Class_28new - PHYS809 Class 28 Notes Time varying fields...

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1 PHYS809 Class 28 Notes Time varying fields and Maxwell’s equations Faraday experimentally determined that a transient current is induced in a circuit if (a) a steady current in a nearby circuit is switched on or off, (b) a nearby circuit with a steady current is moved relative to the first circuit, or (c) a permanent magnet is moved in or out of the circuit. These experiments showed that changes in magnetic field can cause current flow. Faraday’s result can be summarized as a relation between the magnetic flux threading the circuit , S F dS = B n (28.1) and a quantity called the electromotive force around the circuit . C d = E l E (28.2) Note that for an electrostatic situation, 0 ∇× = E implies that the electromotive force (emf) is always zero. In SI units, Faraday’s law is . dF dt = - E (28.3) The physical significance of the minus sign is that the current (if any) that results from the induced emf is the direction that opposes the change in the magnetic flux. Faraday’s law in the form of equation (28.3) is quite general and includes the case in which the circuit
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Class_28new - PHYS809 Class 28 Notes Time varying fields...

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