Electromagnetism PYQ — Page 62

In the circuit shown here, the point '$C$' is kept connected to point $A$ '' till the current flowing through the circuit becomes constant. Afterward, suddenly, point '$C$' is disconnected from point $A$ '' and connected to point '$B$' at time $t=0$. Ratio of the voltage across resistance and the inductor at $t=\frac{L}{R}$ will be equal to : 

During the propagation of electromagnetic wave in a particular medium :-

A metallic rod of length $l$ is tied to a string of length $2l$ and made to rotate with angular speed $\omega$ on a horizontal table with one end of the string fixed. If there is a vertical magnetic field B in the region, the e.m.f. induced across the ends of the rod is: 

Two capacitors ${C}_{1}$ and ${C}_{2}$ are charged to $\text{120 V}$ and $\text{200 V}$ respectively. It is found that by connecting them together the potential on each one can be made zero. Then:

The supply voltage to a room is $120V$. The resistance of the lead wires is $6 \Omega$. A $60W$ bulb is already switched on. What is the decrease of voltage across the bulb, when a $240W$ heater is switched on in parallel to the bulb?

A circular loop of radius $0.3\mathrm{cm}$ lies parallel to a much bigger circular loop of radius $20\mathrm{cm}$. The centre of the small loop is on the axis of the bigger loop. The distance between their centres is$15$ $\mathrm{cm}$. If a current of $2.0A$ flows through the smaller loop, then the flux linked with a bigger loop is:

The amplitude of a damped oscillator decreases to 0.9 times its original magnitude in 5s. In another 10s it will decrease to $\alpha$ times its original magnitude, where $\alpha$ equals :

The magnetic field in a travelling electromagnetic wave has a peak value of $20nT$. The peak value of electric field strength is :

A metal sample carrying a current along $\mathrm{X-}$ axis with density $\mathrm{J}_{\mathrm{x}}$ is subjected to a magnetic field $\mathrm{B}_z$ (along $\mathrm{z-}$axis). The electric field $\mathrm{E}_{\mathrm{y}}$ developed along $\mathrm{Y}$-axis is directly proportional to $\mathrm{J}_{\mathrm{x}}$ as well as $\mathrm{B}_{\mathrm{z}}$. The constant of proportionality has SI unit.

A charge Q is uniformly distributed over a long rod AB of length L as shown in the figure. The electric potential at the point O lying at a distance L from the end A is : 

In an $\text{LCR}$ circuit as shown below both switches are open initially. Now switch ${\text{S}}_{1}$ is closed, ${\text{S}}_{2}$ kept open. ($\text{q}$ is charge on the capacitor and $\tau = \text{RC}$ is capacitive time constant). Which of the following statement is correct? 

Consider a finite insulated, uncharged conductor placed near a finite positively charged conductor. The uncharged body must have a potential :

In a series $\mathrm{L}-\mathrm{C}-\mathrm{R}$ circuit, $\mathrm{C}=10^{-11}$ Farad, $\mathrm{L}=10^{-}$ ${ }^5$ Henry and $\mathrm{R}=100 \mathrm{Ohm}$, when a constant D.C. voltage $\mathrm{E}$ is applied to the circuit, the capacitor acquires a charge $10^{-9}$ C. The D.C. source is replaced by a sinusoidal voltage source in which the peak voltage $\mathrm{E}_0$ is equal to the constant D.C. voltage E. At resonance the peak value of the charge acquired by the capacitor will be :

Select the correct statement from the following :

Photons of an electromagnetic radiation has an energy $11 \mathrm{keV}$ each. To which region of electromagnetic spectrum does it belong ?

An electric current is flowing through a circular coil of radius $\mathrm{R}$. The ratio of the magnetic field at the centre of the coil and that at a distance $2 \sqrt{2} R$ from the centre of the coil and on its axis is :

Two coils, $\mathrm{X}$ and $\mathrm{Y}$, are kept in close vicinity of each other. When a varying current, $I(t)$, flows through coil $\mathrm{X}$, the induced $\operatorname{emf}(V(t))$ in coil $\mathrm{Y}$, varies in the manner shown here. The variation of $I(t)$, with time, can then be represented by the graph labelled as graph :  (A) (B) (C) (D)

This question has Statement-1 and Statement-2. Of the four choices given after the Statements, choose the one that best describes the two Statements. Statement 1: No work is required to be done to move a test charge between any two points on an equipotential surface. Statement 2 : Electric lines of force at the equipotential surfaces are mutually perpendicular to each other.

To find the resistance of a galvanometer by the half deflection method the following circuit is used with resistances $\mathrm{R}_1=9970 \mathrm{~W}, \mathrm{R}_2=30 \mathrm{~W}$ and $\mathrm{R}_3=0$. The deflection in the galvanometer is d. With $\mathrm{R}_3=107 \mathrm{~W}$ the deflection changed to $\frac{d}{2}$. The galvanometer resistance is approximately: 

One of the two small circular coils, (none of them having any self - inductance) is suspended with a V-shaped copper wire, with plane horizontal. The other coil is placed just below the first one with plane horizontal. Both the coils are connected in series with a dc supply. The coils are found to attract each other with a force. Which one of the following statements is incorrect?

A parallel plate capacitor having a separation between the plates $\mathrm{d}$, plate area $\mathrm{A}$ and material with dielectric constant $\mathrm{K}$ has capacitance $\mathrm{C}_0$. Now one-third of the material is replaced by another material with dielectric constant $2 \mathrm{~K}$, so that effectively there are two capacitors one with area $\frac{1}{3} \mathrm{~A}$, dielectric constant $2 \mathrm{~K}$ and another with area $\frac{2}{3} \mathrm{~A}$ and dielectric constant $\mathrm{K}$. If the capacitance of this new capacitor is $\mathrm{C}$ then $\frac{\mathrm{C}}{\mathrm{C}_0}$ is

Choose the correct sketch of the magnetic field lines of a circular current loop shown by the dot $$ \text { and the cross } \otimes \text {. } $$

Two point dipoles of dipole moment $\vec{p}_1$ and $\vec{p}_2$ are at a distance $x$ from each other and $\vec{p}_1 || \vec{p}_2$. The force between the dipoles is :

In a metre bridge experiment null point is obtained at $40 \mathrm{~cm}$ from one end of the wire when resistance $\mathrm{X}$ is balanced against another resistance $\mathrm{Y}$. If $\mathrm{X} < \mathrm{Y}$, then the new position of the null point from the same end, if one decides to balance a resistance of $3 \mathrm{X}$ against $\mathrm{Y}$, will be close to :