Chapter21

Alternating Current Circuits and Electromagnetic Waves Chapter 21 AC Circuit An AC circuit consists of a combination of circuit elements and an AC generator or source The output of an AC generator is sinusoidal and varies with time according to the following equation ?v = ?Vmax sin 2?ƒt ?v: instantaneous voltage ?Vmax is the maximum voltage of the generator ƒ is the frequency at which the voltage changes, in Hz Resistor in an AC Circuit Consider a circuit consisting of an AC source and a resistor The graph shows the current through and the voltage across the resistor The current and the voltage reach their maximum values at the same time The current and the voltage are said to be in phase The direction of the current has no effect on the behavior of the resistor Resistor in an AC Circuit The rate at which electrical energy is dissipated in the circuit is given by i: instantaneous current The heating effect produced by an AC current with a maximum value of Imax is not the same as that of a DC current of the same value The maximum current occurs for a small amount of time rms 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 current Alternating voltages can also be discussed in terms of rms values The average power dissipated in resistor in an AC circuit carrying a current I is Ohm’s Law in an AC Circuit rms values will be used when discussing AC currents and voltages AC ammeters and voltmeters are designed to read rms values Many of the equations will be in the same form as in DC circuits Ohm’s Law for a resistor, R, in an AC circuit ?VR,rms = Irms R The same formula applies to the maximum values of v and i Chapter 21 Problem 4 The figure shows three lamps connected to a 120-V AC (rms) household supply voltage. Lamps 1 and 2 have 150-W bulbs; lamp 3 has a 100-W bulb. Find the rms current and the resistance of each bulb. Capacitors in an AC Circuit Consider a circuit containing a capacitor and an AC source The current starts out at a large value and charges the plates of the capacitor There is initially no resistance to hinder the flow of the current while the plates are not charged As the charge on the plates increases, the voltage across the plates increases and the current flowing in the circuit decreases Capacitors in an AC Circuit The current reverses direction The voltage across the plates decreases as the plates lose the charge they had accumulated The voltage across the capacitor lags behind the current by 90° The impeding effect of a capacitor on the current in an AC circuit is called the capacitive reactance (ƒ is in Hz, C is in F, XC is in ohms): Ohm’s Law for a capacitor in an AC circuit ?VC,rms = Irms XC Inductors in an AC Circuit Consider an AC circuit with a source and an inductor The current in the circuit is impeded by the back emf of the inductor The voltage across the inductor always leads the current by 90° The effective resistance of a coil in an AC circuit is called its inductive reactance (ƒ is in Hz, L is in H, XL is in ohms): XL = 2?ƒL Ohm’s Law for the inductor: ?VL,rms = Irms XL The RLC Series Circuit The resistor, inductor, and capacitor can be combined in a circuit The current in the circuit is the same at any time and varies sinusoidally with time The RLC Series Circuit The instantaneous voltage across the resistor is in phase with the current The instantaneous voltage across the inductor leads the current by 90° The instantaneous voltage across the capacitor lags the current by 90° Phasor Diagrams To account for the different phases of the voltage drops, vector techniques are used Represent the voltage across each element as a rotating vector, called a phasor The diagram is called a phasor diagram The voltage across the resistor is on the +x axis since it is in phase with the current Phasor Diagrams The 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° The phasors are added as vectors to account for the phase differences in the voltages ?VL and ?VC are on the same line and so the net y component is ?VL - ?VC Phasor Diagrams 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 The equations also apply to rms values Phasor Diagrams ?VR = Imax R ?VL = Imax XL ?VC = Imax XC Impedance of a Circuit The impedance, Z, can also be represented in a phasor diagram Ohm’s Law can be applied to the impedance ?Vmax = Imax Z This can be regarded as a generalized form of Ohm’s Law applied to a series AC circuit Summary of Circuit Elements, Impedance and Phase Angles Problem Solving for AC Circuits Calculate as many unknown quantities