# NCERT Physics Class 12 Exemplar Chapter 3 Current Electricity Part 5

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Q.21 A cell of emf E and internal resistance r is connected across an external resistance R. Plot a graph showing the variation of P.D. across R, verses R.

When the cell

*of*emf E and internal resistance*r*is connected across an external resistance*R,*the relationship between the voltage across*R*is given*R*by

With the increase of

*R, V*approaches closer to*E and*when*E*is infinite, V reduces to O.

The graphical representation of voltage across

*R*and the resistance*R*is as shown.This graph shows that potential difference incrc3scs with resistance after

*a*certain value, it becomes constant equal to*E.*

## Short Answer

Q.22 First a set of n equal resistors of R each are connected in series to a battery of emf E and internal resistance R. A current I is observed to flow. Then the n resistors are connected in parallel to the same battery. It is observed that the current is increased 10 times. What is ‘n’?

Ans:

Q.23 Let there be n resistors with and Show that when they are connected in parallel, the resultant resistance and when they are connected in series, the resultant resistance. Interpret the result physically.

And.

In Fig. (b), provides an equivalent route as in Fig. (a) For current. But in addition there are routes by the remaining resistors.

Current in Fig.(b) > current in Fig. (a). Effective Resistance in Fig. (b) Second circuit evidently affords a greater resistance.

You can use Fig. (c) and (d) and prove

Q.24 The circuit in Figure shows two cells connected in opposition to each other. Cell is of emf and internal resistance; the cell is of emf and internal resistance. Find the potential difference between the points A and B.

Ans:

P.D. across

P.D. across

Point B is at a higher potential than A

Q.25 Two cells of same emf E but internal resistance and are connected in series to an external resistor R what should be the value of R so that the potential difference across the terminals of the first cell becomes zero?

Or

Q.26 Two conductors are made of the same material and have the same length. Conductor A is a solid wire of diameter 1mm. Conductor B is a hollow tube of outer diameter 2mm and inner diameter 1mm. Find the ratio of resistance RA to RB.