmajor classes– conductorsconductors

– insulatorsinsulators

– semiconductorssemiconductors

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Electrical MaterialsElectrical Materials

• conductors have many free electrons which will be drifting in a random manner within the material

• insulators have very few free electrons

• semiconductors falls somewhere between these two extremes

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Electric currentElectric current

• Electric current is the movement, or flow of electrons through a conductive material

• It is measured as the rate at which the charge is moved around a circuit, its unit is ampere (A)

I=Q/t or Q=It

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Electromotive ForceElectromotive Force

• In order to cause the 'free' electrons to drift in a given direction an electromotive force must be applied

• The emf is the 'driving' force in an electrical circuit

• The symbol for emf is E and the unit of measurement is the volt (V)

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Electromotive ForceElectromotive Force

• Typical sources of emf are cells, batteries and generators

• The amount of current that will flow through a circuit is related to the size of the emf applied to it

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Potential Difference (p.d.)Potential Difference (p.d.)

• Whenever current flows through a circuit element in a circuit such as resistor, there will be a potential difference(p.d.) developed across it

• The unit of p.d. is volts(V) and is measured as the difference in voltage levels between two points in a circuit

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• Emf (being the driving force) causes current to flow

• potential difference is the result of current flowing through a circuit element

• Thus emf is a cause and p.d. is an effect

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LOAD

P.D.=1.4 V

E.m.f.=1.5V, Rint.

Equivalent circuit of a battery

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ResistanceResistance

• Resistance is the 'opposition' to the current flow

• measured in ohms (Ω)

• Conductors have a low value of resistance

• Insulators have a very high resistance

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Resistor

• Substances, which offer certain amount of resistance to the flow of electrons, are called resistors

• The resistance of a resistor depends on the material used, the physical construction of the resistor and the temperature

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ResistorResistor

• The resistance value can be determined by The resistance value can be determined by the equationthe equation

– RR is the resistance of a resistor in ohm( is the resistance of a resistor in ohm())– ll is the length of the resistor in meter(m). is the length of the resistor in meter(m).– A A is cross-sectional area of the resistor in (mis cross-sectional area of the resistor in (m22).). is the resistivity of the material in (is the resistivity of the material in (-m)-m)

AlR

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ResistivityResistivity

Material (-m) at 0o C• AluminiumAluminium 2.7x102.7x10-8-8

• BrassBrass 7.2x107.2x10-8-8

• CopperCopper 1.59x101.59x10-8-8

• CarbonCarbon 6500.0x106500.0x10-8-8

• ZincZinc 5.57x105.57x10-8-8

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Ohm’s LawOhm’s Law

• Ohm's law states that the p.d. developed between the two ends of a resistor is directly proportional to the value of current flowing through it, provided that all other factors (e.g. temperature) remain constant

V V II

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Ohm’s LawOhm’s Law

RVI

IVR

IRV

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Resistors in SeriesResistors in Series

E

I

V 1 V 2 V 3

R 1 R 2 R 3

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• By Ohm's lawBy Ohm's lawV1 = IR1 volts;

V2 = IR2 volts; and

V3 = IR3 volts

E = V1 + V2 + V3

E = I (R1 + R2 + R3)

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E E = = IRIReqeq and and

RReqeq = = RR11 ++ R R2 2 + + RR33 ohm ohm

where Req is the total circuit resistance

• when resistors are connected in series the when resistors are connected in series the total resistance is found simply by adding total resistance is found simply by adding together the resistor valuestogether the resistor values

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Resistors in ParallelResistors in Parallel

R 1

R 2

R 3

I1

I2

I3

I

E

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By Ohm’s Law

• I1=E/ R1

• I2=E/ R2

• I3=E/ R3

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• The total current of the circuit The total current of the circuit I I is the sum of is the sum of II11, , II22 and and II33 , thus , thus

I I = = II11 + + II22 + +II33

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• The total resistance or the equivalent The total resistance or the equivalent resistance(resistance(RReqeq) of the circuit is defined to be) of the circuit is defined to be

RReqeq= E/I= E/I

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Resistors in Parallel Resistors in Parallel

By substituting the above expression for the By substituting the above expression for the currents, we havecurrents, we have

E/RE/Reqeq=I=E(1/R=I=E(1/R11+1/R+1/R22+ 1/R+ 1/R33))

• Thus we found

1/R1/Reqeq=(1/R=(1/R11+1/R+1/R22+ 1/R+ 1/R33))

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Power in a resistive circuitPower in a resistive circuit

• Power Power is equal to the is equal to the current current multiplied by multiplied by the the voltage voltage and the unit of power is watt (W)and the unit of power is watt (W)

P P = = IEIE

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• By Ohm's law By Ohm's law EE==IR,IR, the above equation can the above equation can be modify to bebe modify to be

P P = = II22RR

• Power is equal to the current squared, multiplied by the resistance.

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• Use Ohm's law again, where I =E/R, we have

P=E2/R

• Power is equal to the voltage squared, divided by the resistance.

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Example 1Example 1

R B C

E = 10 V

I

V A B V B C V C D

A B C D

R A B R C D

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– VVABAB + V + VBCBC + V + VCDCD is exactly equal to the emf=10V is exactly equal to the emf=10V

– The total resistance of the circuit is 2+5+3=10The total resistance of the circuit is 2+5+3=10

– By Ohm's law V=IR, the current I should be By Ohm's law V=IR, the current I should be equal to 1A equal to 1A

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Example 1Example 1

– VVAB AB = IR= IRAB AB =1 x 2 = 2V.=1 x 2 = 2V.

– VVBC BC = = I RI RBC BC ==1 x 5 = 5V1 x 5 = 5V..

