If we shake a electron back and forth I understand how it radiates electromagnetic wave. But I do not understand how a charge going in circular path or accelerating in a straight line radiates. Please answer it and do not mark it as duplicate. Does only electron in simple harmonic motion radiate?
Any acceleration of a charge leads to radiation. Shaking back and forth accelerates a charge. So does going in a circular path (the magnitude of the velocity remains constant but its direction changes, hence an acceleration). Take a look at How and why do accelerating charges radiate electromagnetic radiation?
If we shake a electron back and forth i understand how it radiates electromagnetic wave...
Ask yourself for a while how you shake the electron forth and back. Finished? Now compare with the next explanation.
To shake an electron one has to use for example an alternating electric field. Switching on the field in the arbitrary direction "- +", the potential energy of this field over goes to the electron and accelerates the electron. The electron gain kinetic energy. switching than on the electric field in the direction "+ -" the electron is braked and the difference in the kinetic energy has to go somewhere. It is an empirical fact that the electron's kinetic energy doesn't completely recuperate into the electric field - if at all - and the energy difference is emitted by photons. The electron radiates.
... but i do not understand how a charge going in circular path or accelerating in a straight line radiates
There are two possibilities to move a charge in circles. One can made a difficult device with a lot of electric fields and switch the fields on by one on and off. On get a n-gon path. Ore one came use a magnetic field which is setup perpendicular to the moving electron.
For the movement of charges in a magnetic field the explanation of the radiation is different from the above said. Read the last paragraphs of the mentioned by WAH question about How and why do accelerating charges radiate electromagnetic radiation?