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kiwiNCFC

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I have been revising reactance formulas etc recently and I just went on Swindon massive to do some quick tests, and I'm not sure if im being really stupid or not but I can't get this answer:

Calculate the Power Dissipated in Resistor 2

Resistor 2 : 10 ohms

Voltage : 20v

I did P=V2/ 10 ohms which gives me 40w

But the answer it gives is 17.8w.

Hmm??

 
series so you would add them together right? But it only asks for value of resistor 2 not resistor 1

Parallel =1/RT = 1/R + 2/R + 3/R etc

Series = ohms + ohms

 
A haha there you go, now how do you work it out?
So as they are in series I can add them together ...10 + 5 = 15 ohms

P=Vsquared /R

20*20 / 15 = 26.66w

20*20 / 10 = 40w

Still none the wiser

 
FirstTotal Resistance Rt

Second

Current through total resistance I=V/Rt

Then when you have the current ?
Total Resistance = 15 ohms

I=V/RT

I= 10v/ 15 = 0.66

P=I2 /R = 0.66x0.66 /10 = 0.04356 headbang

 
20/15 ohms = 1.3

P= Isquared /R 1.3x1.3 = 1.69 / 10 = 0.169

hang on

13.3 x 13.3 = 176.89/10 = 17.69 W :put the kettle on

Thanks for being patient. I get it now. Just couldn't understand where the 13.3v came from? I did 20v / 2 (2 resistors) which is obviously 10 so i thought it was 10v :|

 
1.3333 is correctYour formulae is wrong

P2= Isqrd * R2

Not div /

P2 = (1.3333 x 1.3333) * 10
He can either work out the volt drop at each resistor and then use

P= Vsquared/ r

or he can work out the current I= Supply voltage/ Rt and then

use I squared * R for that particular resistor.

Hope that makes sense???

 
Thanks for being patient. I get it now. Just couldn't understand where the 13.3v came from? I did 20v / 2 (2 resistors) which is obviously 10 so i thought it was 10v :|
resistors are 10 & 5. very basic maths for this one - add together, 15. R2 is 10. so is 2/3 of the resistance. and so 2/3 of the voltage. there are other ways of working it out which may be needed for other values.

 
Yes, more than one way to skin a cat so to speak, I wasnt thinking the volt drop route If you look at my posts and hintsThanks :Salute
As the resistors are in series then current remains equal throughout the circuit, so yes i agree your way is easier :Salute

 
Thanks,

I did get it. I have not got a scientific calculator on me at the moment so i think that did not help! Using the windows basic one is awful!

Cheers guys!

 
In calculator go toView... and select Scientific.

HTH
:^O LOL I never knew that and after how many years of using windows....well its true you always learn something new each day.

thanks

 
Thanks,I did get it. I have not got a scientific calculator on me at the moment so i think that did not help! Using the windows basic one is awful!

Cheers guys!
i only used a basic calc... scientific calc would require effort to get out of drawer... normal calc lives on the desk...

 
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