Question

7. A resistance of 30 ohm is connected across 240v supply. If a resistance R ohm is connected in
parallel with 30ohm resistor across the same supply, the current drawn becomes triple of original
one The unknown resistor R is
a. 15ohm b. 10ohm c. 5ohm d.30ohm​

Answer 1

Answer:

a. 15 ohm

Explanation:

see in first case

V= 240V , R = 30 ohm

so , I = V/R= 8A

now when a resistance of R ohm is connected in parallel , current is triple

so current in this case will be 8×3 = 24 A

and voltage is same that is 240V

so total resistance = V/I = 10 ohm

now we know that equivalent resistance in parallel = R1R2/R1+ R2

10 = 30 × R/30+ R

300 + 10R = 30R

300= 20R

R = 15 ohm

hope it helps you

Answer 2

Given:

Voltage (V) = 240V

Resistance (R) = 30 ohm

To find:

The unkown resistor R

Solution:

Current (I) = V/R

                 = 240/30

                 = 8A

• When the resistance of R ohm is connected in parallel, the current is triple

Current, in this case, will be 8×3 = 24 A  

Total resistance = V/I

                           = 240/24

                           = 10 ohm

Equivalent resistance in parallel = R1 x R2/R1+ R2

                                                  10 = 30 × R/30+ R

                                                 300 + 10R = 30R

                                                             300= 20R

                                                              R = 15 ohm

Hence, the unkown resistor R is a) 15 ohm.