 JEE Mains Previous Years Questions with Solutions

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1

AIEEE 2004

Alternating current can not be measured by $D.C.$ ammeter because
A
Average value of current for complete cycle is zero
B
$A.C.$ Changes direction
C
$A.C.$ can not pass through $D.C.$ Ammeter
D
$D.C.$ Ammeter will get damaged.

Explanation

$D.C.$ ammeter measure average current in $AC$ current, average current is zero for complete cycle. Hence reading will be zero.
2

AIEEE 2004

In an $LCR$ series $a.c.$ circuit, the voltage across each of the components, $L,C$ and $R$ is $50V$. The voltage across the $L.C$ combination will be
A
$100V$
B
$50\sqrt 2$
C
$50$ $V$
D
$0$ $V$ (zero)

Explanation

Since the phase difference between $L$ & $C$ is $\pi ,$

$\therefore$ net voltage difference across $LC=50-50=0$
3

AIEEE 2003

The core of any transformer is laminated so as to
A
reduce the energy loss due to eddy currents
B
make it light weight
C
make it robust and strong
D
increase the secondary voltage

Explanation

Laminated core provide less area of cross-section for the current to flow. Because of this, resistance of the core increases and current decreases thereby decreasing the eddy current losses.
4

AIEEE 2003

In an oscillating $LC$ circuit the maximum charge on the capacitor is $Q$. The charge on the capacitor when the energy is stored equally between the electric and magnetic field is
A
${Q \over 2}$
B
${Q \over {\sqrt 3 }}$
C
${Q \over {\sqrt 2 }}$
D
$Q$

Explanation

When the capacitor is completely charged, the total energy in the $L.C$ circuit is with the capacitor and that energy is $E = {1 \over 2}{{{Q^2}} \over C}$

When half energy is with the capacitor in the form of electric field between the plates of the capacitor we get ${E \over 2} = {1 \over 2}{{Q{'^2}} \over C}$ where $Q'$ is the charge on plate of the capacitor

$\therefore$ ${1 \over 2} \times {1 \over 2}{{{Q^2}} \over C} = {1 \over 2}{{Q{'^2}} \over C}$

$\Rightarrow Q' = {Q \over {\sqrt 2 }}$