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 thezentree 
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Joined: Dec 2007  NC, USA
 Posts: 1434 Feedback: 0% (0) |
Posted: 4/1/2012 4:51:03 PM
THE IMAGE ABOVE IS A PAID ADVERTISEMENT 
What I THINK I'm supposed to do is find the magnetic flux ( =magnetic field times area enclosed), use EMF (voltage) = - (derivative of magnetic flux with respect to time), then take that EMF value and plug it in to V=IR and solve for I. However, I keep getting 1.2499E-6 V, giving 6.249E-7 A for t=1.0E-3 seconds. The given answers are (in order): 0.02 A, 0.017 A, and 0 A. Am I using the wrong formulas? Taking the derivative incorrectly? Or are my teachers just failing at this? |
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Posted: 4/1/2012 9:28:58 PM
bump for the night crew
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FP2000H
I'm awesome
Life Member
Joined: Dec 2005 TX, USA
Posts: 4441 Feedback: 100% (136) Link To This Post |
Posted: 4/1/2012 9:30:47 PM
Um, bullets go fast.
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macro
Bitter Pennsylvanian clinging to his guns
Team Member
Joined: Apr 2003 USA
Posts: 16021 Feedback: 100% (14) Link To This Post |
Posted: 4/1/2012 9:31:10 PM
I'll give you a bump and a tag for the answer.
I'm 20 years late to the thread....I've since forgotten all those formulas  |
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TheNamelessOne
Douchebag First Class.
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Joined: Jun 2007 AR, USA
Posts: 7750 Feedback: 0% (0) Link To This Post |
Posted: 4/1/2012 9:32:32 PM
1.21 gigawatts
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scotchymcdrinkerbean
Alcohol is my anti-drug
Team Member
Joined: Oct 2003 LA, USA
Posts: 12372 Feedback: 100% (3) Link To This Post |
Posted: 4/1/2012 9:33:49 PM
Magic.
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Posted: 4/1/2012 9:35:57 PM
I don't miss being in school.
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FP2000H
I'm awesome
Life Member
Joined: Dec 2005 TX, USA
Posts: 4442 Feedback: 100% (136) Link To This Post |
Posted: 4/1/2012 9:36:48 PM
Originally Posted By scotchymcdrinkerbean:
Magic.  |
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ARDestructo
Member
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Joined: Nov 2009 OK, USA
Posts: 4702 Feedback: 0% (0) Link To This Post |
Posted: 4/1/2012 9:38:52 PM
Originally Posted By thezentree:
I'm studying for a physics exam and cannot make my numbers match the answers for this study problem: http://img.photobucket.com/albums/v248/thezentree/currentinduced.jpg What I THINK I'm supposed to do is find the magnetic flux ( =magnetic field times area enclosed), use EMF (voltage) = - (derivative of magnetic flux with respect to time), then take that EMF value and plug it in to V=IR and solve for I. However, I keep getting 1.2499E-6 V, giving 6.249E-7 A for t=1.0E-3 seconds. The given answers are (in order): 0.02 A, 0.017 A, and 0 A. Am I using the wrong formulas? Taking the derivative incorrectly? Or are my teachers just failing at this? I've only done E&M 1 so we haven't dealt much with time-variant E&M, but I'm having a bit of trouble getting off the ground because I don't understand how the field can be completely uniform inside the coil. I also don't know how resistance factors in, but can't you take the equation for the magnetic field generated by current, and move backwards from there? |
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Posted: 4/1/2012 9:55:46 PM
Originally Posted By ARDestructo:
Originally Posted By thezentree:
