Tiêu đề 22.Alternating Current.pdf ✅

NEET-2022 Ultimate Crash Course PHYSICS ALTERNATING CURRENT

POINTS TO REMEMBER 1. The average value of ac current (or voltage) over the other half cycle will be the same as in the first half cycle but in the opposite direction (i.e., of opposite sign). As such, the average value over one cycle is zero. 2. The hot wire meters (used to measure ac current and voltage) which depend on the expansion due to the heat generated in a fine wire because of the current flow in them, always measure the tarns values of ac current and voltage. 3. Form factor. It gives an indication of the wave shape of an ac current or voltage and for a sinusoidal waveform, it is equal to 1.11 2 2 = = rms rms av av I I    For a rectangular waveform, its value is 1. 4. The plots (not to scale) of R, XL and XC are shown in figure. 5. A simplified version of phasor diagram for a series RLC circuit is shown in Fig. 10.9 (a). 6. (a) X X X X L C L C  − ,( ) is positive. Obviously, voltage leads the current by an angle  as shown in figure. (b) If X X X X L C L C  − ,( ) is negative. In such a case, tan  and consequently  is negative. Obviously, the voltage lags behind the current by an angle  as shown in figure. (c) If X X L C = , tan 0  = or 0  = 0 . In this case the voltage and the current are in the same phase and resonance is said to occur at this stage. 7. If we remove the common factor /0 from each of the three phasors   ( − = − = = ) 0 0 0 0 ( ),   V V I X X V I R and I Z L C L C R  in the figure, we can construct a phasor diagram (a triangle), known as the impedance triangle as shown in the figure. 8. In the given figure the plots of R, , X X L C and Z versus  . Since R is constant, Z will be minimum when X X L C = .
9. The inverse of impedance is called the admittance and is usually denoted by Y, i.e., Y = 1/Z. 10. For any element in a circuit, the product of admittance and the voltage across it gives the current through that element. 11. P av is also called the true power. 12. 0 I cos is called the active or wall-full component and 0 I sin is called the wattles, idle or reactive component of the current 13. The situation when the current in a circuit is wattles is similar to that of a frictionless pendulum where the total work done by gravity upon the pendulum in a cycle is zero. 14. = cos P I av rms rms   From the given figure, 0 0 cos , = = V I R R   i.e., 0 0 cos = I R   Thus, ( )( ) ( ) 0 0 0 0 0 2       = =             av rms rms rms I R I R P I I     ( ) 0 2 2   = =     rms rms I I R I R In other words, the average power delivered by the generator is dissipated as heat in the resistor just as in the case of a dc circuit. There is no power loss in an ideal inductor or a capacitor. 15. Since (   2 1 − =  ) 2 , from eqn. ( 2 1 − =) R L   2 /  =  R L or  =  R L / 2 16. Q-factor may also be defined as the ratio of pd across the inductance (or the capacitance) at resonance and the applied voltage. 17. Q-factor is also called figure of merit or simply, Q. 18. A high Q-factor circuit responds to a very narrow range of frequencies whereas a low Q-factor circuit responds to much broader range of frequencies. 19. Typical values of Q in electronic circuits range from 10 to 100. For high frequency circuits, Q may be as large as 200. 20. The maximum energy stored in the capacitor (when I = 0) is 2 0 2 Q C and is equal to the maximum energy stored in the inductor (when Q = 0), i.e., 2 0 1 2 LI . Thus, 21. In fact, even when the energy losses due to wire resistance are neglected, energy is also lost due to e.m. waves radiated by the tank circuit (i.e., LC circuit) 22. In the given figure shows graphs of UE versus time and UB versus time for a resistanceless LC circuit. The sum of the two energies ( UE and UB ) is a constant and is equal to the total energy (U) stored in the circuit. 23. Nikola Tesla (1856-1943) was a key figure in the development of alternating current electricity, high- voltage transformers, and the transport of electrical power using ac transmission lines. Tesla's