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Old Nov 6th 2008, 10:30 PM   #1
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Resistance

The following questions were taken from a quiz held earlier this year. An answer sheet has been provided, but I 'm a little confused as to how and why the answers are correct.

1. What is the effective resistance between the points P and Q? (Refer to the attachment.) Ans: 3 Ohms

2. 8 light bulbs (black dots in the diagram) rated 12V and 24W are connected to a fuse in the circuit as shown in the diagram. What is the most suitable fuse which can be used in the circuit? Ans: 8 Ohms

3. 5 identical resistors are connected to a 45V battery as shown in the diagram. The resistance of each resistor is R Omhs. The total power produced by the resistors is 58W. Find R. Ans:24 Ohms.

Any help would be greatly appreciated.
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Old Nov 7th 2008, 08:07 PM   #2
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Originally Posted by allim View Post
The following questions were taken from a quiz held earlier this year. An answer sheet has been provided, but I 'm a little confused as to how and why the answers are correct.

1. What is the effective resistance between the points P and Q? (Refer to the attachment.) Ans: 3 Ohms


Any help would be greatly appreciated.
Let me tell you for the first problem. Consider the leftmost two 3 ohm resistors, they are actually in series. Group them together and you got 6 ohm. This equivalent resistance is actually in parallel with the 6 ohm resistor. Then 1/(3+3) + 1/6 = 1/R where R is the new equivalnet resistance. As the process continues, you would get you answer



For question 3, again group all the resistors. Consider the 3 resistors on the right. Let R' be the equivalent resistance.1/R'= 1/R + 1/2R
R'=2R/3
then let new equivalent resistance be R''
1/R''=1/R + 1/(2R/3)
R''=2R/5

ler final equivalent resistance be R'''
R'''=2R/5+R = 7R/5

P = V^2/R'''
58 = (45)^2/(7R/5)
R=25 ohm
I guess the answer of mine is correct instead of 24 ohm

Last edited by werehk; Nov 7th 2008 at 08:19 PM.
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