

Subject:
General Physics Explaination
Category: Science > Physics Asked by: dsgoldsga List Price: $7.00 
Posted:
22 Apr 2006 16:11 PDT
Expires: 06 May 2006 16:12 PDT Question ID: 721820 
Is there any reasoning that I could use to defend my answer to the following question: Question: A hydrogen atom has a proton and an electron. If the orbital radius of the electron increases, the potential energy of the electron ________ My answer: Depends on the zero point of potential Claimed correct answer: increases 

There is no answer at this time. 

Subject:
Re: General Physics Explaination
From: kottekoega on 22 Apr 2006 17:22 PDT 
Your answer is incorrect. The zero point of the potential never matters for anything. You can set it to whatever you want, at your convenience. For the Coulomb potential the convention is to set the potential to zero when the electron is infinitely far away. Suppose you choose instead to set the potential to Vo at infinity. Then the potential at a radius r is given by: V = Vo  e^2/r The larger you make r, the larger the potential energy is, no matter what value I choose for Vo. Think about it. The electron is attracted to the proton. I have to use energy to pull it further away. Thus the potential energy increases. 
Subject:
Re: General Physics Explaination
From: kottekoega on 22 Apr 2006 20:14 PDT 
The first time I read your question, I was confused. I didn't realize that the separator line was meant to be understood as a blank to be filled in. Now I see that your answer is not an incorrect statement, it just evades the obvious intent of the question. Yes, certainly the potential energy depends on the zero point, but clearly the question is asking what happens when the radius increases. 
Subject:
Re: General Physics Explaination
From: dsgoldsga on 22 Apr 2006 23:20 PDT 
I need a good way to explain the reasoning for my answer. do you have any examples i could use to prove the question wrong or make my answer seem more reasonable (help me explain my reasoning to others)? 
Subject:
Re: General Physics Explaination
From: kottekoega on 23 Apr 2006 08:23 PDT 
I can't help you explain your own reasoning. I hope you won't take offense, but if I were grading this answer it would not receive any credit because it is a trivial statement that evades the intent of the question, which is asking what happens when the radius increases. You might as well have filled the blank by saying: "... the potential energy of the electron is a form of energy ..." . In the words of Wolfgang Pauli, "It is not even wrong." 
Subject:
Re: General Physics Explaination
From: electandphysicsguyga on 23 Apr 2006 13:41 PDT 
You must make the distinction between absolute potential energy and relative potential energy. Your question is a bit like a hiker on a hill. What is his absolute potential energy? Well now, that depends on where you define the zero point to be. For example if the base of the hill is defined as zero and he is at 100 meters above it. Then his potential energy, U is U=mgh Where: m=mass, of hiker let us say 50kg g=gravitational constant=9.8m/s^2 h=height of hiker 100m then: U= 50kg ( 9.8m/s^2) 100m =approximately 50kJ Let us say he descends to 50m. Then his potential energy changes to: U= 50kg (9.8m/s^2) 50m =approximately 25kJ. His relative potential energy decreased, but at all times he had a positive absolute potential energy. If you set the reference level very much above him, he will have negative potential energy but he can climb and descend in other words, gain and loose potential energy. So it is with the attraction between an electron and proton. Instead of U=mgh(for the gravitational field on earth), We have U=  k e^2/r Note the negative sign Where: k=coulombs constant e= the fundamental unit charge r= the distance between the electron and proton You can see from the equation as r increases U increases. (I figure you are okay with that.) Here, I think, is the real meat of what you are looking for. By convention for particles of opposite charge (electrons and protons), U=0 is defined to be at infinity. You can see this by inspection of the above equation. Now if we pick some arbitrary distance to be zero, let us call it x, then the potential energy equation becomes: U= (k e^2/x) ? (k e^2/r) = k e^2( 1/x  1/r) Now if you make r>x then the potential energy is positive. But these does not change the fact that as r increases the potential energy also increase. Is anyone still reading this? 
Subject:
Re: General Physics Explaination
From: electandphysicsguyga on 23 Apr 2006 13:43 PDT 
Oh yes BTW the answer to your question I am sorry to say is no :( 
Subject:
Re: General Physics Explaination
From: ankur80ga on 23 Apr 2006 16:26 PDT 
Okay here's the thing. I know that potential energy calculations can get confusing because you can select any point of reference. But you should consider this : Since positive and negative charges attract each other "naturally" so a more favorable state (less potential energy) of the system would be when the charges are close together. So the correct answer to the question would be that the potential energy "increases". No matter what reference you choose , if you apply the rules for potential energy calculations correctly then on increasing the orbital radius for the H atom in question the potential energy will always increase. Trust me its not a very high funda question .. its basic physics. Thanks, Ankur 
Subject:
Re: General Physics Explaination
From: physciga on 23 Apr 2006 18:36 PDT 
You might keep in mind that the definition of the electric potential at a point is the work done in bringing a unit positive test charge from infinity (obviously zero potential at infinity) to that point. It is also immediately obvious that negative work is done in bringing a negative charge from infinity to proximity of a positive charge. 
Subject:
Re: General Physics Explaination
From: marcuslga on 27 Apr 2006 18:11 PDT 
kottekoe is completely correct. "zero" potential is defined as infinitely far away. 
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