BASIC CONCEPTSSystem of Units: The SI standard system; prefixesBasic Quantities: Charge, current, voltage, power and energyCircuit Elements

SI DERIVED BASIC ELECTRICAL UNITS

CURRENT AND VOLTAGE RANGES

Strictly speaking current is a basic quantity and charge is derived. However, physically the electric current is created by a movement of charged particles.What is the meaning of a negative value for q(t)?PROBLEM SOLVING TIP IF THE CHARGE IS GIVEN DETERMINE THE CURRENT BYDIFFERENTIATIONIF THE CURRENT IS KNOWN DETERMINE THE CHARGE BYINTEGRATIONA PHYSICAL ANALOGY THAT HELPS VISUALIZE ELECTRICCURRENTS IS THAT OF WATER FLOW. CHARGES ARE VISUALIZED AS WATER PARTICLES

Here we are given thecharge flow as functionof time.DETERMINE THE CURRENTTo determine current we must take derivatives.PAY ATTENTION TOUNITS

CONVENTION FOR CURRENTSIT IS ABSOLUTELY NECESSARY TO INDICATETHE DIRECTION OF MOVEMENT OF CHARGED PARTICLES.THE UNIVERSALLY ACCEPTED CONVENTION INELECTRICAL ENGINEERING IS THAT CURRENT ISFLOW OF POSITIVE CHARGES.AND WE INDICATE THE DIRECTION OF FLOWFOR POSITIVE CHARGES -THE REFERENCE DIRECTION-

This example illustrates the various waysin which the current notation can be used

CONVENTIONS FOR VOLTAGES ONE DEFINITION FOR VOLT TWO POINTS HAVE A VOLTAGE DIFFERENTIAL OFONE VOLT IF ONE COULOMB OF CHARGE GAINS (OR LOSES) ONE JOULE OF ENERGY WHEN IT MOVES FROM ONE POINT TO THE OTHERVOLTAGE IS ALWAYS MEASURED IN A RELATIVE FORM AS THE VOLTAGE DIFFERENCE BETWEEN TWO POINTSIT IS ESSENTIAL THAT OUR NOTATION ALLOWS US TO DETERMINE WHICH POINT HAS THE HIGHER VOLTAGE

THE + AND - SIGNS DEFINE THE REFERENCEPOLARITYIF THE NUMBER V IS POSITIVE POINT A HAS VVOLTS MORE THAN POINT B.IF THE NUMBER V IS NEGATIVE POINT A HAS|V| LESS THAN POINT B

ENERGYVOLTAGE IS A MEASURE OF ENERGY PER UNIT CHARGECHARGES MOVING BETWEEN POINTS WITH DIFFERENT VOLTAGE ABSORB ORRELEASE ENERGY THEY MAY TRANSFER ENERGY FROM ONE POINT TO ANOTHERBASIC FLASHLIGHTConverts energy stored in batteryto thermal energy in lamp filamentwhich turns incandescent and glowsThe battery supplies energy to charges.Lamp absorbs energy from charges.The net effect is an energy transfer

THE CHARGES MOVE TO A POINT WITH HIGHERVOLTAGE -THEY GAINED (OR ABSORBED) ENERGYTHE CIRCUIT SUPPLIED ENERGY TO THE CHARGESENERGYVOLTAGE IS A MEASURE OF ENERGY PER UNIT CHARGECHARGES MOVING BETWEEN POINTS WITH DIFFERENT VOLTAGE ABSORB ORRELEASE ENERGY

THE ELECTRIC POWER RECEIVED BY THEELEMENT IS 6[W]HOW DO WE RECOGNIZE IF AN ELEMENTSUPPLIES OR RECEIVES POWER?

PASSIVE SIGN CONVENTIONPOWER RECEIVED IS POSITIVE WHILE POWERSUPPLIED IS CONSIDERED NEGATIVEA CONSEQUENCE OF THIS CONVENTION IS THATTHE REFERENCE DIRECTIONS FOR CURRENT ANDVOLTAGE ARE NOT INDEPENDENT -- IF WE ASSUME PASSIVE ELEMENTS

AABBUNDERSTANDING PASSIVE SIGN CONVENTION

Sheet1

Voltage(V)Current A - A'S1S2

positivepositivesuppliesreceives

positivenegativereceivessupplies

negativepositivereceivessupplies

negativenegativesuppliesreceives

DETERMINE WHETHER THE ELEMENTS ARE SUPPLYING OR RECEIVING POWERAND HOW MUCHWHEN IN DOUBT LABEL THE TERMINALSOF THE COMPONENT

SELECT HERE THE CURRENT REFERENCE DIRECTIONBASED ON VOLTAGE REFERENCE POLARITYWHICH TERMINAL HAS HIGHER VOLTAGE AND WHICH IS THE CURRENT FLOW DIRECTION

P1 = 12WP2 = 36WP3 = -48WIMPORTANT: NOTICE THE POWER BALANCE IN THE CIRCUITCOMPUTE POWER ABDORBED OR SUPPLIED BY EACH ELEMENT

CIRCUIT ELEMENTSVOLTAGE DEPENDENTSOURCESCURRENTDEPENDENTSOURCES

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