The equation of state of real gas is given by \(\displaystyle \left( p + \frac{a}{V^{2}} \right)(V-b)=RT \), where \( P \), \( V \) and \( T \) are pressure, volume and temperature respectively and \( R \) is the universal gas constant. The dimensions of \(\displaystyle \frac{a}{b^{2}} \) is similar to that of: