Tiêu đề 21.Electromagnetic Induction.pdf ✅

NEET-2022 Ultimate Crash Course PHYSICS ELECTROMAGNETIC INDUCTION

Topic-wise analysis of NEET 2014-2021 Topic name/ year 2014 2015 2016 2017 2018 2019 2019 (Orissa) 2020 2020 (Covid-19) 2021 T1: Magnetic Flux, Faraday’s and Lenz’s Law 1 T2 & T3 : Motional & Static EMI and Applications of EMI 1 1 1 1 1 POINTS TO REMEMBER 1. It must be borne in mind that electric field E is : (i) a non-conservative and (ii) time varying field that is generated by a changing magnetic field. This field is quite different from an electrostatic field produce by stationary charges. 2. Induced emf ( ) always exists whether the circuit is closed or not. But the induced current (I) will exist only if the circuit is closed. 3. If R is the resistance of the circuit, induced current through the circuit. = = Blv I R R  4. F IlB = and I Blv R = / 2 2   = =     Blv B l v F lB R R Power required to pull the conductor ab (to the right) with velocity v, i.e., 2 2 2 = = B l v P Fv R Thermal power dissipated in R, i.e., ( ) 2 2 2 2 2 R = = = Blv B l v P R R R  It is obvious that power supplied by the applied force (which is equal to F but is in opposite direction) is equal to that dissipated as heat in the resistor. Thus, energy is conserved, as always 5. As = = , dQ I I R dt  and = B d dt   , 1 = B dQ d dt R dt  or 1 = B dQ d R  or ( ) | 1 1 |  = = = =  B B B B B B Q d R R R       Thus, Q is independent of time. 6. Since = = . cos   B B A BA , an emf can be induced in the coil in several ways: (i) the magnitude of B can vary with time. (ii) the area (A) of the coil can change with time. (iii) the angle (  ) between B and the normal to the plane of the coil can change with time. (iv) any combination of the above can occur. 7. If v = 0,  = 0 . Thus, the induced emf in the loop will last so long as it keeps on moving. In fact, it is the energy spent in moving the loop that is changed into electric current. 8. It should be clearly noted that the Fleming's right hand rule is applied when we are given :
(i) the direction of the magnetic field, and (ii) the direction of the motion of the conductor and we are to find the direction of the induced current. 9. It may be recalled that the Fleming's left hand rule is used when we are given : (i) the direction of the magnetic field, and (ii) the direction of flow of current and we are to find the direction of force on the conductor, i.e., the direction of motion of the conductor. 10. The current so produced is known as the single phase ac and the generator is called the single phase ac generator. 11. Two phase ac generator : In this generator. there are two coils which are at right angles to each other. Each coil has its own pair of slip-rings and brushes. When this arrangement is rotated in a magnetic field; if the emf in one coil is maximum, it will be minimum in the other and vice-versa. Thus, such a generator gives two alternating currents. one of which lags behind the other by 90°. The resultant of both these alternating currents is known as two phase ac and the generator producing it is called two phase ac generator. 12. Three phase ac generator : In this generator. there are three sets of coils inclined to each other at an angle of 60°. Each set of coil has its own pair of slip rings and brushes. From such an arrangement, we shall obtain three ac currents, each lagging behind the other by 60°. The combination of such alternating currents is called three phase ac and the generator producing it is called three phase ac generator. 13. For a toroidal solenoid, i.e., a coil of radius R, l R = 2 , and as such 2 0 2 = N A L R   14. As Al V= (volume of the solenoid), 2 L n V = 0 15. Here, we have assumed that there is no mutual inductance between the two coils. In case there is any mutual inductance (say M), it can be shown that 2 1 2 1 2 2 − = + + p L L M L L L M 16. If a material of relative permeability r fills the entire space in the inner solenoid, then 0 1 2 1 2 = = r N N A N N A M l l    17. If the length of both the solenoids is the same, i.e., l, then N n l 1 1 = and N n l 2 2 = and 0 1 2 M n n Al =  , where n1 and n2 are the numbers of turns per unit length of PP1 2 and 1 2 SS respectively. 18. In case the cross-sectional areas of two solenoids are different, for calculating M, the smaller value of area of cross-section should be taken into account. This is due to the reason that the flux produced by the primary coil will be linked to the secondary to the extent of the smaller of the two cross-sectional areas. 19. If we were to assume the current in the secondary and to calculate the flux linking the primary, we would have a complicated calculation. This is due to the reason that different amount of flux will link each different turn of the primary. On the other hand, if the primary is long and the secondary is located far from the ends of the primary, a current flowing through the primary will produce uniform field through the secondary and the flux linkage win be easy to calculate as we have already done. 20. The calculation of mutual inductance is quite complicated even in it the case of considerable symmetry. 21. Let 1 1, 2 = = B I U L or = 2 L UB . Thus, the inductance of a circuit is numerically equal to twice the work done in increasing the current from zero to unity. 22. If L is in henry, I in ampere, UB is in joule. 23. The expression 1 2 2 U LI B = is analogous to 1 2 2 mv (mechanical kinetic energy of a particle of mass m moving with speed v) and shows that L is analogous tom. In other words, L is electrical inertia and opposes growth and decay of current in a circuit. 24. Though we have derived the expression for B u , for the special case of a solenoid, it is valid for any region of space in which a magnetic field exists. 25. This expression is similar to electric energy density (energy stored per unit volume in an electric field), i.e., 2 0 1 2 u E E =  .