Question

Bulbs B1 and B2 are exactly identical. When the key K is pressed, the reading of the ammeter will
(a) remain unchanged
(b) be doubled
(c) be halved
(d) become four times

Answer 1

Answer:

c)halved

Explanation:

in parallel current are different

Answer 2

Answer:

b) be doubled

Explanation:

Let the source voltage be V and the resistance of each bulb be R.

Then the current in the circuit when the key k is open:

$I_o=\frac{V}{R}$ ......................(1)

Now when the key k is closed:

$I_c=\frac{V}{R_{eq}}$ ..............(2)

Now the equivalent of the two resistances in series:

$\frac{1}{R_{eq}} =\frac{1}{R} +\frac{1}{R}$

$R_{eq}=\frac{R}{2}$

Now putting this value in eq. (2)

$I_c=\frac{V}{\frac{R}{2} }$

$I_c=\frac{2V}{R}$ ...................(3)

On comparing eq. (1)&(3):

$I_c=2I_o$

hence the current is doubled.

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TOPIC: resistance in parallel

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