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db_lec12_post - Lecture 12 Electrostatics motion of"q in...

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Lecture 12 • Electrostatics • motion of “ q ” in external E-field • E-field generated by q • Magnetostatics • motion of “ q ” and “ I ” in external B-field • B-field generated by “ I • Electrodynamics • time dependent B-field generates E-field –ac circuits, inductors, transformers, etc –time dependent E-field generates B-field 2/20/12 1 D. Bortoletto PHYS221

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2/20/12 2 Induction: DEMONSTRATION ammeter Current flows only if there is relative motion between the loop and the magnet Current disappears when the relative motion ceases Faster motion produces a greater current Caution – this picture is not an example of right hand rule! D. Bortoletto PHYS221
2/20/12 3 Induction Effects from Currents a b • Conclusion: A current is induced in a loop when: there is a change in magnetic field through it. This can happen many different ways. How can we quantify this? •When the switch is closed (or opened) current induced in coil b •Steady state current in coil a no current induced in coil b D. Bortoletto PHYS221

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2/20/12 4 Faraday’s Law An emf is induced in a loop when the number of magnetic field lines that pass through the loop is changing. A B B Define the flux of the magnetic field through an open surface A as: Φ B = BA cos θ Unit: 1 Weber=1Wb=1 Tm 2 Only components perpendicular to loop D. Bortoletto PHYS221
2/20/12 5 Faraday’s Law Restated The magnitude of the emf ε induced in a conducting loop is equal to the rate at which the magnetic flux Φ B through the loop changes. ε = ΔΦ B Δ t A B B •The minus sign indicates opposition •Direction of induced current always opposes any changes in the magnetic flux.

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