Rechargeable batteries The rechargeable batteries for a laptopcomputer need a mu

Question

Rechargeable batteries The rechargeable batteries for a laptopcomputer need a much smaller voltage than what a wall socketprovides. Therefore, a transformer is plugged into the wall socketand produces the necessary voltage for charging the batteries. (a)Is the transformer a step-up or a step-down transformer? step-downtransformer step-up transformer (b) Is the current that goesthrough the batteries greater than, equal to, or smaller than thecurrent coming from the wall socket? greater than less than equalto (c) If the transformer has a negligible resistance, is theelectrical power delivered to the batteries greater than, equal to,or less than the power coming from the wall socket? equal togreater than less than The batteries of a laptop computer are ratedat 9.0 V, and a current of 175 mA is used to charge them. The wallsocket provides a voltage of 120 V. (d) What is the algebraicexpression for the turns ratio Ns/Np of the transformer, where Nsand Np are, respectively, the number of turns in the secondary andprimary. Express your answer in terms of the voltage Vs across thesecondary and the voltage Vp across the primary. (Answer using V_pfor Vp and V_s for Vs.) Ns/Np = 4 (e) Determine the turns ratioNs/Np of the transformer. Ns/Np = 5 (f) What is the algebraicexpression for the current Ip in the primary? Express your answerin terms of the current Is in the secondary, the number Ns of turnsin the secondary, and the number Np of turns in the primary.(Answer using N_p for Np, N_s for Ns, and I_s for Is.) Ip = 6 (g)What is the current Ip coming from the wall socket? Number Unit Ip= 7 8 ---Select--- C W A V Ohm (h) What is the algebraic expressionfor the average power p delivered by the wall socket? Express youranswer in terms of the current Ip in the primary and the voltage Vpacross the primary. (Answer using V_p for Vp and I_p for Ip.) p = 9(i) Find the average power p delivered by the wall socket. NumberUnit p = 10 11 ---Select--- Ohm A W C V (j) Find the average powers sent to the batteries. Number Unit s = 12 13 ---Select--- A V WOhm C

Explanation / Answer

     (a)    since the secondary voltage (the voltage tocharge the batteries) is less than the primary voltage (the voltageat the wall
   socket), the transformer is a step-downtransformer (b)    in a step-down transformer, the voltage acrossthe secondary coil is less than the voltage across the primarycoil
   however, the current in the secondary coil isgreater than the current in the primary coil, thus, the currentthat goes through    the batteries is greater than the current fromthe wall socket (c)
   if the transformer has negligible resistance, thepower delivered to the batteries is equal to the power coming fromthe
   wall socket    the current from the wall socket is given by theequation    Is / Ip =Np / Ns    Ip = (Ns / Np)Is        = ........... A    the power delivered by the wall socket isthe product of the primary current and voltage given by    Pp = IpVp         = ......... W    the power delivered to the batteries is the sameas that coming from the wall socket so we can write
   Ps = Pp
        = .........W

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