In an isothermal expansion,what happens to the internal energy of the gas?

Obtain an expression for work done by an ideal gas in an isothermal expansion.

If an ideal gas is compressed isothermally,then:

For an isothermal expansion of a perfect gas,the value of $\frac{\Delta P}{P}$ is equal to:

$A$ cylinder fitted with a piston contains $0.2 \, moles$ of air at a temperature of $27^{\circ}C$. The piston is pushed so slowly that the air within the cylinder remains in thermal equilibrium with the surroundings. Find the approximate work done by the system if the final volume is twice the initial volume (in $J$).

$Assertion :$ 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.