Chapter 21

A uniformly wound solenoidal coil of self-inductance \(1.8 \times 10^{-4} \mathrm{H}\) and resistance \(6 \Omega\) is broken up into two identical coils. These identical coils are then connected in parallel across a \(12 \mathrm{~V}\) battery of negligible resistance. The time constant of the current in the circuit and the steady state current through battery is (a) \(3 \times 10^{-5} \mathrm{~s}, 8 \mathrm{~A}\) (b) \(1.5 \times 10^{-5}\) s, \(8 \mathrm{~A}\) (c) \(0.75 \times 10^{-4} s, 4 \mathrm{~A}\) (d) \(6 \times 10^{-5} \mathrm{~s}, 2 \mathrm{~A}\)

Match the following Column I to Column II. \begin{tabular}{c|l|c|l} \hline \multicolumn{2}{c|} { Column I } & \multicolumn{2}{c} { Column II } \\ \hline L. & Condenser & A. & increases AC \\ II. & Inductor & B. & reduces \(A C\) \\ III. & Energy dissipation is due to & C. & is conductor for DC \\ IV. & A transformer & D. & resistance only \\ \hline \end{tabular} (a) \(1-B, \|-B, C M I-D, I V-A, B\) (b) \(1-\mathrm{D}, \|-\mathrm{C}, \mathrm{D}, \mathrm{III}-\mathrm{B}, \mathrm{IV}-\mathrm{B}, \mathrm{C}\) (c) \(1-\mathrm{A}, \mathrm{II}-\mathrm{B}, \mathrm{C}, \mathrm{III}-\mathrm{D}, \mathrm{IV}-\mathrm{B}\) (d) \(1-\mathrm{C}, 11-\mathrm{B}, \mathrm{III}-\mathrm{D}, \mathrm{IV}-\mathrm{A}\)

When a voltage measuring device is connected to \(\mathrm{AC}\) mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar] (a) input voltage cannot be AC voltage, but a DC voltage. (b) maximum input voltage is \(220 \mathrm{~V}\) (c) the meter reads not \(v\) but \(\) and is calibrated to read \(\sqrt{\left\langle v^{2}\right\rangle}\) (d) the pointer of the meter is stuck by some mechanical defect.

When a voltage measuring device is connected to \(\mathrm{AC}\) mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar] (a) input voltage cannot be AC voltage, but a DC voltage. (b) maximum input voltage is \(220 \mathrm{~V}\) (c) the meter reads not \(v\) but \(\) and is calibrated to read \(\sqrt{\left\langle v^{2}\right\rangle}\) (d) the pointer of the meter is stuck by some mechanical defect.

A \(60 \mu \mathrm{F}\) capacitor is connected to a \(110 \mathrm{~V}, 60 \mathrm{~Hz} \mathrm{AC}\) supply. Determine the \(\mathrm{rms}\) value of the current in the circuit. (a) \(2.5 \mathrm{~A}\) (b) \(2.1 \mathrm{~A}\) (c) \(3.1 \mathrm{~A}\) (d) \(3.5 \mathrm{~A}\)

Assertion Inductance coil are made of copper. Reason Induced current is more in wire having less resistance.

A \(60 \mu \mathrm{F}\) capacitor is connected to a \(110 \mathrm{~V}, 60 \mathrm{~Hz} \mathrm{AC}\) supply. Determine the \(\mathrm{rms}\) value of the current in the circuit. (a) \(2.5 \mathrm{~A}\) (b) \(2.1 \mathrm{~A}\) (c) \(3.1 \mathrm{~A}\) (d) \(3.5 \mathrm{~A}\)

Assertion The armature current in DC motor maximum when the motor has just started. Reason Armature current is given by \(i=\frac{E-e}{R_{\alpha}}\), where \(e=\) the back emf and \(R_{a}=\) resistance of armature.

The turns ratio of transformer is given as \(2: 3 .\) If the current passing through the primary coil is \(3 \mathrm{~A}\). Find the current through the load resistance. (a) \(4.5 \mathrm{~A}\) (b) \(1.5 \mathrm{~A}\) (c) \(2 \mathrm{~A}\) (d) \(\mid \mathrm{A}\) The \(\bar{n}\) tuxe 4

The primary winding of a transformer has 200 turns and its secondary winding has 50 turns, If the current in the secondary winding is \(40 \mathrm{~A}\), the current in the primary is (a) \(10 \mathrm{~A}\) (b) \(80 \mathrm{~A}\) (c) \(160 \mathrm{~A}\) (d) \(800 \mathrm{~A}\)

The number of turns in a secondary coil is twice the number of turns in primary. A leclanche cell of \(1.5 \mathrm{~V}\) is connected across the primary. The voltage across secondary is (a) \(1.5 \mathrm{~V}\) (b) \(3.0 \mathrm{~V}\) (c) \(240 \mathrm{~V}\) (d) zero

Assertion In a series \(R-L-C\) circuit the voltage across resistor, inductor and capacitor are \(8 \mathrm{~V}, 16 \mathrm{~V}\) and \(10 \mathrm{~V}\) respectively. The resultant emf the circuit is \(10 \mathrm{~V}\). Reason Resultant emf of the circuit is given by the relation \(E=\sqrt{V_{R}^{2}+\left(V_{L}-V_{C}\right)^{2}}\)

A step-up transformer is used on a \(120 \mathrm{~V}\) line to provide a potential difference of \(2400 \mathrm{~V}\). If the primary coil has 75 turns, the number of turns in the secondary coil is (a) 150 (b) 1200 (c) 1500 (d) 1575

Assertion In a series \(R-L-C\) circuit the voltage across resistor, inductor and capacitor are \(8 \mathrm{~V}, 16 \mathrm{~V}\) and \(10 \mathrm{~V}\) respectively. The resultant emf the circuit is \(10 \mathrm{~V}\). Reason Resultant emf of the circuit is given by the relation \(E=\sqrt{V_{R}^{2}+\left(V_{L}-V_{C}\right)^{2}}\)

The ratio of turns in primary and secondary coils of a transformer is \(1: 20 .\) The ratio of currents in primary and secondary coils will be (a) \(1: 20\) (b) \(20: 1\) (c) \(1: 400\) (d) \(400: 1\) A

Assertion In series \(L-C-R\) circuit resonance can take place. Reason Resonance takes place if inductive and capacitive reactances are equal and opposite.

