Electricity and Magnetism Electricity and Magnetism Physics Help Forum Nov 6th 2008, 10:30 PM #1 Junior Member   Join Date: Nov 2008 Posts: 5 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.   Nov 7th 2008, 08:07 PM   #2
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 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.  Tags resistance Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post gordonmckee Electricity and Magnetism 2 Oct 13th 2014 08:44 AM gordonmckee Electricity and Magnetism 1 Aug 14th 2014 07:27 AM ppsolanke Electricity and Magnetism 0 Aug 23rd 2012 02:03 AM anwaar Electricity and Magnetism 5 Feb 9th 2011 01:34 PM vyomictor Electricity and Magnetism 3 Aug 13th 2010 11:56 PM