Posted: 4/1/2012 6:26:10 PM EDT
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Cross posted from GD:
I'm studying for a physics exam and cannot make my numbers match the answers for this study problem: 
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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Emf = -d phi / dt
B*area = phi area = constant, d phi / dt = [ -Bo * a * exp(-a*t) ] * area ––––––––––––> plug in givens like time here V = IR ETA: this gave me the right answer for the first given time value. do unit analysis. use meters when determining area of coil. |
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Quoted:
Emf = -d phi / dt B*area = phi area = constant, d phi / dt = [ -Bo * a * exp(-a*t) ] * area ––––––––––––> plug in givens like time here V = IR ETA: this gave me the right answer for the first given time value. do unit analysis. use meters when determining area of coil. Teach a man to fish... |
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Quoted:
Quoted:
Emf = -d phi / dt B*area = phi area = constant, d phi / dt = [ -Bo * a * exp(-a*t) ] * area ––––––––––––> plug in givens like time here V = IR ETA: this gave me the right answer for the first given time value. do unit analysis. use meters when determining area of coil. Teach a man to fish... i'm not a teacher, just a wannabe nerd with a huge chip on my shoulder 
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Emf = -d phi / dt B*area = phi area = constant, d phi / dt = [ -Bo * a * exp(-a*t) ] * area ––––––––––––> plug in givens like time here V = IR ETA: this gave me the right answer for the first given time value. do unit analysis. use meters when determining area of coil. Teach a man to fish... i'm not a teacher, just a wannabe nerd with a huge chip on my shoulder
Sorry for your unhappy life. It must be hell to be you. |
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I forgot about this thread, but not that stupid problem.
My mistake was that I was seeing the "200s^-1" as 1/200s (or 0.005s^-1), not 200/1s, which would make the units cancel and the function actually work. So, as it turns out, I have more trouble with basic algebra than I do with calculus. 
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Emf = -d phi / dt B*area = phi area = constant, d phi / dt = [ -Bo * a * exp(-a*t) ] * area ––––––––––––> plug in givens like time here V = IR ETA: this gave me the right answer for the first given time value. do unit analysis. use meters when determining area of coil. Teach a man to fish... i'm not a teacher, just a wannabe nerd with a huge chip on my shoulder
Sorry for your unhappy life. It must be hell to be you. thanks brah |
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Quoted:
I forgot about this thread, but not that stupid problem. My mistake was that I was seeing the "200s^-1" as 1/200s (or 0.005s^-1), not 200/1s, which would make the units cancel and the function actually work. So, as it turns out, I have more trouble with basic algebra than I do with calculus. that's where it usually get ya.... |