Physics 142: Homework 1 Solutions
Ch. 23, Q: 4, 6, 9, 15, 18, 20
Ch. 23, P: 4, 6, 7, 10, 11, 14, 15, 16, 18, 54
Q4: If hospital personnel wore insulating rubber-soled shoes, charge could build up on the
shoes which might lead to a spark, which could start a fire in the oxygen rich environment.
If the shoes are conductive, charge would not be able to build up.
Q6: The charge rubber rod (lets say the charge is positive, it is an arbitrary choice), the
charge on the rod will induce a polarization within the metal sphere. The negative charges
within the sphere will be attracted to the side near the rod, and the positive charges will be
pushed to the side away from the rod. Since the negative charge on the metal sphere is
closer to the positive rod than the positive charge on the sphere, the attractive force between
the positive and negative charges is stronger than the repulsive force so there is a net
attraction. But once the metal sphere touches the rod, some of the positive charge rod will
be annihilated with some of the negative charge on the surface of the sphere. Now the rod is
still positive, but now the metal sphere has a net positive charge, so they will repel.
Q9: The wall does not need to be positively charged since the charge on the balloon could
be inducing a polarization within the wall. The balloon eventually falls off because the
charge eventually disperses into the wall or the air.
Q15: If the distance from the charge distribution to the point at which you are interested in
find the electric force or field is much greater than the distances within the charge
Q18: Since the magnitude of the charge on an electron and a free proton is the same they