24. A police car with a siren of frequency 8 kHz is moving with uniform velocity $36 \mathrm {~km} / \mathrm { hr }$ towards a tall building which reflects the sound waves. The speed of sound in air is $320 \mathrm {~m} / \mathrm { s }$. The frequency of the siren heard by the car driver is\\
(A) 8.50 kHz\\
(B) 8.25 kHz\\
(C) 7.75 kHz\\
(D) 7.50 kHz

\section*{ANSWER:A}
\begin{enumerate}
  \setcounter{enumi}{24}
  \item 5.6 liter of helium gas at STP is adiabatically compressed to 0.7 liter. Taking the initial temperature to be $T _ { 1 }$, the work done in the process is\\
(A) $\frac { 9 } { 8 } R T _ { 1 }$\\
(B) $\frac { 3 } { 2 } R T _ { 1 }$\\
(C) $\frac { 15 } { 8 } R T _ { 1 }$\\
(D) $\quad \frac { 9 } { 2 } R T _ { 1 }$
\end{enumerate}

\section*{ANSWER:A}
\begin{enumerate}
  \setcounter{enumi}{25}
  \item Consider an electric field $\vec { E } = E _ { 0 } \hat { x }$, where $E _ { 0 }$ is a constant. The flux through the shaded area (as shown in the figure) due to this field is\\
\includegraphics[max width=\textwidth, alt={}, center]{cdfce4af-7f3d-4d66-ab09-7da7ecb927ff-10_482_728_1558_729}\\
(A) $2 E _ { 0 } a ^ { 2 }$\\
(B) $\sqrt { 2 } E _ { 0 } a ^ { 2 }$\\
(C) $E _ { 0 } a ^ { 2 }$\\
(D) $\frac { E _ { 0 } a ^ { 2 } } { \sqrt { 2 } }$
\end{enumerate}

\section*{ANSWER: C}
\begin{enumerate}
  \setcounter{enumi}{26}
  \item The wavelength of the first spectral line in the Balmer series of hydrogen atom is $6561 \AA$. The wavelength of the second spectral line in the Balmer series of singly-ionized helium atom is\\
(A) $1215 \AA$\\
(B) $1640 \AA$\\
(C) $2430 \AA$\\
(D) $4687 \AA$
\end{enumerate}

ANSWER: A\\