# faraday's law - Michael Faraday 1791-1867 Last Week...

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1 1 Magnetically Induced EMF: Faraday’s Law Michael Faraday 1791-1867 This lecture: HRW 30.1-30.5 Next time: HRW 30.6-30.8 2 Last Week Electric currents create B fields Calculate B using Biot-Savart Calculate B using Ampere’s Law Faraday’s Law: what happens when B changes in time? Generators, eddy currents, electromagnetic brakes, maglev trains etc. Today Static B fields. 3 Some Experimental Observations V N S Currents create B-fields. Can B-fields induce currents? Yes, a changing B-field can induce a current! Move a magnet towards or away from a closed loop of wire What do we observe? •A potential difference (“induced EMF”) is measured in the loop even though there is NO battery present. E is induced ONLY while the magnet MOVES •Reversing the motion reverses E . •Reversing the magnet reverses the E . •The E is larger when the magnet moves faster. 4 Faraday’s Law d A B n ˆ ≡• r B S Φ Bn dA Magnetic flux is defined just like electric flux: Faraday’s Law : A time varying magnetic flux creates an induced EMF B Φ =- t E Lenz’s Law: An induced E tries to produce a current that opposes the change in flux . Hence, the minus sign.

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## This note was uploaded on 04/05/2008 for the course PHYS 212L taught by Professor Huang,tai-yin during the Spring '07 term at Pennsylvania State University, University Park.

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faraday's law - Michael Faraday 1791-1867 Last Week...

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