An electret is similar to a magnet, but rather than being permanently magnetized, it has a permanent electric dipole moment. Suppose a small electret with electric dipole moment 1.0×10^−7 C m is 25 cm from a small ball charged to +25 nC , with the ball on the axis of the electric dipole. What is the magnitude of the electric force on the ball?

A 3.00-cm-long spring has a small plastic bead glued to each end. Charging each bead to −25 nC expands the spring by 0.50 cm. What is the value of the spring constant?

A small glass bead charged to +6.0 nC is in the plane that bisects a thin, uniformly charged, 10-cm-long glass rod and is 4.0 cm from the rod's center. The bead is repelled from the rod with a force of 840 μN . What is the total charge on the rod?

INT The surface charge density on an infinite charged plane is −2.0×10^−6 C/m^2 . A proton is shot straight away from the plane at 2.0×10^6 m/s . How far does the proton travel before reaching its turning point?

INT In a classical model of the hydrogen atom, the electron orbits the proton in a circular orbit of radius 0.053 nm. What is the orbital frequency in rev/s? The proton is so much more massive than the electron that you can assume the proton is at rest.

Consider an oil droplet of mass m and charge q. We want to determine the charge on the droplet in a Millikan-type experiment. We will do this in several steps. Assume, for simplicity, that the charge is positive and that the electric field between the plates points upward. (c) A spherical object of radius r moving slowly through the air is known to experience a retarding force Fₔᵣₐ₉ = −6πηrv where η is the viscosity of the air. Use this and your answer to part b to show that a spherical droplet of density ρ falling with a terminal velocity vₜₑᵣₘ has a radius . r = √9ηvₜₑᵣₘ / 2pg