Unformatted text preview: 96W! V1.5 1008 PHY2054 Spring 2008 Prof. P. Kurnar
Prof. P. Avery
March 5. 2008 Exam 2 Solutions 1. Two cylindrical resistors are made of the same material and have the same resistance. The
resistors, R1 and R2, have different radii, n and r;, and different lengths, L1 and L 2. Which of the
following relative values for radii and lengths would result in equal resistances? @2n=rzand4L1=Lz
)2r1=r2and L]: 21.2
(3) r1=r2and 4L1=L2
(4) r1=r2 and L1: 2L2
(5) none of these The resistance is given by R = pL /A , where p is the resistivity, L is the length and A is the cross sectional area. T 0 keep R unchanged, the ratio of L to A must be unchanged. Since A = 717'}, If r
is doubled then L must be multiplied by 4. Only the answer (1) works. 2. If a 9.0V battery with internal resistance 0.2 Q and an 18 Q resistor are connected together,
what is the amount of electrical energy (in I) transformed to heat per coulomb of charge that
ﬂows through the circuit? ® 9.0 (2) 3.0
(3) 0.50
(4) 72
(5) 4.5 The amount of heat released is PAt, where P = Vi. The total amount of charge is q = iAt. Thus
the amount of heat energy released per coulomb of charge is PA! / iAt = P/i = V = 9V. M "e W’ . ~ ,,. , 7 , ...
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