Presume electrons move through a copper wire at speed v. Call the linear charge densities in the Earth reference frame Ep for the fixed positive ions in the wire and En for the (negative) electrons. Observer E in the Earth reference frame (which is also the reference frame of the positive ions) gauges the wire to be electrically neutral. Observer M is moving along with the electrons (in the same direction as the electrons and at speed v).
1) Compute the linear charge density of the wire as gauged by M in terms of En if the speed is 4.5×10^5 m/s .M/En =
2) Compute the linear charge density of the wire as gauged by M in terms of En if the speed is 7.5 mm/s(an achievable speed for electrons in copper wire).M/En =