Electromagnetic sum rules

The sum rules (SR) are introduced showing their connection with the linear response of a nucleus to an external electromegnetic field. Their relation to some macroscopic “collective” aspects of the nucleus are briefly outlined. Energy weighted sum rules (EWSR) for electron scattering at constant momentum transfer are studied for light and medium weight nuclei. The q-dependence of the different contributions to these SR permits to discuss the role of the nuclear potential and of correlations in the electroexcitation process.