The ratio of turns in primary and secondary coils of a transformer is \(1: 20 .\) The ratio of currents in primary and secondary coils will be (a) \(1: 20\) (b) \(20: 1\) (c) \(1: 400\) (d) \(400: 1\) A

A low-loss transformer has \(230 \mathrm{~V}\) applied to the primary and gives \(4.6 \mathrm{~V}\) in the secondary. Secondary is connected to a load, which draws \(5 \mathrm{~A}\) of current. The current (in ampere) in the primary is (a) \(0.1\) (b) \(1.0\) (c) 10 (d) 250

In AC series circuit, the resistance, inductive reactance and capacitive reactance are \(3 \Omega, 10 \Omega\) and \(14 \Omega\) respectively. The impedance of the circuit is [Orissa JEE 2011] (a) \(5 \Omega\) (b) \(4 \Omega\) (c) \(7 \Omega\) (d) \(10 \Omega\)

A transformer is used to light \(140 \mathrm{~W}, 24 \mathrm{~V}\) lamp from \(240 \mathrm{~V}\) AC mains, The current in the mains is \(0.7 \mathrm{~A}\). The efficiency of transformer is nearest to (a) 909 (b) 8096 (c) 7096 (d) \(60 \%\)

The reduce the resonant frequency in an \(L-C-R\) series circuit with a generator \(\quad\) [NCERT Exemplar] (a) the generator frequency should be reduced (b) another capacitor should be added in parallel to the first [c) the iron core of the inductor should be removed (d) dielectric in the capacitor should be removed

The reduce the resonant frequency in an \(L-C-R\) series circuit with a generator \(\quad\) [NCERT Exemplar] (a) the generator frequency should be reduced (b) another capacitor should be added in parallel to the first [c) the iron core of the inductor should be removed (d) dielectric in the capacitor should be removed

An AC voltage source of variable angular frequency @ and fixed amplitude \(V\) connected in series with a capacitance \(C\) and an electric bulbs of resistance \(R\) (inductance zero) when wis increased [IIT JEE 2010] (a) The bulb glows dimmer (b) The bulb glows brighter (c) Total impedence of the circuit is unchanged (d) Total impedence of the circuit increases

A transformer is having 2100 turns in primary and 4200 turns in secondary. An AC source of \(120 \mathrm{~V}, 10 \mathrm{~A}\) is connected to its primary. The secondary voltage and current are (a) \(240 \mathrm{~V}, 5 \mathrm{~A}\) (b) \(120 \mathrm{~V}, 10 \mathrm{~A}\) (c) \(240 \mathrm{~V}, 10 \mathrm{~A}\) (d) \(120 \mathrm{~V}, 20 \mathrm{~A}\)

A conducting circular loop is placed in a uniform magnetic field \(0.04 \mathrm{~T}\) with its plane perpendicular to the magnetic field. The radius of the loop starts sinking at \(2 \mathrm{mms}^{-1}\). The induced emf in the loop when the radius is \(2 \mathrm{~cm}\) is [Kerala CET 2009] (a) \(1.6 \pi \mu \mathrm{V}\) (b) \(3.2 \pi \mu \mathrm{V}\) (c) \(4.8 \pi \mu \mathrm{V}\) [d) \(0.8 \pi \mu \mathrm{V}\)

An AC voltage source has an output of \(\Delta V=(200 \mathrm{~V}) \sin 2 \pi f t\). This source is connected to a \(100 \mathrm{~W}\) resistor. Rms current in the resistance is [Kerala CET 2008] (a) \(1.41 \mathrm{~A}\) (b) \(2.41 \mathrm{~A}\) (c) \(3.41 \mathrm{~A}\) (d) \(0.71 \mathrm{~A}\)

A transformer is used to light a \(100 \mathrm{~W}-110 \mathrm{~V}\) lamp from \(220 \mathrm{~V}\) mains. If main current is \(0.5 \mathrm{~A}\), efficiency of transformer is (a) \(90 \%\) (b) 9596 (c) 9696 (d) \(99 \%\)

If the self-inductance of 500 turn coil is \(125 \mathrm{mH}\), then the self- inductance of similar coil of 800 turns is [Kerala CET 2008] (a) \(48.8 \mathrm{mH}\) (b) \(200 \mathrm{mH}\) (c) \(187.5 \mathrm{mH}\) (d) \(320 \mathrm{mH}\)

A coil of inductance \(300 \mathrm{mH}\) and resistance \(2 \Omega\) is connected to a source of voltage \(2 \mathrm{~V} .\) The current reaches half of its steady state value in [Kerala CET 2008] (a) \(0.05 \mathrm{~s}\) (b) \(0.1 \mathrm{~s}\) (c) \(0.15 \mathrm{~s}\) (d) \(0.3 \mathrm{~s}\)

The flux linked with a circuit is given by \(\phi=t^{3}+3 t-7 .\) The graph between time \((x\)-axis) and induced emf ( \(y\)-axis) will be (a) a straight line through the origin (b) straight line with positive intercept (c) straight line with negative intercept (d) parabola not through the origin