Alternating Currents ( A.C.) - Physics

Electric Generator It is a device that uses induction to convert mechanical energy to electrical energy. In ac generator, while a rotating loop is in a magnetic field, the induced emf is zero when the loop is perpendicular to the magnetic field, and is at a maximum when the loop is parallel to the field.  A transformer is a static piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the  voltage in a circuit but with a corresponding decrease or increase in current.       Principle :                  In brief, a transformer is a device that                     transfers electric power from one circiut to another.                       it does so without a change of frequency.                       it accomplishes this by electromagnetic induction and                       where the two circuit are in mutual inductive influence of each                          other.            From the above equations (i) and (ii), we get                                                             =  K  Voltage transformation ratio (i)   If    K>1, then the transformer is called step-up transformer. (ii)   If    K<1, then the transformer is called step-down transformer.  Iron loss is the electrical energy loss which is lost in the condition of alternate-current magnetization. The hysteresys loss which occurs when changing the magnetic domain direction and eddy current loss which occurs due to eddy currents constitute iron loss. Compared with conventional thick electrical steels, ultra-thin-gauge electrical steel strips have very low core losses because of very small eddy current occurrence. The advantages of ultra-thin-gauge electrical steel strips is apparent at high frequencies, and they assist in energy saving and down-sizing high-frequency reactors, transformers, and motors. An a.c. voltage E = E0 Sin ωt is applied across an inductor L. Obtain an expression for current I. An ac generator has a coil of N turns each of area A, rotating with angular speed ω in a uniform magnetic field B. (i) What is the maximum emf between its slip rings? (ii) What is the flux associated with the coil when the emf across it is zero? A town situated 20 km away from a power plant generating power at 440V, requires 600 kW of electric power at 220 V. The resistance of the two wire line carrying power is 0.4 Ωper km. The town gets power from the line through a 3000 – 220 V step down transformer at a substation in the town. (i) Find the line power losses in the form of heat. (ii) How much power must the plant supply, assuming there is negligible power loss due to leakage? Calculate the current drawn by the primary of a transformer, which steps down 200 V to 20 V to operate a device of resistance 20 Ω. Assume the efficiency of the transformer to be 80%. An 8.2mH inductor is connected to an ac generator of 10V and 620Hz. Determine the peak value of the current supplied by the generator. E max = NBA flux = N= NBA.1= NBA Current in secondary Is= =1A Efficiency = = 0.8 = Primary current =A = 0.125A REASONING The rms current can be calculated from Equation Vrms =Irms XL , provided that the inductive reactance is obtained first. Then the peak value of the current supplied by the generator can be calculated from the rms current by using Equation Irms=, Io= SOLUTION At the frequency of f=620 Hz, XL=2πfL=32 Ω Irms= =0.31A we find that the peak value Io==0.44A Alternating Current By: Mohammad Abdul Mumeed. +966507433412 7 © Md. Abdul Mumeed, Senior Secondary Teacher in PHYSICS at I. I. School, Riyadh, K.S.A.