The current flowing through the 60 Æ and the can be worked out in either one of two ways: 30 Æ
Method 1: Calculate the potential difference across the resistors, and then use Ohm’s law to calculate current through them.
The potential difference across the 50 Æ resistor is: V IRQV1 a
35b(50) 60 6
7
This implies that the potential difference across the parallel part of the circuit is: The current flowing through each resistor can be calculated using Ohm’s law:
V IRQI2 VP R2
The current flowing through the 60 Æ resistor is 35 A . 2
V IRQI3 VP R3
The current flowing through the 30 Æ resistor is
I2 : I3 R3 : R2Q I2 I3
QI2 1 3 I & I3 2
QI2 a
1 3ba
R3 R2
3 I
35b 2 6
35 & I3 a
2 3ba
The current flowing through the 60 Æ resistor is 35 A and through the 30 Æ resistor is 2
35b 4 6
35
4 . 35 A
Q I2 I3
35 A . 4
Method 2: The currents through the parallel part of the circuit divide in the ratio of the resistances in the parallel part:
30 60QI2 1
2 I3
24 7 30 4
35
24 7 60 2
35
V V1 V2QVP V V1 12 60 7 24
7
EXERCISE 18.1 RESISTANCE
Q1 A circuit consists of a 20 resistor and an 80 resistor connected in parallel to a 3 V d.c. supply as illus- trated in Fig 18.6. Calculate: (i) the total resistance of the two resistors (ii) the current flowing in the circuit
(iii) the current in the 20 resistor.
Q2 A circuit consists of a 100 fixed resistor and a variable resistor set at its maximum resistance of 300 connected in series to a 12 V d.c. supply as illustrated in Fig 18.7. (i) Calculate the total resistance of the circuit. (ii) Calculate the current flowing in the circuit. (iii) Calculate the potential difference across the variable resistor. (iv) What happens to the potential difference across the variable resistor as its resistance is lowered?
