Transcript Slide 1
Chapter 26 AC Electricity © 2010 Pearson Education, Inc. PowerPoint® Lectures for College Physics: A Strategic Approach, Second Edition 26 AC Electricity © 2010 Pearson Education, Inc. Slide 26-2 © 2010 Pearson Education, Inc. Slide 26-3 © 2010 Pearson Education, Inc. Slide 26-4 © 2010 Pearson Education, Inc. Slide 26-5 An AC Voltage Source The circuit symbol for an AC voltage source © 2010 Pearson Education, Inc. Slide 26-10 Resistor Circuits © 2010 Pearson Education, Inc. Slide 26-11 Resistor Voltages and Currents Lowercase symbols represent instantaneous values of current or voltage. They change sinusoidally with time. Uppercase symbols represent peak values of current or voltage. They are fixed quantities. vR VRcos(2πft ) VR VRcos(2πft ) iR IRcos(2πft ) R R © 2010 Pearson Education, Inc. Slide 26-12 AC Power in Resistors 1 2 R= 2 R P © 2010 Pearson Education, Inc. I R Slide 26-13 Root-Mean-Square Current and Voltage If we define IR Irms 2 and VR Vrms 2 then we can write IR 2 PR R ( I ) rms R 2 2 and The expressions for AC power are identical to those used for DC currents if rms currents and voltages are used. © 2010 Pearson Education, Inc. Slide 26-14 Transformers © 2010 Pearson Education, Inc. Slide 26-15 Checking Understanding If the primary coil of wire on a transformer is kept the same and the number of turns of wire on the secondary is increased, how will this affect the voltage observed at the secondary? A. The voltage will increase. B. The voltage will stay the same. C. The voltage will decrease. © 2010 Pearson Education, Inc. Slide 26-16 Answer If the primary coil of wire on a transformer is kept the same and the number of turns of wire on the secondary is increased, how will this affect the voltage observed at the secondary? A. The voltage will increase. B. The voltage will stay the same. C. The voltage will decrease. © 2010 Pearson Education, Inc. Slide 26-17 Example Problem A 120 V (rms) AC power supply is connected to a motor, which is rated at 100 W. A. What is the rms current in the circuit? Now suppose that the wires used to connect the motor to the power supply have a resistance of 7.0 . B. Assume that the rms current stays the same. What is the voltage drop across the resistance of the wires? What is the voltage at the motor now? © 2010 Pearson Education, Inc. Slide 26-18 Example Problem Clearly this is untenable. Now suppose the power supply is 1200 V, and the motor is rated at 100 W at this higher voltage. C. What is the current in the circuit, assuming no resistance in the wires? D. If the wires have a resistance of 7.0 , what is the voltage drop across the wires? The voltage at the motor? Now suppose the power supply is increased to 12,000 V, and the motor is rated at 100 W at this still higher voltage. E. F. What is the current in the circuit, assuming no resistance in the wires? If the wires have a resistance of 7.0 , what is the voltage drop across the wires? The voltage at the motor? © 2010 Pearson Education, Inc. Slide 26-19 Transmission of Electrical Power © 2010 Pearson Education, Inc. Slide 26-20 Two-Phase Power to Your Home © 2010 Pearson Education, Inc. Slide 26-21 Example Problem The following devices are plugged into outlets on the same 120 V circuit in a house. This circuit is protected with a 15-A circuit breaker. Device Power Computer 250 W Heater 900 W Lamp 100 W Stereo 120 W Is there too much current in the circuit—that is, does the circuit breaker blow? © 2010 Pearson Education, Inc. Slide 26-22 Physiological Effects and Electrical Safety © 2010 Pearson Education, Inc. Slide 26-23 Electrical Safety V V 480 V I 48 mA Req Rboots 10 MV © 2010 Pearson Education, Inc. Slide 26-24 Example Problem Suppose a person is standing, barefoot, on wet ground. The resistance of the skin on each foot is 300 . He now grabs an improperly wired device with his right hand—establishing a connection to the hot wire of the electric supply. His hand is a bit sweaty, so the resistance of the skin is only 1.2 . A. B. What current will flow through his body? Will this be large enough for him to feel? Large enough to be dangerous? © 2010 Pearson Education, Inc. Slide 26-25 Capacitor Circuits vC VC cos(2πft ) © 2010 Pearson Education, Inc. Slide 26-26 Voltage, Charge, and Current for a Capacitor in an AC Circuit where 1 XC 2πfC © 2010 Pearson Education, Inc. Slide 26-27 Capacitive Reactance © 2010 Pearson Education, Inc. Slide 26-28 Inductance and Inductors iL vL L t Units of inductance: 1 henry 1 henry 1 H 1 H V s/A nry 1 H 1 V1s/A 1 1s s V s/A 1 s The circuit symbol for an inductor © 2010 Pearson Education, Inc. Slide 26-29 Inductor Circuits © 2010 Pearson Education, Inc. Slide 26-30 Inductor Currents and Voltages © 2010 Pearson Education, Inc. Slide 26-31 Inductive Reactance VL L(2πfIL) (2πfL) IL or where XL 2πfL © 2010 Pearson Education, Inc. Slide 26-32 Checking Understanding An isolation transformer—basically a 1:1 transformer—is connected into a circuit with identical light bulbs on both the primary and secondary circuits. What will happen to the brightness of the bulb on the primary if you remove the bulb on the secondary? A. B. C. D. E. It will become much brighter. It will become slightly brighter. It will remain the same brightness. It will become slightly dimmer. It will become much dimmer. © 2010 Pearson Education, Inc. Slide 26-33 Answer An isolation transformer— basically a 1:1 transformer— is connected into a circuit with identical light bulbs on both the primary and secondary circuits. What will happen to the brightness of the bulb on the primary if you remove the bulb on the secondary? A. B. C. D. E. It will become much brighter. It will become slightly brighter. It will remain the same brightness. It will become slightly dimmer. It will become much dimmer. © 2010 Pearson Education, Inc. Slide 26-34 LC Circuits © 2010 Pearson Education, Inc. Slide 26-35 Energy of an LC Circuit and a Block on a Spring Block on spring: 1 f 2π k m LC circuit: © 2010 Pearson Education, Inc. Slide 26-36 The RLC Circuit © 2010 Pearson Education, Inc. Slide 26-37 The Driven RLC Circuit 1 f0 2π LC © 2010 Pearson Education, Inc. e0 Imax R Slide 26-38 The Driven RLC Circuit for Different Values of R © 2010 Pearson Education, Inc. Slide 26-39 Additional Question Suppose that an ideal transformer has 400 turns in its primary coil and 100 turns in its secondary coil. The primary coil is connected to a 120-V (rms) electric outlet and carries an rms current of 10 mA. What are the rms values of the voltage and current for the secondary? A. B. C. D. 480 V, 40 mA 480 V, 2.5 mA 30 V, 40 mA 30 V, 2.5 mA © 2010 Pearson Education, Inc. Slide 26-40 Answer Suppose that an ideal transformer has 400 turns in its primary coil and 100 turns in its secondary coil. The primary coil is connected to a 120-V (rms) electric outlet and carries an rms current of 10 mA. What are the rms values of the voltage and current for the secondary? A. B. C. D. 480 V, 40 mA 480 V, 2.5 mA 30 V, 40 mA 30 V, 2.5 mA © 2010 Pearson Education, Inc. Slide 26-41