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Chapter 19: Q9CQ (page 694)
In what region of space is the potential due to a uniformly charged sphere the same as that of a point charge? In what region does it differ from that of a point charge?
Both electric potential are similar for regions outside the sphere and differ within the sphere.
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The voltage across a membrane forming a cell wall is \(80.0{\rm{ }}mV\) and the membrane is \(9.00{\rm{ }}nm\) thick. What is the electric field strength? (The value is surprisingly large, but correct. Membranes are discussed in Capacitors and Dielectrics and Nerve Conduction鈥擡lectrocardiograms.) You may assume a uniform electric field.
A bare helium nucleus has two positive charges and a mass of \(6.64 \times {10^{ - 27}}{\rm{ }}kg\) .
(a) Calculate its kinetic energy in joules at \(2.00\% \) of the speed of light.
(b) What is this in electron volts?
(c) What voltage would be needed to obtain this energy?
(a) Sketch the equipotential lines near a point charge \( + q\) . Indicate the direction of increasing potential. (b) Do the same for a point charge \( - 3q\) .
Give the reason why a dielectric material increases capacitance compared with what it would be with air between the plates of a capacitor. What is the independent reason that a dielectric material also allows a greater voltage to be applied to a capacitor? (The dielectric thus increases \(C\) and permits a greater \(V\).
Which methods of radiation protection were used in the device shown in the first photo in Figure 32.35? Which were used in the situation shown in the second photo?
Figure 32.35 (a)
Figure 32.35(b)
(a) This x-ray fluorescence machine is one of the thousands used in shoe stores to produce images of feet as a check on the fit of shoes. They are unshielded and remain on as long as the feet are in them, producing doses much greater than medical images. Children were fascinated with them. These machines were used in shoe stores until laws preventing such unwarranted radiation exposure were enacted in the 1950s. (credit: Andrew Kuchling ) (b) Now that we know the effects of exposure to radioactive material, safety is a priority. (credit: U.S. Navy)
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