PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Electric Force Field and Potential · Electric Force Field and Potential
Problem
After being rubbed on a wool sweater, a rubber balloon acquires a net charge of −3.0μC and is pressed gently against a vertical concrete wall. The balloon remains stuck to the wall. For the purposes of estimation, treat the balloon as a thin spherical shell of radius 15cm whose excess charge is uniformly distributed over the hemisphere in contact with the wall. Assume the portion of the wall interacting with the balloon can be modeled as a circular patch of equal radius.
(1) Qualitatively explain why the balloon adheres to the wall even though the wall is overall electrically neutral. In your explanation, discuss the relevant charge-transport properties of rubber and concrete.
(2) Estimate the average surface charge density (magnitude and sign) induced on the wall patch while the balloon is in contact. Clearly outline any assumptions you make in your calculation.
(3) The mass of the balloon (including air) is 5.0g. Compare the estimated magnitude of the electric force holding the balloon to the wall with the weight of the balloon. State whether the electrostatic attraction is sufficient to support the balloon and justify your conclusion quantitatively.