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Homework #3 Phys 112-1 F12 Page 1 of 1
Homework #3
Chapter 22: Electric Potential
p 385
#6: Must the electric field be zero at any point where the potential is zero? Explain.
#22: The potential difference across a typical cell membrane is about 80mV. How much work is done on
a singly ionized potassium ion moving through the membrane?
#25: The potential at the surface if a10cm sphere is 4.8kV. What’s the sphere’s total charge, assuming
charge is distributed in a spherically symmetric way?
Chapter 23: Electrostatic Energy and Capacitors
p 399
#17: A crude model of the water molecule has a negative charged oxygen atom and two protons, as
shown in the figure below. Calculate the electrostatic energy of this configuration, which is therefore the
magnitude of energy released in forming this molecule. (Note: Your answer is an overestimate because
electrons are actually “shared” among the three atoms, spending more time near the oxygen.)
#25: Find the capacitance of a parallel-plate capacitor with circular plates 20cm in radius separated by
1.5mm.
#31: (a) Find the equivalent capacitance of the combination shown below. Find (b) the charge and (c) the
voltage on each capacitor when a 100V battery is connected across the combination (i.e. to the two dots in
the drawing).