CHAPTER VIIDIRECT CURRENT CIRCUITS

A. DEFINITION OF ELECTRIC CURRENTElectric current illustrated as a motion of positive charges passing through from the higher potential to the lower potential.Electric current (i) defined as the amount of charge passing through in every unit of time ( second ).n = the amount of electrone = the electron charge/the elementary charge = 1,6 x 10 -19 Ci = Electric current ( ampere )q = charge ( coulomb )t = unit of time ( second )

B. RESISTANCE OF CONDUCTING WIREThe resistance of a conducting wire depends on :Length of the wireCross section AreaKind of the wireTemperatureFormula : R = Resistance ( Ohm, ) = Resistivity of the material ( m)A = Cross-section Area ( m2) L = Length (m)

Resistivities and TemperaturecCoefficients of Resistivity fo various Materials

MATERIALSRESISTIVITY(m)TEMPERATURE COEFFICIENT(1/OC)SilverCopperGoldAluminumTungstenIronPlatinumLeadNichromeCarbonGermaniumSiliconGlass1,59 x 10-81,7 x 10-82,44 x 10-82,82 x 10-85,6 x 10-810 x 10-811 x 10-822 x 10-81,50 x 10-63,5 x 10-50,466401010 - 10143,8 x 10-33,9 x 10-33,4 x 10-33,9 x 10-34,5 x 10-35,0 x 10-33,92 x 10-33,9 x 10-30,4 x 10-3-0,5 x 10-3-48 x 10-3-75 x 10-3

Temperature Influence for resistivity and resistanceIf temperature of wire is increase, so the resistivity and the resistance of it is increaseororO = initial of resistivity (m)t = final of resistivity (m)Ro = initial of resistance()Rt = final of resistance ()T = the change of temperature (oC) = temperature coefficient of resistivity (/oC)R = The change of resistance = The change of resistivity

C. OHMS LAWThe ratio of the voltage (V) across a conductor to the current (i) that flows through it is equal to a constant. This constant is called resistance (R)

R = tan A = AmmeterV = VoltmeterL = Lampi = Current (A)V = Voltage/the potential difference (V)R = Resistance ()

MEASUREMENT OF CURRENT AND VOLTAGE

SERIES AND PARALLEL CIRCUIT Kirchhoffs first rule:The sum of the currents entering the any junction must equal the sum of the currents leaving the junction.Example : I1i5i6I2i3i4i1 + i2 + i4 = i3 + i5 + i6

SERIES CIRCUIT (VOLTAGE DIVIDER)i1 = i2 = i3 = IRS = R1 + R2 + R3V = V1 + V2 + V3V1 : V2 : V3 = R1 : R2 : R3Characteristic :The current passing through every resistor is equal.The potential difference on every resistor is different.

PARALLEL CIRCUIT (ELECTRIC CURRENT DIVIDER)

Characteristics : The current passing through the junction is different. The potential difference of every junction is equal.

If in the galvanometer (G) there are no electric current passed, called a galvanometer in equilibrium conditionE. WHEAT STONES BRIDGER1 . RB = R2 . RAR1 . LB = R2 . LAbecauseso;RA= wire resistance of part ARB= wire resistance of part BLA= wire length of part ALB= wire length of part B

The forms of Wheat stone bridge:R1 . R3 = R2 . R4 @@@If:so, R5 can reduced and then the wheat stone bridge circuit can simplify to be:

R1 . R3 R2 . R4 Ifso, the circuits can be transforms to Y form (transformation of to Y forms)

F. SOURCE OF ELECTROMOTIVE FORCE (EMF)Current in conductor is produced by an electric field, and electric field is formed by the potential difference, devices such as batteries and dynamos should be connected to the circuit. These sources of electric energy are called source of electromotive force () When the switch K is open, the voltmeter reads is EMF () When the switch K is closed, the voltmeter reads is clamping voltage (V)V= i RV= clamping voltage = potential difference on the external resistance

= EMF (volt)r = internal resistor ( )R = external resistor ( ) = i R + i r = i ( R + r )If the batteries are identical, and each has an EMF , and an internal resistance r Batteries connected in series = n r = n ri = 1 + 2 +3r = r1 + r2 + r3

r3r1r2 Batteries connected in parallel123 = For identical batteries:iR

G.KIRCHHOFFS SECOND RULESor = (i. R)(i.R) = Dropping Potential difference = EMF ( electromotive force ) + (i . R) = 0The sum of the drops in potential difference in a close circuit is equal to zero. In applying Kirchhoffs rules, the following rules should be noted:Assign a symbol and direction to the currents in each part of the circuitLoops are chosen and the direction around each loop is designatedThe sign of the current are taken + when they are in the same direction of loops, and taken - when they are in the opposite direction of loopsThe sign of the EMF are taken + when loops inside polar (+) of elements, and taken - when loops inside polar (-) of elements

WORK DONE BY THE ELECTRIC CURRENT ( JOULES LAW)W = q V W = electrical energy (J)V = potential difference (volt)q = charge (C) i = electric current (A)t = time ( s )The amount of heat dissipated from a current carrying conductor is proportional to the resistance of the conductor, the square of current and the time needed for the current to pass trough the conductorSince q = i t, W = V i t And V= I R W = i2 R t W = i2 R t

P = Electric Power (Watt)The electrical energy dissipated per unit time (second) is called electrical power.

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