# MCAT Physical : Electrostatics

## Example Questions

### Example Question #1 : Electrostatics

A neutral conducting sphere rests on an insulating styrofoam block. A student then places his hand on the sphere, maintaining this contact as a negatively charged plastic rod is brought near the sphere without touching. While the plastic rod is still nearby, the student removes his hand. Finally, the rod is also removed. Which of the following best describes the final charge on the sphere?

Net positive charge, and polarized

Zero net charge, but polarized

Net positive charge, distributed evenly over the sphere

Net negative charge, and polarized

Net negative charge, distributed evenly over the sphere

Net positive charge, distributed evenly over the sphere

Explanation:

The student's hand on the sphere "grounds" it, allowing charges to flow in or out. No charge flows through the styrofoam insulator. When the negatively charged rod is brought nearby, negative charges in the sphere are repelled and exit via the student's hand, giving the sphere a net positive charge. After the student removes his hand, no more charges can flow into or out from the sphere, so the net positive charge remains after the rod is removed. Since this is a conducting sphere, the charges will spread out evenly over the surface.

### Example Question #2 : Electrostatics

How much energy is required by a  lightbulb running for  minutes?