– VVCD CD = = IRIRCD CD ==1 x 3 = 3V.1 x 3 = 3V.

– Power dissipation in RPower dissipation in RAB AB = = II22RRAB AB = 1= 122 x 2 = 2W.x 2 = 2W.

– Power dissipation in RPower dissipation in RBC BC = = II22RRBC BC = 1= 122 x 5 = 5W.x 5 = 5W.

– Power dissipation in RPower dissipation in RCD CD = = II22RRCD CD = 1= 122 x 3 = 3W.x 3 = 3W.

– Total power dissipated = 2+5+3 =10WTotal power dissipated = 2+5+3 =10W

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Example 2Example 2

R 1

R 2

R 3

I1

I2

I3

I

E = 6V

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Example 2Example 2

– the potential difference across each of the three the potential difference across each of the three resistors is equal to the battery emf 6Vresistors is equal to the battery emf 6V

– Apply Ohm's LawApply Ohm's Law

– EE==II11 R R1 1 ; I; I11=E/R=E/R1 1 = 6/2 = 3A= 6/2 = 3A

– EE==II22 R R2 2 ; I; I22=E/R=E/R2 2 = 6/3 = 2A= 6/3 = 2A

– EE==II33 R R3 3 ; I; I33=E/R=E/R3 3 = 6/6 = 1A= 6/6 = 1A

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Example 2Example 2

– The total current The total current I I is equal to the sum of currents is equal to the sum of currents II11++II22++II33 = 3+2+1 =6A.= 3+2+1 =6A.

– Power dissipation in RPower dissipation in R1 1 = = II1122RR1 1 = 3= 322

x 2 = 18W.x 2 = 18W.


x 3 = 12W.x 3 = 12W.


x 6= 6W.x 6= 6W.

– Total power dissipated =18+12+6 =36WTotal power dissipated =18+12+6 =36W

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CapacitanceCapacitance

T w o m e ta l p la te sse p a ra te d b y ad ie le c tr icm a te r ia l

A

BE

C h a rg in g c u rre n t

+ Q

-QV A B

E le c tro n s f lo w

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• The property of a capacitor to store an electric charge when its plates are at different potentials is referred to as its capacitance(C). The unit of capacitance is farad(F)

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• The total charges stored in the capacitor (Q) is The total charges stored in the capacitor (Q) is equal to the capacitance of the capacitor (C) equal to the capacitance of the capacitor (C) multiplied by the potential across the multiplied by the potential across the capacitor(V).capacitor(V).

Q = CV coulomb

• it is more usual to express capacitance values in it is more usual to express capacitance values in microfarads (microfarads (µµF), nanofarads (nF), or picofarads F), nanofarads (nF), or picofarads (pF).(pF).

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Charging of a RC circuitCharging of a RC circuit

Switch SR

CE VC

I+

-

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Charging of a RC circuitCharging of a RC circuit

E

Time, second

Voltage across the capacitor, VC

Charging current, I

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Discharging of a RC circuitDischarging of a RC circuit

Switch

Discharge Current

R

C

+

-VC

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Discharging of a RC circuitDischarging of a RC circuit

Initial discharge current

Vc/R

I

Time, secondsDischarge current

IVEEnergy Stored in CapacitanceEnergy Stored in Capacitance

P.D (volt)

V

Charge (C)

Q

Area

IVEEnergy Stored in CapacitanceEnergy Stored in Capacitance

• The area under the graph =The area under the graph =QV/2QV/2

• But But Q Q == CVCV coulombcoulomb

so energy stored, W = so energy stored, W = CVCV22 /2 joule/2 joule.

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Capacitors connected in seriesCapacitors connected in series

DC DBA

V3V2V1

CEQ

AC1 C2 C3

Vs

Vs

+Q+Q+Q +Q

+ +- -

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Capacitors connected in parallelCapacitors connected in parallel

As As QQ, total charges stored on a capacitor , total charges stored on a capacitor

= C= C11VV11 = C = C22VV22 = C = C33VV33 = V = VSSCCEQEQ

or or VV11=Q/C=Q/C11 , V , V22=Q/C=Q/C22 , V , V33=Q/C=Q/C33 and V and VSS=Q/C=Q/CEQEQ

and and VVSS=V=V11 +V +V2 2 +V+V33 = Q ( 1/C = Q ( 1/C11 +1/C +1/C22 + 1/C + 1/C33) = Q/C) = Q/CEQEQ

so, so, 1/C1/CEQEQ= ( 1/C= ( 1/C11 +1/C +1/C22 + 1/C + 1/C33))

where CEQ is the equivalent capacitance of the capacitors in

series.

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Capacitors connected in parallelCapacitors connected in parallel

+Q1 +Q2 +Q3 +(Q1+Q2+Q3)

C1 C2 C3CEQ

VsVs

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Capacitors connected in parallelCapacitors connected in parallelThe voltage across the three capacitors The voltage across the three capacitors = = VVSS

The charges stored on capacitor, CThe charges stored on capacitor, C11 , ,QQ11 = = VVSSCC11



Therefore, the total amount of charges storedTherefore, the total amount of charges stored

= = QQ11 +Q +Q22 +Q +Q33 = = VVSS (C (C11 +C +C22 +C +C33)) = = VVSSCCEQEQ

ThereforeTherefore CCEQEQ = = (C(C11 +C +C22 +C +C33))

where CEQ is the equivalent capacitance of the capacitors in parallel

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Basic Concepts of electricityBasic Concepts of electricity

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Documents

IVE Basic Concepts of electricity. IVE SI Units In science and engineering the International System of Units (SI units) form the basis of all units used