A repulsive force force of 220 N exists between an unknown charge and a charge of +4.7mC.

1. A repulsive force force of 220 N exists between an unknown charge and a charge of +4.7mC. If they are separated by 2.5cm, what is the magnitude of the unknown charge?

A uniformly charged CD spins counter clockwise in the x-y plane. Determine magnetic moment.

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Poynting vector and power dissipation

Consider the integral of the inwardly directed normal component of the Poynting vector over a surface S bounding a volume V of a linear, isotropic, homogeneous, conducting medium. Show that this quantity can be indentified with the sum of the rate of change of the electromagnetic energy and the power dissipated when there is a free current density J within V. You may assume that electric and magnetic energy densities are (1/2)E.D and (1/2)B.H respectively (aside: how might these definitions be justified?) Could you please show me how to prove this? Thank you.

To find the value and direction of the Poynting vector at the surface of a current carrying wire and show that the total power entering the wire per unit length is (I^2)R.

A straight conducting wire of circular cross section, radius a, has a resistance R per unit length and carries a constant current I. Find the value and direction of the Poynting vector at the surface of the wire, and hence show that the total power entering the wire per unit length is (I^2)R. By considering the divergence of the poynting vector inside the wire, show that the energy is dissipated uniformly through it's volume.

Show that the field inside a spherical cavity cut in a uniform dielectric medium is uniform and of magnitude Ecav = 3erEm/(2Er+1), where er is the relative permittivity of the medium and Em is the uniform field in the dielectric at a point distant from the cavity.

Show that the field inside a spherical cavity cut in a uniform dielectric medium is uniform and of magnitude Ecav = 3erEm/(2Er+1), where er is the relative permittivity of the medium and Em is the uniform field in the dielectric at a point distant from the cavity.