Home Physics Current Electricity General In the circuit shown the three ammeters (mar…
Physics Current Electricity General MCQ (Single Correct)

In the circuit shown the three ammeters (marked as 1, 2, 3) are identical, each have a resistance
R 0 = 2 Ω Ω . Between points A and B there is a constant potential difference of 19V. The first and second ammeter read I 1 = 2.5A and I 2 = 1.5A respectively.

A
What is the reading of third ammeter?
B
Calculate value of resistance R.
C
Investigate what happens to current I 3 if the value of R x is changed. Show approximately graphical variation of I 3 vs R x .

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Text Solution

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The correct answer is:
CHECK THE SOLUTION.

[ 1 A, 4 Ω Ω , 1.1 A]

Sol.

R 0 = 2 Ω Ω , I 1 = 2.5A

I 2 =1.5A

V P = 19 – R 0 I

= 19 – (2) × (2.5)

V P = 14V

Similarly

V Q = 16V

⇒ I 3 = ⇒ I 3 = 1A

I 4 = = I 1 + I 3

⇒ I 4 = = I 1 + I 3

= 2.5+1

R = 4 Ω Ω

Applying KVL in FEQPF

– I 1 R 0 – I 3 R 0 + (I– I 1 ) ×R 0 = 0

I = I 3 + 2I 1 ... (i)

Applying KVL in AFEDCBA

V–I 1 R 1 – (I 1 – I 3 ) R X = 0

I 1 = .........(ii)

Applying KVL in AFCBA

V – (I–I 1 ) R 0 – (I–I 1 +I 3 ) R = 0

V – I (R 0 +R) +I 1 (R 0 +R) –I 3 R = 0

V– (I 3 +2I 1 ) (R 0 +R 1 ) +I 1 (R 0 +R) – I 3 R = 0 Using (i)

V – I 3 (R 0 +2R) –I 1 (R 0 +R) = 0

V– I 3 (R 0 +2R) – (R 0 +R) = 0 Using (ii)

The graph of the function is a hyperbola. Its special points are: at R x = 0, I 3 = –3.8 A ; at R x ; = 4I 3 = 0; at
R x = 32 Ω Ω , I 3 = 1A ; If R x → ∞ , I 3 tends to 19/16 = 1.1875A.

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