Homework #2 Physics 112-4 Fall 2011 p1
HOMEWORK #2
Chapter 20: Electric Charge, Force and Field
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#1: The gravitational force between an electron and a proton is 10-40 times weaker than the electric force between them. Since matter consists largely of protons and electrons, why is the gravitational force important?
#11: Why should there be a force between two dipoles, which each have a zero net charge?
#18: You break apart a piece of Styrofoam packing material, and it releases lots of little spheres whose electric charge makes them stick annoyingly to you. If two of the spheres carry equal charges and repel with a force of 21mN when they’re 15mm apart, what’s the magnitude of the charge on each?
#26: The electron in a hydrogen atom is 52.9p from the proton. What’s the proton’s electric field strength at this distance.
#30: Find the line charge density on a long wire if the electric field 45cm from the wire has magnitude 260kN/C and points toward the wire.
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#40: A charge 3q is at the origin and a charge -2q is on the positive x-axis at x=a. Where would you place a third charge so it would experience no net electric force.
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#66: The electric field on the axis of a uniformly charged ring has magnitude 380kN/C at a point 5.0cm from the ring center. The magnitude 15cm from the center is 160kN/C; in both cases the field points away from the ring. Find (a) the ring’s radius and (b) it’s charge.
Chapter 21: Gauss’s Law
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#1: Can electric field lines ever cross? Why or why not?
#2: The electric flux through a closed surface is zero. Must the electric field be zero on that surface? If not, give an example.
#9: A point charge is located a fixed distance outside of a uniformly charged sphere. If the sphere shrinks in size without losing any charge, what happens to the force on the point charge?
#15: Why must the electric field be zero inside a conductor in electrostatic equilibrium?
#18: Charges +2q and -1 are near each other. Sketch some field lines for this charge distribution, using eight lines for a charge of magnitude q.
#24: A sock comes out of the dryer with a trillion (1012) excess electrons. What’s the flux through a surface surrounding the sock?
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#28: The electric field at the surface of a 5.0cm radius uniformly charged sphere is 90kN/C. What’s the field strength 10cm from the surface?
#35: A rod 50cm long and 1.0c in radius carries a 2.0μC charge distributed uniformly over its length. Find the approximate magnitude of the electric field (a) 4.0mm from the rod surface, not near either end, and (b) 23m fro the rod.
#45.: A study shows that mammalian red blood cells (RBCs) carry electric charge resulting from 4.4 million to 15 million excess electrons spread over their surfaces. Approximating RBCs as spheres, calculate the electric field strengths at the cells’ surfaces for rabbits (4.4 million electrons, radii=30μm) and humans (15 million electrons, radii=36μm).
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#58: A solid sphere 10cm in radius carries a 40μC charge distributed uniformly throughout its volume. It’s surrounded by a concentric shell 20cm in radius, also uniformly charged with 40μC. Find the electric field (a) 5.0cm, (b) 15cm and (c) 30cm from the center.