82CHAPTER 17 17.29(a) From RLAρ=, the initial resistance of the mercury is ( )( )()7239.410m1.000 0 m1.2 1.0010m4iiiLRAπ−−×Ω⋅===Ω×(b) Since the volume of mercury is constant, ffiL ALiVA=⋅=⋅gives the final cross-sectional area as ( )fiifAALL=⋅. Thus, the final resistance is given by 2ffLLRAALρρ⋅. The fractional change in the resistance is then ( )21fiLL=−211fi ffiiiiRRRLALRRLA−⋅∆==− =−24100.0418.010100.00−×or a 0.080% increase 17.30The resistance at 20.0°C is ()( )01-30200.0 217 11+ 3.9210 C0 C20.0 CRRTTα−Ω=+−×°°−°(ΩSolving )001=+−for Tgives the temperature of the melting potassium as
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