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# act07ans - _ANSWERS PHYS-1200 PHYSICS II Class 7 Activity...

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___________ ANSWERS __________________ PHYS-1200 PHYSICS II SPRING 2006 Class 7 Activity: Faraday’s Law The relation between an induced electromotive force (emf) and a changing magnetic field is given by Faraday’s Law. Today, you will use Faraday’s Law, a coil of wire, and a computer to measure the strength of a magnetic field. Let us examine Faraday’s Law to see how it can be used to measure a magnetic field. If a coil of wire, consisting of N turns, experiences a changing magnetic field, the magnetic flux through it will change. That will induce an electromotive force, E , as described by Faraday’s Law: E E = - = - N d dt dt N d B B Φ Φ , so If both sides of the equation on the right are integrated, the result is, ( 29 E dt N d N t B B f Bi Bi B f 0 = - = - - Φ Φ Φ Φ Φ That is, the integral of E over time is equal to the change in flux through the coil times the number of turns in the coil. Now imagine that the coil is initially in a strong magnetic field, and ends up in a magnetic field of zero. Then the equation becomes, ( 29 i B i B t N N dt Φ = Φ - - = 0 0 E . However, the flux through the coil, when it is in the magnetic field, is equal to the average value of the magnetic field in the coil, B av , times the area of the coil, A . Since the coil is circular, this can be written as, 2 r B A B av av i B π = = Φ . Then the equation for the integral becomes, 2 0 r NB dt av t π = E . Finally, the equation can be solved for B av to give, B dt N r av t = E 0 2 π With the help of LoggerPro, t dt 0 E can be measured, and B av can be calculated. 1

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