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Chapter 20: Q9CQ (page 731)
In which of the three semiconducting materials listed in Table 20.1 do impurities supply free charges?
(Hint: Examine the range of resistivity for each and determine whether the pure semiconductor has the higher or lower conductivity.)
Germanium is a semiconducting material.
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Find the current through a person and identify the likely effect on her if she touches a \({\rm{120 - V AC}}\)source: (a) if she is standing on a rubber mat and offers a total resistance of \({\rm{300k\Omega ;}}\) (b) if she is standing barefoot on wet grass and has a resistance of only \({\rm{4000k\Omega }}{\rm{.}}\)
Electron guns are used in X-ray tubes. The electrons are accelerated through a relatively large voltage and directed onto a metal target, producing X-rays. (a) How many electrons per second strike the target if the current is 0.500 mA ? (b) What charge strikes the target in 0.750 s?
Using an ohmmeter, a student measures the resistance between various points on his body. He finds that the resistance between two points on the same finger is about the same as the resistance between two points on opposite hands—both are several hundred thousand ohms. Furthermore, the resistance decreases when more skin is brought into contact with the probes of the ohmmeter. Finally, there is a dramatic drop in resistance (to a few thousand ohms) when the skin is wet. Explain these observations and their implications regarding skin and the internal resistance of the human body.
(a) A defibrillator passes \({\bf{12}}{\bf{.0}}\;{\bf{A}}\) of current through the torso of a person for\({\bf{0}}{\bf{.0100}}\;{\bf{s}}\). How much charge moves?
(b) How many electrons pass through the wires connected to the patient? (See figure two problems earlier.)
Some makes of older cars have \(6.00 - V\)electrical systems.
(a) What is the hot resistance of a \(30.0 - W\)headlight in such a car?
(b) What current flows through it?
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