I'm studying for a physics exam and cannot make my numbers match the answers for this study problem: http://img.photobucket.com/albums/v248/thezentree/currentinduced.jpg What I THINK I'm supposed to do is find the magnetic flux ( =magnetic field times area enclosed), use EMF (voltage) = - (derivative of magnetic flux with respect to time), then take that EMF value and plug it in to V=IR and solve for I. However, I keep getting 1.2499E-6 V, giving 6.249E-7 A for t=1.0E-3 seconds. The given answers are (in order): 0.02 A, 0.017 A, and 0 A. Am I using the wrong formulas? Taking the derivative incorrectly? Or are my teachers just failing at this? I've only done E&M 1 so we haven't dealt much with time-variant E&M, but I'm having a bit of trouble getting off the ground because I don't understand how the field can be completely uniform inside the coil. I also don't know how resistance factors in, but can't you take the equation for the magnetic field generated by current, and move backwards from there? To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. |
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ARDestructo
Member
Team Member
Joined: Nov 2009 OK, USA
Posts: 4704 Feedback: 0% (0) Link To This Post |
Posted: 4/1/2012 9:58:53 PM
Originally Posted By thezentree:
Originally Posted By ARDestructo:
Originally Posted By thezentree:
I'm studying for a physics exam and cannot make my numbers match the answers for this study problem: http://img.photobucket.com/albums/v248/thezentree/currentinduced.jpg What I THINK I'm supposed to do is find the magnetic flux ( =magnetic field times area enclosed), use EMF (voltage) = - (derivative of magnetic flux with respect to time), then take that EMF value and plug it in to V=IR and solve for I. However, I keep getting 1.2499E-6 V, giving 6.249E-7 A for t=1.0E-3 seconds. The given answers are (in order): 0.02 A, 0.017 A, and 0 A. Am I using the wrong formulas? Taking the derivative incorrectly? Or are my teachers just failing at this? I've only done E&M 1 so we haven't dealt much with time-variant E&M, but I'm having a bit of trouble getting off the ground because I don't understand how the field can be completely uniform inside the coil. I also don't know how resistance factors in, but can't you take the equation for the magnetic field generated by current, and move backwards from there? To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. But isn't the equation for magnetic field due to current symmetrical? Wouldn't you be able to plug in the field and solve for the current that caused it? |
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burnprocess
Pissed off and ugly.
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Joined: Nov 2008 FL, USA
Posts: 1259 Feedback: 0% (0) Link To This Post |
Posted: 4/1/2012 9:59:29 PM
I would recommend posting this in the Math and Science forum. Asking something like this in GD will get you blank stares, and possibly handfuls of shit thrown at you, which would then make this a poop thread, which will get locked anyway.
Just a suggestion.  |
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Posted: 4/1/2012 10:01:35 PM
[Last Edit: 4/1/2012 10:03:15 PM by kelone]
Originally Posted By thezentree:
...snip... To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. a=200s^(-1) means a=200/second, which cancels the 'second' from t it does not mean 1/200 seconds B =0.25e^(-200t) or maybe I am to drunk to play
ETA: to clarify - it is 200(s^-1), not (200s)^(-1) - I think. If that is backwards, ignore my post |
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Posted: 4/1/2012 10:05:13 PM
Is the coil in the Northern or Southern hemisphere?
