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November 6th, 2018, 06:47 AM  #1 
Member Joined: Aug 2017 From: India Posts: 54 Thanks: 2  General question on Vectors
A general question which I want to clarify, it is regarding the vector addition. We have a certain methodology to do vector addition. Let us take the example of AC currents or voltages which are vectors and 120 Deg apart and if we add them there is a resultant vector which will be in a direction matching as per our calculations. But in real situations are there any other signals which are vectors but still do not follow our mathematical vector addition? Is it possible or everything follows mathematics?
Last edited by skipjack; November 6th, 2018 at 12:58 PM. 
November 6th, 2018, 06:59 AM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 2,530 Thanks: 1390 
If they don't follow vector addition they aren't vectors.

November 6th, 2018, 07:10 AM  #3 
Senior Member Joined: Jun 2015 From: England Posts: 915 Thanks: 271 
This is a really good question as strictly speaking phasors are not true vectors and do not follow the parallelogram law of addition. Equally one phasor may be divided by another, but vector division is undefined. However this does not belong in elementary math. 
November 6th, 2018, 01:18 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 20,926 Thanks: 2205 
Two things may have different ways of being added. If they can be added by use of vector addition, they are vectors in relation to that method of addition. If they can be added by use of some other method to give a different result, they aren't vectors in relation to that other method of addition. It's quite possible that both possibilities can make sense in a "real world" situation. For example, the two things may be two actions that you can take, and their sum may correspond to taking both actions. It may turn out that this sum can be found by vector addition if the two actions are taken simultaneously, but not by vector addition if the actions are taken at separate times. 
November 6th, 2018, 01:21 PM  #5  
Senior Member Joined: Aug 2012 Posts: 2,356 Thanks: 738  Quote:
You can multiply a phasor by a scalar, but they add funny. I skimmed the article but my eyes glazed pretty fast. Studiot can you explain like I'm 5?  
November 6th, 2018, 03:19 PM  #6  
Senior Member Joined: Sep 2015 From: USA Posts: 2,530 Thanks: 1390  Quote:
$A \angle \phi\sim A e^{i (2\pi f t + \phi)}$ It's assumed when using them that all the signals have the same frequency $f$ They are vectors, in the same sense a complex number is a vector. They multiply as you'd expect $A_1 \angle \phi_1 A_2 \angle \phi_2 = A_1 A_2 \angle (\phi_1 + \phi_2)$ but in order to add them you have to unpack them into cartesian form. Just as with any complex number. Last edited by romsek; November 6th, 2018 at 03:40 PM.  
November 7th, 2018, 12:02 PM  #7  
Senior Member Joined: Sep 2016 From: USA Posts: 635 Thanks: 401 Math Focus: Dynamical systems, analytic function theory, numerics  Quote:
A similar example is $n \times n$ real matrices which form a vector space of dimension $n^2$ over $\mathbb{R}$. However, they also form a ring so you can multiply elements which is more structure than you get from the vector space. Similarly, you can divide (some) matrices but as you have pointed out, this is not possible from the vector structure alone.  

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