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PHYSICS 3 (ELECTRICITY AND MAGNETISM) - CHAPTER 5

ELECTROMAGNETIC INDUCTION 5,1 Faraday Định luật cảm ứng 1) Trong hình. 5,1 ampe kế được kết nối trong các mạch của một vòng lặp thực hiện. Khi các thanh nam châm được di chuyển gần, hoặc xa hơn từ hơn, vòng lặp, một lực điện động (emf) được cảm ứng trong vòng lặp. Ampe kế chỉ ra dòng điện trong các hướng khác nhau tùy thuộc vào chuyển động tương đối của nam châm và vòng lặp. Chú ý rằng, khi nam châm ngừng di chuyển, trở về hiện tại để không chỉ bởi ampe kế. | Electricity and Magnetism 53 Chapter 5 ELECTROMAGNETIC INDUCTION 5.1 Faraday s Law of Induction 1 In Fig. 5.1 an ammeter is connected in the circuit of a conducting loop. When the bar magnet is moved closer to or farther from the loop an electromotive force emf is induced in the loop. The ammeter indicates currents in different directions depending on the relative motion of magnet and loop. Notice that when the magnet stops moving the current returns to zero as indicated by the ammeter Fig. 5.1 Electromagnetic Induction The magnetic flux Ob through an area A in a magnetic field B is defined as Fig. 5.2 Ob IB .dA Wb 5.1 where the integration is taken over the area. Faraday s Law of Induction The induced electromotive force dOB dt 5.2 2 Lenz s Law An induced current has a direction such that the magnetic field due to the current opposes the changes in the magnetic flux that induced the current. The induced emf has the same direction as the induced current. 3 Electromotive force and the induced electric field An emf is induced by a changing magnetic flux even if the loop through which the flux is changing is not a physical conductor. The induced emf is related to the electric field E by e I Eds 5.3 Electricity and Magnetism 54 where the integration is taken around the loop. It follows from 5.2 and 5.3 that i lids - d B 5.4 dt a changing magnetic field B induces an electric field E. 5.2 Inductors 1 Inductance NOb L N-B H 5.5 1 The inductance per unit length near the middle of a long solenoid of cross sectional area A and n turns per unit length L pon2A 5.6 2 Self-induction If the current in a coil changes with time an emf is induced in the coil e -L 5.7 dt The direction of e is found from the Lenz s law the direction of e acts to oppose the changes that produces it. 5.3 Series RL circuit 1 Rise of current Fig. 5.3 For t 0 the switch K is at 2 i 0A. For t 0 the switch K is at 1. Apply the loop rule E lA Ri dt i E 1-e-t T R 5.8 where T inductive time constant. R Fig. 5.3