STD-X, ELECTRICITY, PROBLEMS ON OHM'S LAW

 

1.  Calculate the potential difference required across a conductor of resistance 5 Ω to flow a current of 1.5 A through it.

 

ANS: Given, R = 5 Ω

                       I = 1.5 A

From Ohm’s law, V = IR = 1.5 x 5 = 7.5 V

 

2.  How much electric current is produced on applying p.d of 30 volts on a wire of resistance 20 Ω?

 

ANS: Given, R = 20 Ω

                      V = 30 V

According to Ohm’s law, V=IR

Therefore, I = V/R = 30/20 = 1.5 A

 

3.  If 5 ampere current flowing through a wire, develops a potential difference of 50 volts across its ends, find the resistance of the wire.

 

ANS: Given, I = 5 A

                      V = 50 V

Using Ohm’s law, V = IR

Therefore, R = V/I

                       = 50/5 = 10 Ω

 

4.  The p.d. between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the p.d. is increased to 120 V?

 

( As, current is directly proportional to Potential difference

So, as V is doubled (from 60 V to 120 V), current will also be doubled. So, the answer will be 2 x 4 = 8 A)

ANS: Given, V = 60 V

                       I = 4 A

According to Ohm’s law, V = IR

                                 So, R = V/I = 60/4 = 15 Ω

When the potential is increased to 120 V, the current is given by,

                                       I = V/R = 120/15 = 8 A

The current through the heater becomes 8 A.

 

5.  A relay with a resistance of 12 Ω is in an electric circuit with one side at a potential of 85 volts and the other at 71 volts. How much current is in the relay?

 

ANS: The p.d. across the relay = V₂ – V₁ = 85 V – 71 V =  14 V

Therefore, the current in the relay, I = V/ R = 14/12 = 1.2 A

PROBLEMS FOR PRACTICE

6.  A lamp of resistance 80 Ω draw a current of 0.75 A. Find the line voltage.

7.  An electric heater draw a current of 5 A when connected to 220 V mains. Calculate the resistance of its filament.

8.  How much current will an electric heater draw from a 200 V source, if the resistance of the filament is 100 Ω?

9.  How much current does an electric heater draw from a 220 V line, if the resistance of the heater (when hot) is 50 Ω?

10.  A bulb when cold has 1 Ω resistance. It draws a current of 0.3 A when glowing from a source of 3 V. Calculate the resistance of the bulb when it is glowing and explain the reason for the difference in resistance.