CIRCUIT THEORY

Basic circuit

OHM’S LAW

• The law stating that the direct current flowing in a conductor is directly proportional to the potential difference and undirectly proportional to the resistance.

• It is usually formulated as V = IR, where V is the potential difference, or voltage, I is the current, and R is the resistance of the conductor

Types of circuit connection

i. Series circuit

Formula; i. RT = R1 + R2 + R3

ii. VT = V1+V2+V3 where V1=IR1, V2=IR2, V3=IR3 iii. I = I1 = I2 =I3 where I = VT/RT

I V1 V2

V3

Types of circuit connected

ii. Parallel circuit

Formula ; i. 1/RT = 1/R1 + 1/R2 + 1/R3 ,jika 2 rintangan sahaja

selari maka RT = R1xR2 R1+R2 i. VT = V1 = V2 = V3 where V1=I1R1, V2=I2R2,

V3=I3R3 iii. IT = I1 + I2 + I3 atau IT = VT / RT

I1 I2

I3 IT

RT9V

V1 V2 V3

Types of circuit connected iii. Series-Parallel circuit

Ra = R1xR2 R1+R2 Rb = R3xR4 R3+R4 RT = Ra + Rb

RT24V

IT

IT

E =IT x RT

Ra

Rb

Problem 1

Refer to the Figure, find VT if the current given as 0.5 A

Example ; (series circuit)

Solution

RT = R1 + R2 + R3

RT = 50 + 75 + 100

RT = 225

Find VT. Use Ohm’s law.

VT = IRT

VT = (0.5 amps)(225 )

VT = 112.5 volts

Find RT.

Problem 2

Refer to the circuit shown, determine ;

i. The battery voltage

ii. The total resistance of circuit

iii. The values of resistance of resistors R1, R2 and R3 if the voltage drop across R1, R2 and R3 are 5V, 2V and 6V.

V

R1 R2 R3

I=4A

V1 V2 V3

Solution

i. V1+V2+V3

5+2+6 = 13V

ii. Total resistance RT = VT/IT

= 13/4 = 3.25

iii. R1 = V1/I = 5/4 = 1.25

R2= V2/I = 2/4 = 0.5

R3 = V3/I = 6/4 = 1.5

Example (parallel circuit)

Problem 1;

Two resistors, of resistance 3 and 6 are connected in parallel across a battery having a voltage of 12V, determine ;

i. The total circuit resistance

ii. The current flowing in the 3 resistor

Solution

i. Total resistance

1/RT = 1/R1+1/R2 = 1/3+1/6 RT = 2

or use RT =R1 x R2

(R1+R2)

ii. Current flowing

I flowing through 3 = 12/3 = 4 A

Problem 2

For the circuit shown, find ;

i. The value of supply voltage, V

ii. The value of the current,I

Solution

i. The value of supply voltage, V

V=V2=I2 x R2

=3 x 20 = 60V

ii. The value of the current,I

given I2 = 3A,

find I1 = V/R1=60/10=6 A

find I3 = V/R3=60/60=1 A

IT=I1 + I2 + I3

= 6 + 3+ 1 = 10 A

Example 1; (Series-Parallel Circuit)

• Refer to the circuit shown, find ;

• i) Equivalent resistance, RT

• ii)Total current, IT

i)Req 1 = 20Ω parallel with 30Ω

• = 20x30/(20+30) = 12Ω (litar B)

• RT = Req 1 + R1

• = 12Ω + 8Ω = 20Ω (litar C)

Solution ;

• ii) IT = VT / RT

= 60V / 20Ω

= 3 Amp (rujuk litar C)