4Resistors in an AC Circuit The direction of the current has no effect on the behavior of the resistorThe rate at which electrical energy is dissipated in the circuit is given byP = i2 R
5rms Current and Voltage The rms current is the direct current that would dissipate the same amount of energy in a resistor as is actually dissipated by the AC currentAlternating voltages can also be discussed in terms of rms values
6Ohm’s Law in an AC Circuit Ohm’s Law for a resistor, R, in an AC circuitΔVrms = Irms R
7Problem 21.2.aWhat is the resistance of a lightbulb that uses an average power of 75 W when connected to a 60 Hz power source with a peak voltage of 170V?
8QUICK QUIZ 21.1Which of the following statements might be true for a resistor connected to an AC generator? (a) Paverage = 0 and iaverage = 0 (b) Pav = 0 and iav > 0 (c) Pav > 0 and iav = 0 (d) Pav > 0 and iav > 0.
9Capacitors in an AC Circuit The current starts out at a large value and charges the plates of the capacitor, initially there is no resistanceAs the charge on the plates increases, the voltage across the plates increases and the current flowing in the circuit decreases
10Capacitors in an AC Circuit The voltage lags the current by 90o
11Capacitive Reactance and Ohm’s Law Ohm’s Law for a capacitor in an AC circuitΔVrms = Irms XC
12Inductors in an AC Circuit The current in the circuit is impeded by the emf of the inductorThe voltage across the inductor always leads the current by 90°
13Inductive Reactance, XL, and Ohm’s Law Ohm’s Law for the inductorΔVrms = Irms XLXL = 2ƒL
14The RLC Series CircuitThe current in the circuit is the same at any time and varies sinusoidally with time
15Current and Voltage Relationships in an RLC Circuit The instantaneous voltage across the resistor is in phase with the currentThe instantaneous voltage across the inductor leads the current by 90°The instantaneous voltage across the capacitor lags the current by 90°
16Phasor DiagramsRepresent the voltage across each element as a rotating vector, called a phasorIts projection on the y-axis represents the varying instantaneous voltage in the circuitThe diagram is called a phasor diagram
17Phasor Diagram for RLC Series Circuit The voltage across the resistor is on the +x axis since it is in phase with the currentThe voltage across the inductor is on the +y since it leads the current by 90°The voltage across the capacitor is on the –y axis since it lags behind the current by 90°
18Phasor DiagramThe phasors are added as vectors to account for the phase differences in the voltagesΔVL and ΔVC are on the same line and so the net y component is ΔVL - ΔVC
19ΔVmax From the Phasor Diagram The voltages are not in phase, so they cannot simply be added to get the voltage across the combination of the elements or the voltage source is the phase angle between the current and the maximum voltage
20QUICK QUIZ 21.2For the circuit of the figure below, is the voltage of the source equal to (a) the sum of the maximum voltages across the elements, (b) the sum of the instantaneous voltages across the elements, or (c) the sum of the rms voltages across the elements?
21Impedance and Ohm’s Law Ohm’s Law can be applied to the impedanceΔVmax = Imax Z
22Problems 11 and 23Problem What value of capacitor must be inserted in a 60 Hz circuit in series with a generator of 170 V maximum voltage to produce an rms current output of 0.75 A?Problem A 60.0 Q resistor, a 3.00 mF capacitor, and a H inductor are connected in series to a 90.0 V 60 Hz source. Find the voltage across the LC combination. Repeat for the RC combination.
25Power in an AC CircuitNo power losses are associated with capacitors and pure inductors in an AC circuitThe average power delivered by the generator is converted to internal energy in the resistorPav = IrmsΔVR = IrmsΔVrms cos cos is called the power factor of the circuitWe may maximize P by adjusting the power factorVVIrmsLrmsVRVC
26Resonance in an AC Circuit Resonance occurs at the frequency, ƒo, where the current has its maximum valueTo achieve maximum current, Z must be minimumThis occurs when XL = XC
27TransformerThe use of iron core results in a the same magnetic flux in bothPrimary and secondary windingsThe power input into the primary equals the power output at the secondaryI1ΔV1 = I2ΔV2
29Conceptual questions2. What is the impedance of an RLC circuit at the resonance frequency?3. When a dc voltage is applied to a transformer, the primary coil sometimes overheats and burns. Why?Why are the primary and secondary coils of a transformer wrapped on an iron core that passes through both coils?
30Hertz’s Basic LC Circuit When the switch is closed, oscillations occur in the current and in the charge on the capacitorWhen the capacitor is fully charged, the total energy of the circuit is stored in the electric field of the capacitorAt this time, the current is zero and no energy is stored in the inductor
34Electromagnetic Waves are Transverse Waves The E and B fields are perpendicular to each otherBoth fields are perpendicular to the direction of motionTherefore, em waves are transverse waves
35Properties of EM Waves Speed of ALL electromagnetic waves is The ratio of the electric field to the magnetic field is equal to the speed of lightElectromagnetic waves carry energy as they travel through space
36QUICK QUIZ 21.4In an apparatus such as that in the figure below, suppose the black disk is replaced by one with half the radius. Which of the following are different after the disk is replaced? (a) radiation pressure on the disk; (b) radiation force on the disk; (c) radiation momentum delivered to the disk in a given time interval.
37The Spectrum of EM Waves Wavelengths for visible light range from 400 nm to 700 nmThere is no sharp division between one kind of em wave and the next
38Questions 15. Does a wire connected to a battery emit an EM wave? When light (or any EM wave) travels across a given region, what is that moves?11. Suppose a creature from another planet had eyes that were sensitive to infrared radiation. Describe what he would see if he looked around the room you are in now. What would be bright and what would be dim?
39Question 21.5Receiving radio antennas can be in the form of conducting lines or loops. What should the orientation of each of these antennas be, relative to a broadcasting antenna that is perpendicular to the Earth?
40ProblemAssume that the solar radiation incident on the Earth is 1340 W/m2. Calculate the power radiated by the Sun. The average Sun-Earth separation is m.
41MCAD A time varying magnetic field A transformer is used to change 1. Produces an electric field2. Provides power to rotate an ac generator3. Can induce currents to flow in a conductive loop4. Both 1 and 3A transformer is used to changeVoltagePowerCurrentVoltage and current
42Which of the following most accurately describes light? An electric and magnetic wave parallel to each other and perpendicular to the direction of propagationAn electric and magnetic wave parallel to each other and parallel to the direction of propagationAn electric and magnetic wave perpendicular to each other and perpendicular to the direction of propagationAn electric and magnetic wave perpendicular to each other and parallel to the direction of propagation