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Posted: 4/1/2012 10:23:53 PM
Originally Posted By ARDestructo:
Originally Posted By thezentree:
Originally Posted By ARDestructo:
I've only done E&M 1 so we haven't dealt much with time-variant E&M, but I'm having a bit of trouble getting off the ground because I don't understand how the field can be completely uniform inside the coil. I also don't know how resistance factors in, but can't you take the equation for the magnetic field generated by current, and move backwards from there? To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. But isn't the equation for magnetic field due to current symmetrical? Wouldn't you be able to plug in the field and solve for the current that caused it? I'm not sure, to be honest. The only equation that I know of from which I'd be able to solve directly for current deals with the magnetic field at the center of a circular current loop, and I'm not sure I can apply that in this case. That equation assumes that the conductor is equidistant from the point where the magnetic field is measured, and it's not in this case. Which equation are you talking about? Originally Posted By burnprocess:
I would recommend posting this in the Math and Science forum. Asking something like this in GD will get you blank stares, and possibly handfuls of shit thrown at you, which would then make this a poop thread, which will get locked anyway. Just a suggestion. Good point. I will cross post this. Originally Posted By kelone:
Originally Posted By thezentree:
...snip... To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. a=200s^(-1) means a=200/second, which cancels the 'second' from t it does not mean 1/200 seconds B =0.25e^(-200t) or maybe I am to drunk to play
ETA: to clarify - it is 200(s^-1), not (200s)^(-1) - I think. If that is backwards, ignore my post By using -200t instead of t/-200, I'm getting answers that are a lot closer in magnitude to the given answers, even though they're still not correct, so I think you're right. I'm so burned out that I can't even do basic algebra anymore. This bodes well for the test. |
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Posted: 4/1/2012 10:26:40 PM
[Last Edit: 4/1/2012 10:27:12 PM by kelone]
Originally Posted By thezentree:
...snip... Originally Posted By kelone:
Originally Posted By thezentree:
...snip... To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. a=200s^(-1) means a=200/second, which cancels the 'second' from t it does not mean 1/200 seconds B =0.25e^(-200t) or maybe I am to drunk to play
ETA: to clarify - it is 200(s^-1), not (200s)^(-1) - I think. If that is backwards, ignore my post By using -200t instead of t/-200, I'm getting answers that are a lot closer in magnitude to the given answers, even though they're still not correct, so I think you're right. I'm so burned out that I can't even do basic algebra anymore. This bodes well for the test. (D[0.25*E^(-a*t), t] /. {a -> 200, t -> 1*10^-3} // N )*(0.05*0.02)/2 -0.0204683 thats the right answer using 200, so that was your problem. |
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ARDestructo
Member
Team Member
Joined: Nov 2009 OK, USA
Posts: 4711 Feedback: 0% (0) Link To This Post |
Posted: 4/1/2012 10:30:07 PM
[Last Edit: 4/1/2012 10:30:30 PM by ARDestructo]
Originally Posted By thezentree:
Which equation are you talking about? uh... The one where you use the divergence theorem to solve for the total magnetic field at a distance from a current cross-section. I took my B and walked away, man. |
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Posted: 4/1/2012 10:36:39 PM
Originally Posted By kelone:
Originally Posted By thezentree:
...snip... Originally Posted By kelone:
Originally Posted By thezentree:
...snip... To my understanding, the only current in the coil is the current induced by the magnetic field. Maybe I'm doing the derivative wrong. If a = 200 s^(-1), then B would be 0.25e^(t/-200), right? I feel like for as many times as I've gotten the same numbers I'm doing something wrong. a=200s^(-1) means a=200/second, which cancels the 'second' from t it does not mean 1/200 seconds B =0.25e^(-200t) or maybe I am to drunk to play
ETA: to clarify - it is 200(s^-1), not (200s)^(-1) - I think. If that is backwards, ignore my post By using -200t instead of t/-200, I'm getting answers that are a lot closer in magnitude to the given answers, even though they're still not correct, so I think you're right. I'm so burned out that I can't even do basic algebra anymore. This bodes well for the test. (D[0.25*E^(-a*t), t] /. {a -> 200, t -> 1*10^-3} // N )*(0.05*0.02)/2 -0.0204683 thats the right answer using 200, so that was your problem. Yep, I forgot to multiply .00025 by 200 when I took the derivative.
Thanks so much for the help. |
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Posted: 4/1/2012 10:38:12 PM
Originally Posted By ARDestructo:
Originally Posted By thezentree:
Which equation are you talking about? uh... The one where you use the divergence theorem to solve for the total magnetic field at a distance from a current cross-section. I took my B and walked away, man. Haha I don't recognize some of those words. You can keep your B, I don't want anymore B. Please no more B.
(thanks for replying though, you were one of the physics geeks I was looking for) |
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