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Unformatted text preview: 7B.S11.C Quiz 3 DL section _ Last 6 digits of student ID
______ Grade: 04/20/11 Last name , first name:
________________ ________________ First three letters of your last name ___ N o b oo k s o r n o t e s . C a l c u l a t o r s O K . S h o w a l l y o u r w o r k b e l o w . A n s w e r s a l o n e w i l l r e c e i v e n o c r e d i t !
R3 = 20 a) In the circuit to the right, which resistor, R1 or R2, will have the greater thermal power?
Explain briefly. (No calculation needed).
P = I Delta(V) and Delta(V) = I R so P = I R^2 bigger than R_1, then R_2 will have the greater thermal power. R5 = 2 R4 = 3 Because I is the same (it's the same wire from R_2 to R_1) and R_2 is R2 = 9 b) Find the equivalent resistance of the circuit. Show your work. R1 = 7 ! R_4 + R_5 are in series, then R_3 is in paraller with the 4,5 pair.
R_45 = R_4 + R_5
1/R_par = 1/R_45 + 1/R_3
R_par = (R_45 x R_3)/(R_45 + R_3) = [(R_4 + R_5) x (R_3)]/(R_4 + R_5 + R_3) = 4 Ohms
To the parallel part you need to add R_1 and R_2 that are in series (with respect to the parallel part).
R_eq = R_1 + R_2 + [(R_4 + R_5) x (R_3)]/(R_4 + R_5 + R_3) =
= R_1 + R_2 + R_par = (7 + 9 + 4) Ohms = 20 Ohms c) You measure the current through R3 to be 2 Amps. What is the voltage of the battery? Show your work. if you consider the upper parallel part you have Delta(V)_par = I_3 R_3 = 40 V
then you apply the same voltage to the bottom parallel part (the voltage is the same in parallel circuits)
Delta(V)_par = I_45 (R_4 + R_5)
which gives you I_45 = 40 V / 5 Ohms = 8 Amps
The total current is then I_tot = I_3 + I_45 = 10 Amps
Appling the ciruit equation to the whole circuit you get
E  I_tot R_eq = 0
then you plug in the R_eq from the previews question
E = I_tot R_eq = 10 Amps x 20 Ohms = 30 Volts R5 d) The circuit to the right contains the same resistors and battery as the circuit above, but the
wires and resistors have been rearranged. Are the two circuits equivalent (same currents
through the battery and all resistors), or can you not tell with the given information? C i r c l e
you r c hoi c e b e l ow c l e a r l y . (No work needed). R4 R2 Equivalent Not Equivalent R1 R3 Can't Tell !
(you can tell because R_1 is now in the parallel part with R_4 and R_5) ...
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This note was uploaded on 10/24/2011 for the course PHYSICS 7B taught by Professor Johnconway during the Spring '07 term at UC Davis.
 Spring '07
 JOHNCONWAY

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