Assertion & Reason

Question 11 Mark

Assertion $\quad:$ In series $L C R$ circuit resonance can take place.
Reason : Resonance takes place if inductance and capacitive reactances are equal and opposite.

Answer

(a) At resonant frequency, $X_L=X_C \quad \therefore \quad Z=R \quad$ (minimum) there for current in the circuit is maximum.

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Question 21 Mark

Assertion : A capacitor of suitable capacitance can be used in an ac circuit in place of the choke coil.
Reason : A capacitor blocks dc and allows ac only.

Answer

(a) Capacitive reactance $X_C=\frac{1}{\omega C}$. When capacitance $(C)$ increases, the capacitive reactance decreases. Due to decrease in its values, the current in the circuit will increases $\left(I=\frac{E}{\sqrt{R^2+X_C^2}}\right)$ and hence brightness of source (or electric lamp) will also increases.

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Question 31 Mark

Assertion : An inductance and a resistance are connected in series with an ac circuit. In this circuit the current and the potential difference across the resistance lag behind potential difference across the inductance by an angle $\pi / 2$.
Reason : In $L R$ circuit voltage leads the current by phase angle which depends on the value of inductance and resistance both.

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Question 41 Mark

Assertion : An electric lamp connected in series with a variable capacitor and ac source, its brightness increases with increase in capacitance.
Reason : Capacitive reactance decrease with increase in capacitance of capacitor.

Answer

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Question 51 Mark

Assertion : When ac circuit contain resistor only, its power is minimum.
Reason : Power of a circuit is independent of phase angle.

Answer

(d) The power of a ac circuit is given by $P=E I \cos \phi$where $\cos \phi$ is power factor and $\phi$ is phase angle. In case of circuit containing resistance only, phase angle is zero and power factor is equal to one. Therefore power is maximum in case of circuit containing resistor only.

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Question 61 Mark

Assertion : The division are equally marked on the scale of ac ammeter.
Reason : Heat produced is directly proportional to the current.

Answer

(d) An ac ammeter is constructed on the basics of heating effect of the electric current. Since heat produced varies as square of current $\left(H=I^2 R\right)$. Therefore the division marked on the scale of ac ammeter are not equally spaced.

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Question 71 Mark

Assertion : Average value of ac over a complete cycle is always zero.
Reason : Average value of ac is always defined over half cycle.

Answer

(b) The mean average value of alternating current (or emf) during a half, cycle is given by $I_m=0.636 I_0$ (or $E_m=0.636 E_0$ )During the next half cycle, the mean value of ac will be equal in magnitude but opposite in direction.For this reason the average value of ac over a complete cycle is always zero. So the average value is always defined over a half cycle of ac.

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Question 81 Mark

Assertion $\quad:$ ac is more dangerous than $dc$
Reason : Frequency of ac is dangerous for human body.

Answer

(a) The effect of ac on the body depends largely on the frequency. Low frequency currents of 50 to $60 \mathrm{~Hz}$ (cycles/sec), which are commonly used, are usually more dangerous than high frequency currents and are 3 to 5 times more dangerous than $\mathrm{dc}$ of same voltage and amperage (current). The usual frequency of $50 \mathrm{cps}$ (or $60 \mathrm{cps}$ ) is extremely dangerous as it corresponds to the fibrillation frequency of the myocardium. This results in ventricular fibrillation and instant death.

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Question 91 Mark

Assertion : The dc and ac both can be measured by a hot wire instrument.
Reason : The hot wire instrument is based on the principle of magnetic effect of current.

Answer

(c) Both ac and dc produce heat, which is proportional to square of the current. The reversal of direction of current in ac is immaterial so far as production of heat is concerned.

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Question 101 Mark

Assertion : An alternating current does not show any magnetic effect.
Reason : Alternating current varies with time.

Answer

(b) Like direct current, an alternating current also produces magnetic field. But the magnitude and direction of the field goes on changing continuously with time.

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Question 111 Mark

Assertion : A bulb connected in series with a solenoid is connected to ac source. If a soft iron core is introduced in the solenoid, the bulb will glow brighter.
Reason : On introducing soft iron core in the solenoid, the inductance increases.

Answer

(e) On introducing soft iron core, the bulb will glow dimmer. This is because on introducing soft iron core in the solenoid, its inductance $L$ increases, the inductive reactance, $X_L=\omega L$ increases and hence the current through the bulb decreases.

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Question 121 Mark

Assertion : If the frequency of alternating current in an ac circuit consisting of an inductance coil is increased then current gets decreased.
Reason : The current is inversely proportional to frequency of alternating current.

Answer

(a) When frequency of alternating current is increased, the effective resistance of the inductive coil increases. Current $\left(X_L=\omega L=2 \pi f L\right)$ in the circuit containing inductor is given by $I=\frac{V}{X_L}=\frac{V}{2 \pi f L}$. As inductive resistance of the inductor increases, current in the circuit decreases.

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Question 131 Mark

Assertion : Chock coil is preferred over a resistor to adjust current in an ac circuit.
Reason : Power factor for inductance is zero.

Answer

(a) If resistor is used in controlling ac supply, electrical energy will be wasted in the form of heat energy across the resistance wire. However, ac supply can be controlled with choke without any wastage of energy. This is because, power factor $(\cos \phi)$ for resistance is unity and is zero for an inductance. $[P=E I \cos \phi]$.

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Question 141 Mark

Assertion : When capacitive reactance is smaller than the inductive reactance in $L C R$ current, e.m.f. leads the current .
Reason : The phase angle is the angle between the alternating e.m.f. and alternating current of the circuit.

Answer

(b) The phase angle for the $L C R$ circuit is given by$\tan \phi=\frac{X_L-X_C}{R}=\frac{\omega L-1 / \omega C}{R}$Where $X, X$ are inductive reactance and capacitive reactance respectively when $X_{+}>X_c$ then $\tan \phi$ is positive i.e. $\phi$ is positive (between 0 and $\pi / 2$ ). Hence emf leads the current.

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Question 151 Mark

Assertion : Capacitor serves as a block for dc and offers an easy path to ac.
Reason : Capacitive reactance is inversely proportional to frequency.

Answer

(a) The capacitive reactance of capacitor is given by$X_C=\frac{1}{\omega C}=\frac{1}{2 \pi f C}$So this is infinite for $\mathrm{dc}(f=0)$ and has a very small value for ac. Therefore a capacitor blocks dc.

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Question 161 Mark

Assertion : The alternating current lags behind the e.m.f. by a phase angle of $\pi / 2$, when ac flows through an inductor.
Reason : The inductive reactance increases as the frequency of ac source decreases.

Answer

(c) When ac flows through an inductor current lags behind the emf., by phase of $\pi / 2$, inductive reactance, $X_L=\omega L=\pi \cdot 2 f . L$, so when frequency increases correspondingly inductive reactance also increases.

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