CHAPTER 5 ELECTROMAGNETIC INDUCTION 1 Quick Summary on Previous Concepts Electrostatics Magnetostatics Electromagnetic Induction 2 Cases of Changing Magnetic Field Changing Field Strength in a Loop A Loop Leaving a Magnetic Field Page 1
3 Explanation: Conductor Cutting Magnetic Field Lines Induced EMF for Conductor Cutting B-field Lines Fleming s Right Hand Rule 4 Easy Summary of Electromagnetic Induction Faraday s Law of Electromagnetic Induction (Strength of Induced EMF) Lenz s Law of Electromagnetic Induction (Strength of Induced EMF) Page 2
[HKCEE] [Induced Current] A metal ring is released and falls vertically around a magnet as shown above. Draw the direction of induced current at positions X and Y. [HKCEE] [Right Hand Rule] In the above diagram, a metal rod is placed inside a magnetic field pointing into the paper. In which direction should the rod be moved in order to produce an induced current as shown in the diagram? [HKCEE] [Lenz s Law] A bar magnet is placed near a solenoid. Which of the following correctly shows the direction of the induced current in the solenoid? Page 3
[HKALE] [Induced EMF] In the above figure, when the metal rod PQ moves with constant velocity across a uniform magnetic field, a p.d. is induced across the rod. Which of the following statements is/are correct? (1) The magnitude of the p.d. depends on the length of the rod. (2) Q is at a higher potential than P. (3) A force is acting on the rod to oppose its motion. A. (1), (2) and (3) B. (1) and (2) only C. (2) and (3) only D. (1) only E. (3) only [HKALE] [Faraday s Law] The figure shows a rectangular coil PQRS moving from left to right with a uniform speed across an insulated metal wire in the plane of the coil. The wire carries a steady current I. Which of the following gives the correct sequence for the direction if the current induced in the coil PQRS? A. Clockwise and then anticlockwise B. Anticlockwise and then clockwise C. Clockwise, then anticlockwise and finally clockwise again D. Anticlockwise, then clockwise and finally anticlockwise again Page 4
5 Mathematical Description of Induced EMF [FX only] Magnetic Flux, Flux Density and Flux Linkage Faraday s Law Page 5
[HKALE] [Cutting Field Lines] [FX only] A metal rod of length l is inclined at 60 o to rail PQ as shown. It is moved across a uniform magnetic field along the direction of the two horizontal rails PQ and RS. If the rod moves with a uniform velocity v and the flux density of the field is B, the e.m.f. induced in the rod is A. Blv 2 B. Blv C. 2Blv 3 D. 3Blv 2 [HKALE] [Faraday s Law] [FX only] A solenoid of n 1 turns per metre and cross-sectional area A 1 carries a current I. It is inserted into the core of another larger solenoid of N 2 turns and cross-sectional area A 2. If the current in the smaller solenoid drops uniformly to zero in time t, what is the e.m.f. induced in the larger solenoid? 2µ n IA N t µ n IA N t A. 0 1 1 2 D. 0 1 2 2 2µ n IA N t µ n IA N t B. 0 1 2 2 E. 0 1 1 2 µ n IA A t C. 0 1 1 2 Page 6
6 Application of Electromagnetic Induction: Generator Induced EMF in rotating coil Simple A.C. Generator Draw the graph of induced current against time: Page 7
n Simple D.C. Generator Factors Affecting Induced EMF 1. 2. 3. 4. 5. Page 8
7 Eddy Current Concept 8 Application of Induced EMF Moving Coil Microphone Page 9
Induction Cooker FINAL REMARKS Electromagnetic induction is the linkage between electricity and magnetism. We found that a changing magnetic field can lead to an induced electric field. A charge that moves along the electric field can acquire an amount of energy (per unit charge) which is called the induced EMF. To be precise, the changing magnetic field is quantified by the changing magnetic flux over through a coil. A changing magnetic flux can be caused by (i) changing magnetic field strength, (ii) changing area of the loop or (iii) changing angle between the area and the B field. The rate of change of magnetic flux determines the induced EMF, the mathematical statement is given by the Faraday s law. Page 10