In which of the processes does the internal energy of the system remain constant?

$Assertion :$ The isothermal curves intersect each other at a certain point.
$Reason :$ The isothermal changes take place rapidly,so the isothermal curves have very little slope.

$A$ certain amount of heat supplied to an ideal gas under isothermal conditions will result in:

The isothermal elasticity of a gas is equal to

An ideal gas is trapped inside a test tube of cross-sectional area $20 \times 10^{-6} \, m^2$ as shown in the figure. The gas occupies a height $L_1$ at the bottom of the tube and is separated from air at atmospheric pressure by a mercury column of mass $0.002 \, kg$. If the tube is quickly turned isothermally,upside down,the gas now occupies height $L_2$ in the tube. Find the ratio $L_2/L_1$. [Take atmospheric pressure $P_0 = 10^5 \, Nm^{-2}$ and $g = 10 \, ms^{-2}$]

In isothermal expansion,the pressure is determined by