resistance values

[mpc4000]

hi i was juist watching this youtube clip about resistance in series and in parrallel and @ 2:48 of the clip he comes up with some values 60/3 & for the love of god i cannot work out
anyone can put me right i would be most gratful

thanks

[ame="http://www.youtube.com/watch?v=W8USCMCW3ww&feature=related"]YouTube - Physics 13.4.2g - Parallel Circuit Example[/ame]

 

[wallyanker]

I have no idea how he has done that but he gets the correct answer. lol. I did it like this:

V=I/R so to find R you need V/I

120v/6a = 20 ohms

 

[Notsosmart]

He is using
1 = 1 + 1 = 1 = 1 + 1 = 2 + 1 (use common denominator); =
Rt~R1~R2~Rt~30~60~60~60
3
60
Therefore;
Rt = 60 = 20 Ohms (he should have shown this first IMO)
(1)~~3

@ 2:48 he could just said that R=
120 = 20 ohms
~6

 

[uksel]

easiest way to get the resistance in parallel

1/Rt = 1/R1 + 1/R2

1/Rt = 1/30 + 1/60

1/Rt = 0.033 + 0.017

1/Rt = 0.05

Transpose this so Rt = 1/0.05

therfore Rt = 20


Now using ohms law V/R = I

120/20 = I

therefore I = 6 Amps

 

[Notsosmart]

Not what the OP was asking

 

[Colonel Hathi]

Its a very good site though isn't it!!

 

[uksel]

Not what the OP was asking

he was asking how the video guy worked out the resistors in parallel

i can be quoted as saying

'easiest way to get the resistance in parallel'


I was showing him another (much simpler in my view) method, one which I would hope helps him

one which doesn't arse about with trying to work out common denominators

 

[jimuk07]

60/3 is an equivalent fraction of 120/6. They both produce the same outcome, and it makes doing these calcs in your head easier.

 

[mpc4000]

thanks everybody all you're replies have been a great help