1. If you quickly run a comb through your hair several times , then bring the co
ID: 1650465 • Letter: 1
Question
1. If you quickly run a comb through your hair several times , then bring the comb up to the right side of a small but smooth stream of running water from a sink faucet, the stream of water will deflect. In which direction wil l the water deflect? Explain, using a complet e explanation of the process on an atomic level.
2. Assuming that two 1 kg balls have equal magnitude charges, how much net charge would each need to have in order for the electric force between the balls to have the same magnitude as the gravitational force betwee n the balls? If you want the balls to remain fixed in place without moving, should they have the same sign of charge or opposite signs?
3 . A +2 nC charge is placed at x = 2 cm, y = 5 cm. A - 8 nC charge is placed at x = 8 cm, y = 2 cm. a. What is the magnitude and direction of the net force on a +4 nC charge at the origin? (See figure to the right). b. What is the magnitude and direction of the electric field at the origin?
4. DNA is negatively charged. In gel electrophoresis, fragments of DNA are driven through a gel by an electric field in order to separate them by size. Should the negative or positive termi nal of the power supply be placed near the DNA? Explain why.
5. Positive charge Q is uniformly distributed around a semicircle of radius a . Find the magnitude and direction of the resulting electric field at point P , the center of curvature of the semicircle.
6 . Given that the electric force (described by Coulomb ’ s law) is a type of conservative force, derive the electric potential energy function corresponding to this force.
Explanation / Answer
Problem 1)
Electrical charges come in two types, positive and negative. Those electrons that ended up on our comb have a negative charge, while water molecules have a positive charge on one side. Positive and negative charges pull each other closer together, so when the comb is close enough, the positive side of the water molecules get tugged in that direction.
When the negatively charged comb comes near the water, it repels the electrons, so that the side of the water nearest the comb then has a positive charge. The attraction between this positive charge and the negatively charged comb results in a net force on the water, bending the stream towards the charged comb.
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