As in Example 27.2, consider a power supply with fixed emf ε and internal resistance r causing current in a load resistance R. In this problem, R is fixed and r is a variable. The efficiency is defined as the energy delivered to the load divided by the energy delivered by the emf. (a) When the internal resistance is adjusted for maximum power transfer, what is the efficiency? (b) What should be the internal resistance for maximum possible efficiency? (c) When the electric company sells energy to a customer, does it have a goal of high efficiency or of maximum power transfer? Explain. (d) When a student connects a loudspeaker to an amplifier, does she most want high efficiency or high power transfer? Explain.
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Chapter 28 Solutions
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- The figure below shows a simple RC circuit with a 1.50-uF capacitor, a 4.40-M0 resistor, a 9.00-V emf, and a switch. What are the following exactly 9.00 s after the switch is closed? (a) the charge on the capacitor pc (b) the current in the resistor (c) the rate at which the capacitor is storing energy uw (d) the rate at which the battery is delivering energy uwarrow_forwardA capacitor that is initially uncharged is connected in series with a resistor and a 700.0 V emf source with negligible internal resistance. Just after the circuit is completed, the current through the resistor is 0.900 mA and the time constant for the circuit is 10.00 s. What are (a) the resistance of the resistor and (b) the capacitance of the capacitor?arrow_forwardThe figure below shows a capacitor, with capacitance C = 7.22 uF, and a resistor, with resistance R = 4.23 Ma, connecte = 28.0 V. The circuit has a switch, which is initially open. (a) What is the circuit's time constant (in seconds)? (b) What is the maximum charge (in uC) on the capacitor after the switch is closed? (c) What is the current (in µA) through the resistor 10.0 s after the switch is closed? HAarrow_forward
- A resistor with resistance R is connected to a battery that has emf 12.0 V and internal resistance r = 0.40 Ω. For what two values of R will the power dissipated in the resistor be 80.0 W?arrow_forwardThe diagram below shows a RC circuit which consists of one capacitor and two identical light bulbs. The resistance of each light bulb is 20 O and the circuit is driven by an ideal emf device with & = 12 Volts. The switch is closed at t = 0. (a) In class, I said the following: "At t = 0, an uncharged capacitor acts as an electrical short and may be conceptually replaced by an ideal wire ". Apply this idea and compare the brightness of each bulb at t =0 right after the switch is closed. (b) In class, I said the following: "If you wait for a long time, the capacitor is fully charged and the current through the branch of the circuit that contains C is zero". Apply this idea to this circuit and determnine if the current through the emf device after a long time is also zero. Please explain. (c) If you wait for a long time, is the brightness of bulb A greater than, less than, or equal to the bright- ness of bulb B? Switch Capacitor Bulb B Bulb A ww- 98% 田 %23arrow_forwardA 6μF capacitor that is initially uncharged is connected in series with a resistor and an emf source of 40V with negligible internal resistance. The time constant is 30μs. At the instant when the resistor is dissipating electrical energy at a rate of 20W, how much energy has been stored in the capacitor?arrow_forward
- Consider an initially uncharged capacitor in an RC circuit. The resistance is 20,000 Ohms and the capacitance is 500 x10• F. If the source potential being used to charge the capacitor is 15 V, how long after charging begins will the power delivered to the resistor be 25 % of its maximum value?arrow_forwardA 0.20-uF capacitor is to be charged through a resistor so that it becomes 63 percent charged in 0.10 s. What should the resistance of the capacitor be? (b) What is the time constant if the capacitor is charged through a 20-MΩ resistor?arrow_forwardUsing your data, answer the following questions: a. To three decimal places, what fraction of the total charge does a capacitor in an RC circuit have after charging for 4.15 time constants; the fraction f such that q(4.15t) = fQmaxarrow_forward
- At time t = 0, an RC circuit consists of a 20.0-V emf device, a 66.0-0 resistor, and a 140.0-uF capacitor that is fully charged. The switch is thrown so that the capacitor begins to discharge. (a) What is the time constant z of this circuit? (b) How much charge is stored by the capacitor at t = 0.5r, 2r, and 4r? q(t = 0.5t) = q(t = 21) = q(t = 47) =arrow_forwardIn the RC circuit below, Vs-8, R =49k2, and the capacitor = 83 µF. if, initially, the switch is at position A and the capacitor is fully charged, what time, in seconds (to two decimal places), will it take V to reach 7 Volts after the switch is moved from position A to position B? Vs A B R Vxarrow_forwardIn a series RC circuit shown, the switch is closed and after a short time, the charge q on the capacitor is 5.2 x 10-5 C. What is the current in the resistor at this time?arrow_forward
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