Physics MCQs for NEET — Practice Questions with Answers

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The peak value of an alternating e.m.f. E is given by E=E0cosω t is 10 volts and its frequency is 50 Hz. At time t=1600sec, the instantaneous e.m.f. is

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Explanation

E=E0cosωt=E0cos2πtT

=10cos2π×50×1600=10 cosπ6=53 volt. 

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If a current I given by I0sin ω t−π2 flows in an ac circuit across which an ac potential of E=E0sinω t has been applied, then the power consumption P in the circuit will be 

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Explanation

Phase angle ϕ=90o, so power P=Vicosϕ=0  

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In an ac circuit, the instantaneous values of e.m.f. and current are e = 200 sin 314 t volt and i=sin 314t+π3 ampere. The average power consumed in watt is 

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Explanation

Vrms=2002,  irms=12

∴P=Vrms irmscosϕ=200212cosπ3=50 watt 

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An alternating current is given by the equation i=i1cosω t+i2sinω t. The r.m.s. current is given by

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Explanation

irms=i12+i222=12i12+i221/2 

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In an ac circuit, the current is given by i=5sin 100 t−π2 and the ac potential is V = 200 sin(100t) volt. Then the power consumption is :

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Explanation

P=Vicosϕ 

Phase difference ϕ=π2⇒P=zero  

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A resistance of 20 ohms is connected to a source of an alternating potential V=220sin(100πt). The time taken by the current to change from its peak value to r.m.s value is 

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Explanation

Peak value to r.m.s. value means current becomes 12 times.

 

Time taken by the current to reach its peak value:i=i0sin100πt⇒i0=i0sin100πt1⇒sin100πt1=sinπ2⇒t1=1200secTime taken by the current to become i2 after reaching its peak value:i=i0sin100πt⇒12×i0=i0sin100πt2⇒sin100πt2=sin3π4⇒t2=3400secTime taken by the current to change from its peak value to r.m.s. value:t2-t1=3400-1200=1400sec=2.5×10−3sec. 

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Voltage and current in an ac circuit are given by V=5sin 100πt−π6 and I=4sin 100πt+π6 

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Explanation

Phase difference Δϕ=ϕ2−ϕ=π6−−π6=π3  

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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 is 

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Explanation

Z=R2+XL2,  XL=ωL and ω=2πf

∴Z=R2+4π2f2L2 

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A resistance of 300 Ω and an inductance of 1π henry are connected in series to a ac voltage of 20 volts and 200 Hz frequency. The phase angle between the voltage and current is :

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Explanation

Phase angle tanϕ=ωLR=2π×200300×1π=43

∴ ϕ=tan−143

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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 :

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Explanation

Amplitude of ac=i0=V0R=ωNBAR=(2πν)NB(πr2)R

⇒i0=2π×20060×1×10−2×π×(0.3)2π2=6 mA

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