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

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.

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.

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