Page 1 of 4 123 ... LastLast
Results 1 to 10 of 31

Thread: Stupid electrical math question

  1. #1
    Join Date
    Oct 2006
    Location
    jonnyGURU forums, of course!
    Posts
    15,865
    Thanks Thanks Given 
    525
    Thanks Thanks Received 
    179
    Thanked in
    129 Posts

    Default Stupid electrical math question

    Question:

    Equation for resistive loss is P = I^2 * R, where P is Wattage, I is Current and R is Resistance.

    Since P = I * V (Voltage), wouldn't I divide both sides by I and come up with the equation of V = I * R? Wouldn't that be the same thing?

  2. #2
    Join Date
    Oct 2006
    Location
    Moderator
    Posts
    6,503
    Thanks Thanks Given 
    148
    Thanks Thanks Received 
    3
    Thanked in
    2 Posts

    Default

    Quote Originally Posted by jonnyGURU View Post
    Since P = I * V (Voltage), wouldn't I divide both sides by I and come up with the equation of V = I * R?
    Ohm's law thinks you're correct

  3. #3
    Join Date
    Oct 2006
    Location
    jonnyGURU forums, of course!
    Posts
    15,865
    Thanks Thanks Given 
    525
    Thanks Thanks Received 
    179
    Thanked in
    129 Posts

    Default

    Right... because Ohm's Law is I = V / R which is the same as V = I * R. Damn... Why bother with the P = I^2 * R. Sheesh!

  4. #4
    Join Date
    Dec 2006
    Location
    UK
    Posts
    514
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Default

    yes correct, its called algebra, you will find all sorts of relations if you keep on dividing and multiplying. but its been done before and the equations we have I think were all done that way, apart from the 'laws' which were experimentally derived from observation, and perhaps from 'first principles' by the clever people.

    why bother? depends what 'variable's' you have available and where you want to get to.

    the one's on the right are called the 'independant' variables, and the left one is 'dependant' as its dependant on the others, or sometimes called a 'function' of the others.

    your favorite people the arabs, came up with al gabra, when some of them were enlightened.
    Last edited by ianm2; 10-02-2007 at 03:42 AM.

  5. #5
    Join Date
    Oct 2006
    Location
    jonnyGURU forums, of course!
    Posts
    15,865
    Thanks Thanks Given 
    525
    Thanks Thanks Received 
    179
    Thanked in
    129 Posts

    Default

    Thanks smart ass. I got an A in algebra, perfect score on the exam and was a programmer for some years. That's not the point of my question.

    I was just wondering if I could apply it to the electrical equations because the paper I was looking at was clearly trying to differentiate P = I^2 * R and V = I * R and I can't for the life of me understand why.

  6. #6
    Join Date
    Aug 2007
    Posts
    41
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Default

    Quote Originally Posted by jonnyGURU View Post
    Thanks smart ass. I got an A in algebra, perfect score on the exam and was a programmer for some years. That's not the point of my question.

    I was just wondering if I could apply it to the electrical equations because the paper I was looking at was clearly trying to differentiate P = I^2 * R and V = I * R and I can't for the life of me understand why.
    If you don't know the voltage, but just need the power the circuit dissipates then you'd have to use two formulas to find the power without P=I^2 * R. Now, in what circumstance you'd not know or care what the voltage supplied to the circuit was... I haven't a clue. But it's good to have something to cover all bases I guess.

  7. #7
    Join Date
    Oct 2006
    Location
    jonnyGURU forums, of course!
    Posts
    15,865
    Thanks Thanks Given 
    525
    Thanks Thanks Received 
    179
    Thanked in
    129 Posts

    Default

    Well, yeah. That's kind of my point. How would you actually know the wattage and not either the voltage or current (of course, if you knew either and the wattage than you can figure out the other.)

    I just think some people make explaining electrical theory overly complicated just for the sake of it.

  8. #8
    Join Date
    Aug 2007
    Posts
    41
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Default

    Quote Originally Posted by jonnyGURU View Post
    Well, yeah. That's kind of my point. How would you actually know the wattage and not either the voltage or current (of course, if you knew either and the wattage than you can figure out the other.)

    I just think some people make explaining electrical theory overly complicated just for the sake of it.

    Well, say you have a circuit that has to stay under 500 watts (because you're using a shared 110v/15a branch circuit), and you have to need to maintain a constant current of 5 amps (for say a lead acid battery charger). You need to find out what the resistance needs to be so you can choose your components. You'd have to use P=I²R to solve the problem.

    500 = 5²R;
    500 = 25R;
    500/25 = 25R/25;
    20 = R.

    Your circuit can have a resistance of no more than 20 Ohms.

  9. #9
    Join Date
    Feb 2007
    Posts
    14
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Default

    I think you already wrote the answer to your question in your first post
    The P = I² * R formula is nothing else then the short version of P=I*I*R which is nothing else then P=I*V | V = I * R (the trick is to view the V as an variable, where you can put in any equivalent therms).

    Basically, this has nothing to do with Ohm`s law, it is pure algebra i would say

  10. #10
    Join Date
    Apr 2007
    Posts
    75
    Thanks Thanks Given 
    0
    Thanks Thanks Received 
    0
    Thanked in
    0 Posts

    Default

    I am pretty sure the point of physics/science is to make things more complicated than they need to be, I'm pretty sure one of my old text books actually says that on the cover...

  11. The Following User Says Thank You to Ripudio For This Useful Post:

    Jon Gerow (12-22-2017)

Similar Threads

  1. [Stupid question] Can i connect 2 CPU cables on this PSU?
    By Gigi86 in forum PC Power Supply Discussion
    Replies: 4
    Last Post: 11-23-2018, 12:30 PM
  2. Psu electrical/technical question.
    By wobblefang in forum PC Power Supply Discussion
    Replies: 5
    Last Post: 07-13-2017, 09:53 AM
  3. Probably a stupid question but...(kabylake)
    By Jerri1240 in forum General PC Hardware
    Replies: 9
    Last Post: 01-13-2017, 04:53 PM
  4. Quick, stupid, repetitive....."Will this PS work for me?" question...
    By C'DaleRider in forum PC Power Supply Discussion
    Replies: 9
    Last Post: 02-04-2009, 10:40 AM
  5. ,,Stupid,, question.
    By Oranje in forum PC Power Supply Discussion
    Replies: 2
    Last Post: 09-23-2007, 04:03 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •