Resistance in parallel

[disinfo]

Hi Guys,

I'm currently revising for my Unit 203 (2330) Exam

Going through the book "Electrical Installation Calculations" I'm struggling on a question.

"Calculate the value of a resistor which, when connected in parallel with the given resistor, will produce the value asked for."

Given resistance = 48

Resistance required = 12

I know the answer to this is 16, but i'm struggling to work out how to get there.

I know how to calculate total resistance in parallel (1/R + 1/R = 1 / R = RT) but i'm not sure how to calculate the missing value.

P.S. Not homework, just revision!

 

[EFLImpudence]

Hi,

1/rt = 1/r1 + 1/r2

1/12 = 1/48 + 1/r2

therefore 1/r2 = 1/12 - 1/48

1/rt = 0.0833 - 0.0208

1/rt = 0.0625

1/0.0625 = 16

I think I typed it all correctly

 

[EFLImpudence]

Not quite I didn't

1/rt = 1/r1 + 1/r2

1/12 = 1/48 + 1/r2

therefore 1/r2 = 1/12 - 1/48

1/r2 = 0.0833 - 0.0208

1/r2 = 0.0625

1/0.0625 = 16

I think I typed it all correctly

 

[holmslaw]

..

 

[EFLImpudence]

That's correct but didn't do it that way because next time there might not be as simple a common denominator so may have confused OP even more.

 

[holmslaw]

..

 

[flameport]

next time there might not be as simple a common denominator

With any 2 fractions, there is always an obvious common denominator, it is obtained by multiplying the denominators together.

 

[ban-all-sheds]

With any 2 fractions, there is always an obvious common denominator, it is obtained by multiplying the denominators together.

Given resistance = 97

Resistance required = 41

Enjoy, ha ha.


But seriously - in this exam the idea of the question will be to test if you know how to calculate parallel resistances - the actual calculations involved will deliberately be quite simple.

That's why (I'm sure) disinfo will have been told to show his workings, i.e. the way holmslaw or EFLI did it (holmslaw's is better, and doesn't need a calculator), rather than just sticking down the answer - that way you still pick up marks even if a daft arithmetic error produces the wrong answer.

 

[riveralt]

That's why (I'm sure) disinfo will have been told to show his workings.... rather than just sticking down the answer - that way you still pick up marks even if a daft arithmetic error produces the wrong answer.

Not if its a multiple choice question BAS -there is only one write (sic) answer

 

[joinerjohn]

With any 2 fractions, there is always an obvious common denominator, it is obtained by multiplying the denominators together.

Given resistance = 97

Resistance required = 41

Enjoy, ha ha.

Very roughly 70 ohms (could be just slightly more.)

 

[empip]

Eleven key presses on Casio gets 71.02 Ohms.

open bracket; 41; inverse; minus; 97; inverse; end bracket; inverse; equals:

-0-

 

[ban-all-sheds]

Not really in the spirit of flameports method of finding a common denominator....

Are calculators allowed in the 2330 exam?

 

[Wontdothatagain]

yes but they are checked before hand so they are no programable PDA's or simular

 

[riveralt]

Are calculators allowed in the 2330 exam?

Yes for both level 2 (multiple choice) and level 3 (written & multiple choice).

You have to memorise 40 plus equations going into these exams - many of them have SI units in the terra or pica range - which makes a calculator handy.

 

[dingbat]

You have to memorise 40 plus equations going into these exams - many of them have SI units in the terra or pica range - which makes a calculator handy.

pmsl. Most sparks have difficulty with Ohm's law!

Memorise 40+ equations, my jacksie.

This is part of the problem - students seem to prefer not to learn principles, from which any formulae can be derived, relying instead on regurgitating rote-learned solutions to predictable problems. Lots of effort for little gain.