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Three point charges are arranged at the vertices of an equilateral triangle ABC of side l. Charges at A, B and C are +q, +4q and -6q, respectively. (i) Calculate electric potential energy of this system. (ii) Charge at A is moved to the midpoint of AB, charge at B is moved to midpoint of BC and charge at C is moved to midpoint of CA. Calculate electric potential energy of this system now. (iii) What work is needed to shift the charges to new locations as described in part (ii)?

When charge is at rest then it creates only electric field not the magnetic field. A moving charge generates electric as well as magnetic field around it. If charge is moving with constant velocity then electric and magnetic field do not change with time, hence, it cannot produce electromagnetic wave. When charge is accelerated then time-varying electric and magnetic fields are created and thus electromagnetic wave is produced by accelerated charge. We know that the current-carrying wire creates magnetic field around it. If alternating current is flowing through the wire then it creates variable magnetic field around it and thus starts radiating electromagnetic waves. When a charged capacitor is connected across an inductor then charge starts oscillating between the plates of the capacitor. Normally one plate of the capacitor is connected to Earth and the other plate is connected to an antenna. Antenna radiates electromagnetic wave. Hertz successfully produced electromagnetic waves using such LC oscillator. Speed of electromagnetic wave in vacuum is constant and equal to 3 xx 10^8 m/s. But inside a medium the speed changes according to electrical and magnetic properties of the medium. IF mu_r and epsi_r are the relative permeability and relative premittivity of the medium then speed of light in that medium can be written as