Page 45 - Physics - XII

Q18. How does the wire vibrate? Identify and explain the rule with the help of which you can
determine the direction of force acting on the wire. (NCERT)
Ans. The current fl owing in the electromagnet core is alternating in nature. So, the force which acts on
the sonometer wire is also oscillatory in nature. Hence, the sonometer wire vibrates. Fleming's left
hand rule is used to fi nd the direction of force acting on the wire.

COMPETENCY BASED QUESTIONS

Multiple Choice Questions

Q1. In an A.C. circuit, the maximum value of voltage is 300 2 volts. Its eff ective voltage is

(a) 400 volt (b) 300 volt (c) 340 volt (d) 600 volt

Q2. When frequency of applied alternating voltage becomes very high then
(a) a capacitor will tend to become SHORT. (b) an inductor will tend to become SHORT.
(c) both (a) and (b). (d) no one will become short.

Q3. An A.C. source is connected to a resistive circuit. Which of the following statements is true?
(a) Current leads the voltage in phase. (b) Current lags the voltage in phase.
(c) Current and voltage are in same phase.

(d) Either (a) and (b) depending on the value of resistance.
Q4. The frequency of A.C. is 50 Hz. How many times the current becomes zero in one second?
(a) 50 times (b) 100 times (c) 200 times (d) 25 times

Q5. R, L, and C represent the physical quantities resistance, inductance, and capacitance

respectively. Which one of the following combinations has dimension of frequency?
1 R 1 C
(a) (b) (c) (d)
RC L LC L

Q6. A capacitor of capacitance C has reactance X . If capacitance and frequency becomes double,
C
then the capacitive reactance will be
X X
(a) 2X (b) 4X (c) C (d) C
C
C
4 2
Q7. When a voltage measuring device is connected to A.C. mains, the meter shows the steady input
voltage of 220 V. This means [NCERT Exemplar]
(a) input voltage cannot be A.C. voltage, but a D.C. voltage.
(b) maximum input voltage is 220 V.
(c) the meter reads not V, but < V > and is calibrated to read V 2 .
2
(d) the pointer of the meter is stuck by some mechanical defect.

Q8. The phase diff erence between the current and the voltage in series LCR circuit at resonance is
(a) π (b) π/2 (c) π/3 (d) zero

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