IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: cashem'up on March 12, 2010, 11:22:23 am

Title: physics- electricity help
Post by: cashem'up on March 12, 2010, 11:22:23 am

why is the power dissipated max when the internal resistance is equal to the load resistance................plz explain

Title: Re: physics- electricity help
Post by: astarmathsandphysics on March 12, 2010, 12:13:05 pm

It is a complicated maths thing. You differentiate and set equal to zero.

Title: Re: physics- electricity help
Post by: cashem'up on March 12, 2010, 04:48:49 pm

its ok can u show me how to do it......is it like finding the stationary point and the maxima and minima...........jst explain....nd then please support with theological statement

Title: Re: physics- electricity help
Post by: astarmathsandphysics on March 12, 2010, 05:08:06 pm

Ok when I get home

Title: Re: physics- electricity help
Post by: astarmathsandphysics on March 12, 2010, 11:13:56 pm

Pout=Iout Vout =V/(R+r)*VR/(R+r)=VR/(R+r)^2
d(P out)/dR =V((R+r)^2 -2(R+r)R)/(R+r)^4 =0
0=(R+r)^2-2(R+r)R
r^2-R^2=0 so r=R

Title: Re: physics- electricity help
Post by: 7ooD on March 13, 2010, 12:31:18 am

do they ask such question in edexcel as

Title: Re: physics- electricity help
Post by: vanibharutham on March 13, 2010, 10:52:04 am

i have attached my method

its a bit different from astar's method, but works out to be the same i.e. R = r

Title: Re: physics- electricity help
Post by: cashem'up on March 13, 2010, 04:15:19 pm

thanks........yea that does make sense mathemetically but how would u explain it theoritacally....... i mean wat is actually happening

Title: Re: physics- electricity help
Post by: astarmathsandphysics on March 13, 2010, 07:07:53 pm

Thats what I meant by it being a mathematical answer. Because p=vi you have to maximise v and I but you cant do both and must use maths to do it.