Page 38 - Physics - XII

Q23. How can you increase the range of the converted galvanometer to 0-60 mA? (NCERT)
Ans. Shunt resistance,
I
S = g G
I − I g

To increase the range of ammeter by n times,
I = nIg
Thus, the resistance of the shunt becomes.

G
S =
n −1
Q24. How can you decrease the range of the converted galvanometer to 0–20 mA? (NCERT)
Ans. We cannot decrease the range of converted galvanometer (ammeter) because for I < I , the value of
g
shunt resistance becomes negative which is not possible.

Q25. Why is a voltmeter always connected in parallel with the circuit?
Ans. A voltmeter has very high resistance. To ensure that it’s connection do not alter the fl ow of current
in the circuit, it is connected in parallel with the circuit.

Q26. If S << G, what is the order of resistance of converted galvanometer? (NCERT)
Ans. If S is negligible in comparison to G, then we get the ideal ammeter. In this situation, the resistance
of converted galvanometer can be calculated by using the following formula:
V
R = – G
Ig
Q27. Why is an ammeter always connected in series with the circuit? (NCERT)
Ans. Because it measures the current passing through the circuit.

COMPETENCY BASED QUESTIONS

Multiple Choice Questions
Q1. In a moving coil galvanometer, current in the coil is

(a) directly proportional to angle of defl ection.
(b) inversely proportional to the angle of defl ection.
(c) directly proportional to the square root of the angle of defl ection.
(d) inversely proportional to the square root of the angle of defl ection.

Q2. The defl ecting torque acting on the coil of a galvanometer is
(a) inversely proportional to number of turns.

(b) inversely proportional to current fl owing.
(c) inversely proportional to area of the coil.
(d) directly proportional to the magnetic fi eld strength.

Q3. To convert a galvanometer to ammeter, a shunt S is to be connected with the galvanometer.
The eff ective resistance of the ammeter then is
(a) GS/(G + S) (b) (G + S)/GS (c) G + S (d) None of these

36