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Current Electricity

Question
CBSEENPH12037559
Wired Faculty App

A potentiometer wire of length 100 cm has a resistance of 10 Ω. It is connected in series with a resistance and a cell of emf 2V and of negligible internal resistace. A source of emf 10 mV is balanced by a length of 40 cm of the potentiometer wire, What is the value of external resistance?

Solution

Given, 
Length of the potentiometer wire, l = 100 cm
Resistance of the wire, R = 10 Ω 
Emf of the cell, connected in series with it, E = 2V
Balancing length is obtained at 40 cm

Let, I be the current through the potentiometer wire, then

I= ER+R'  = 210 + R'       .... (1)

Now, the resistance of the 40 cm length of potentiometer wire is, 

R = 10100 × 40 = 4 Ω 

Emf of the cell balanced by a length of the wire, E' = 10 mV= 10 × 10-3 V. 

Therefore, using V=IR we have,

     10×10-3 = I × 4 ⇒ I= 2.5 × 10-3 A 

Now, putting the value of I obtained in equation 1 we get, 
 
             0.0025 = 210 + R' 0.025 + 0.0025 R' = 20.0025 R' = 2- 0.025R'= 1.9750.0025 = 790 Ω 

Thus, the value of external resistance is R' = 790Ω . 




Some More Questions From Current Electricity Chapter

What conclusion can you draw from the following observations on a resistor made of alloy manganin?

Current A

Voltage V

Current A

Voltage V

0.2

0.4

0.6

0.8

1.0

2.0

3.94

7.87

11.8

15.7

19.7

39.4

3.0

4.0

5.0

6.0

7.0

8.0

59.2

78.8

98.6

118.5

138.2

158.0

Answer the following questions:
A steady current flows in a metallic conductor of non-uniform cross-section. Which of these quantities is constant along the conductor: current, current density, electric field, drift speed?

Answer the following questions:
Is Ohm’s law universally applicable for all conducting elements? If not, give examples of elements which do not obey Ohm’s law.

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