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Today4/23 Magnetic Induction Review HW Questions? Exam 4/24 Force and motion of charged particles B-fields due to wires Force and Torque on current loops Forces from currents (like 4/16 Wires 2) Induced current (like 4/17) Magnetic Induction The current only flows while: the B-field is changing the ring is moving the magnet is moving It’s all about a “changing” B field!!! Look at whether the # of field lines are increasing or decreasing Force on Current Loops Ch 21: Force/Torque arises from a battery operated loop in a static B-field. Ch 22: Current in loops “induced” by a “changing” external field. The loop then reacts as in Ch 21. In fact, battery operated loops resist the changing field caused by themselves! Called “Self Induction” see 22.8 Faraday’s & Lenz’z Law Loop “Voltage” E = - t Lenz’s Law is the # of field lines inside loop can change in many ways: changing B-field changing loop area changing loop angle changing B-field angle This is the “equation” you will see on the exam Using E = - t Identify initial field direction Identify final field direction Determine direction of the “change” Use Lenz’s law to determine B-field due to the current induced around the loop Use RHR #2 to find direction of induced current Energy Considerations It takes energy to push current around a loop Therefore, forces arising from induced currents must always serve to retard (or slow down) the object