CBSE Class 12 · 2012 · Current ElectricityShort Answer

A cell of EMF

E

and internal resistance

r

is connected to two external resistances

R_{1}

and

R_{2}

and a perfect ammeter. The current is measured in four cases:
(i) no external resistance;
(ii) with

R_{1}

only;
(iii) with

R_{1}

and

R_{2}

in series;
(iv) with

R_{1}

and

R_{2}

in parallel. The four readings are

0.42 A, 1.05 A, 1.4 A

and

4.2 A

(not in order). Identify the current in each case.

Answer: (i) $4.2 A$ \;
(ii) $1.05 A$ \;
(iii) $0.42 A$ \;
(iv) $1.4 A$ .

Solution

Current is largest when the resistance is least and smallest when it is greatest. (i) Only the cell: least resistance

\Rightarrow

largest current

4.2 A

. (iii)

R_{1} + R_{2}

in series: greatest resistance

\Rightarrow

smallest current

0.42 A

. (iv)

R_{1} ∥ R_{2}

is smaller than either alone but larger than zero

\Rightarrow 1.4 A

. (ii)

R_{1}

only lies between (iv) and (iii)

\Rightarrow 1.05 A

CBSE Class 12 Board · 2012 · Subjective