(a) You need a 45 resistor, but the stockroom has only 20 and 50 resistors. How can the desired resistance be achieved under these circumstances? (b) What can you do if you need a 35 resistor?

+2 votes
asked Apr 16, 2012 in Physics by moonlight (47 points)
    

1 Answer

+3 votes

Since you only have 20 and 50 ohm resistors, you must place resistors in parallel to achieve the desired resistance.

When resistors are parallel the equivalent resistance is given by:

R1 // R2 = 1 / (1/R1  - 1/R2)   (* // = parallel)

There are more than one way to obtain the 45 total ohm resistance,

For example a (50 // 50) = 25 ohm  in series with a 20 ohm to that and you get 45 ohm.

 

Part B, is solved the same way.

For example use a (20//20) = 10 ohm  in series with another 20 ohm resistor for a total of 35 ohms.

answered Apr 16, 2012 by pokemonmaster ~Expert~ (3,856 points)
Part B asks for 35 ohms. You will need to do another (50//50) = 25 ohm in series with (20//20) = 10 ohm for a total of 35 ohms.

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