A step-down transformer is connected to main supply 200V to operate a 6V, 30W bulb. The current in primary is 

(1) 3 A

(2) 1.5 A

(3) 0.3 A

(4) 0.15 A

Subtopic:  Transformer |
 77%
From NCERT
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A transformer has 100 turns in the primary coil and carries 8 A current. If input power is one kilowatt, the number of turns required in the secondary coil to have 500V output will be 

(1) 100

(2) 200

(3) 400

(4) 300

Subtopic:  Transformer |
 86%
From NCERT
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The potential difference V and the current i flowing through an instrument in an ac circuit of frequency f are given by V=5cosωt volts and I = 2 sin ωt amperes (where ω = 2πf). The power dissipated in the instrument is 

(1) Zero

(2) 10 W

(3) 5 W

(4) 2.5 W

Subtopic:  Different Types of AC Circuits |
 63%
From NCERT
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A generator produces a voltage that is given by V = 240 sin 120 t, where t is in seconds. The frequency and r.m.s. voltage are 

(1) 60 Hz and 240 V

(2) 19 Hz and 120 V

(3) 19 Hz and 170 V

(4) 754 Hz and 70 V

Subtopic:  RMS & Average Values |
 80%
From NCERT
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The peak value of an alternating e.m.f. E is given by E=E0cosωt is 10 volts and its frequency is 50 Hz. At time t=1600sec, the instantaneous e.m.f. is

1. 10 V

2. 53V

3. 5 V

4. 1 V

Subtopic:  Different Types of AC Circuits |
 80%
From NCERT
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If a current I given by I0sinωtπ2 flows in an ac circuit across which an ac potential of E=E0sinωt has been applied, then the power consumption P in the circuit will be 

(1) P=E0I02

(2) P=2E0I0

(3) P=E0I02

(4) P = 0

Subtopic:  Different Types of AC Circuits |
 74%
From NCERT
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A resistance of \(20~ \Omega\) is connected to a source of an alternating potential, \(V=220\sin(100 \pi t).\) The time taken by the current to change from its peak value to its rms value will be: 
1. \( 0.2~\text{sec}\) 2. \( 0.25~\text{sec}\)
3. \(25 \times10^{-3}~\text{sec}\) 4. \(2.5 \times10^{-3}~\text{sec}\)
Subtopic:  RMS & Average Values |
 66%
From NCERT
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An alternating current of frequency \(f\) is flowing in a circuit containing a resistance \(R\) and a choke \(L\) in series. The impedance of this circuit will be:
1. \(R + 2\pi f L\)
2. \(\sqrt{R^{2} + 4 \pi^{2} f^{2} L^{2}}\)
3. \(\sqrt{R^{2} + L^{2}}\)
4. \(\sqrt{R^{2} + 2 \pi f L}\)
Subtopic:  Different Types of AC Circuits |
 87%
From NCERT
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A resistance of \(300~\Omega\) and an inductance of \(\frac{1}{\pi}\) henry are connected in series to an AC voltage of \(20\) volts and a \(200\) Hz frequency. The phase angle between the voltage and current will be:

1. \(\tan^{- 1} \dfrac{4}{3}\) 2. \(\tan^{- 1} \dfrac{3}{4}\)
3. \(\tan^{- 1} \dfrac{3}{2}\) 4. \(\tan^{- 1} \dfrac{2}{5}\)
Subtopic:  Different Types of AC Circuits |
 80%
From NCERT
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In a region of uniform magnetic induction B = 10–2 tesla, a circular coil of radius 30 cm and resistance π2 ohm is rotated about an axis that is perpendicular to the direction of B and which forms a diameter of the coil. If the coil rotates at 200 rpm the amplitude of the alternating current induced in the coil is :

(1) 4π2 mA

(2) 30 mA

(3) 6 mA

(4) 200 mA

Subtopic:  Different Types of AC Circuits |
 57%
From NCERT
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