[en] Periodic pulses of charged particles passing through a superconducting radiofrequency (SRF) cavity can in principle excite all the electromagnetic (EM) eigenmodes of the cavity. Excitation of higher order modes (HOMs) leads to additional heat generation at the cavity surface, which adds to the cryogenic load. This becomes more prominent when one of the Fourier components of the periodic beam current has a frequency close to the natural frequency of HOM. In addition, HOMs may also induce additional energy spread, and transverse emittance in the beam. Although the formulae for calculating the EM fields excited by a pulsed beam current in an SRF cavity, along with its effect on the beam are available in the literature, to the best of our knowledge, the detailed derivation of the formulae is not described. The derivation presented in this paper gives useful insights of the calculations related with HOMs in SRF cavities. Using the derived formulae, numerical calculations have been performed to estimate the HOM heat load in a β_g = 0.9, 650 MHz elliptic cavity. The derived formula requires frequencies and strength of all the excitations terms, which were obtained by performing Fourier expansion of the beam current, and also evaluating the RF parameters of the HOMs. (author)