as possible (e.g., find XL and XC) Be careful with units – use F, H, ? Apply Ohm’s Law to the portion of the circuit that is of interest Determine all the unknowns asked for in the problem Chapter 21 Problem 26 A 60.0-V resistor is connected in series with a 30.0-µF capacitor and a generator having a maximum voltage of 1.20 × 102 V and operating at 60.0 Hz. Find the (a) capacitive reactance of the circuit, (b) impedance of the circuit, and (c) maximum current in the circuit. (d) Does the voltage lead or lag the current? Power in an AC Circuit No power losses are associated with pure capacitors and pure inductors in an AC circuit In a capacitor, during 1/2 of a cycle energy is stored and during the other half the energy is returned to the circuit In an inductor, the source does work against the back emf of the inductor and energy is stored in the inductor, but when the current begins to decrease in the circuit, the energy is returned to the circuit Power in an AC Circuit The average power delivered by the generator is converted to internal energy in the resistor Pav = Irms ?VR,rms ?VR, rms = ?Vrms cos ? Pav = Irms ?Vrms cos ? cos ? is called the power factor of the circuit Phase shifts can be used to maximize power outputs Chapter 21 Problem 35 An inductor and a resistor are connected in series. When connected to a 60-Hz, 90-V (rms) source, the voltage drop across the resistor is found to be 50 V (rms) and the power delivered to the circuit is 14 W. Find (a) the value of the resistance and (b) the value of the inductance. Resonance in an AC Circuit Resonance occurs at the frequency, ƒ0, where the current has its maximum value To achieve maximum current, the impedance must have a minimum value This occurs when XL = XC and Resonance in an AC Circuit Theoretically, if R = 0 the current would be infinite at resonance Real circuits always have some resistance Tuning a radio: a varying capacitor changes the resonance frequency of the tuning circuit in your radio to match the station to be received Transformers An AC transformer consists of two coils of wire wound around a core of soft iron The side connected to the input AC voltage source is called the primary and has N1 turns The other side, called the secondary, is connected to a resistor and has N2 turns The core is used to increase the magnetic flux and to provide a medium for the flux to pass from one coil to the other Transformers The rate of change of the flux is the same for both coils, so the voltages are related by When N2 > N1, the transformer is referred to as a step up transformer and when N2 < N1, the transformer is referred to as a step down transformer The power input into the primary equals the power output at the secondary Chapter 21 Problem 45 An AC power generator produces 50 A (rms) at 3 600 V. The voltage is stepped up to 100 000 V by an ideal transformer, and the energy is transmitted through a long-distance power line that has a resistance of 100 ?. What percentage of the power delivered by the generator is dissipated as heat in the power line? LC Circuit When the switch is closed, oscillations occur in the current and in the charge on the capacitor When the capacitor is fully charged, the total energy of the circuit is stored in the electric field of the capacitor At this time, the current is zero and no energy is stored in the inductor As the capacitor discharges, the energy stored in the electric field decreases At the same time, the current increases and the energy stored in the magnetic field increases LC Circuit When the capacitor is fully discharged, there is no energy stored in its electric field The current is at a maximum and all the energy is stored in the magnetic field in the inductor The process repeats in the opposite direction There is a continuous transfer of energy between the inductor and the capacitor James Clerk Maxwell 1831-1879 Maxwell’s Theory Electricity and magnetism were originally thought to be unrelated Maxwell’s theory showed a close relationship between all electric and magnetic phenomena and proved that electric and magnetic fields play symmetric roles in nature Maxwell hypothesized that a changing electric field would produce a magnetic field He calculated the speed of light – 3x108 m/s – and concluded that light and other electromagnetic waves consist of fluctuating electric and magnetic fields James Clerk Maxwell 1831-1879 Maxwell’s Theory Stationary charges produce only electric fields Charges in uniform motion (constant velocity) produce electric and magnetic fields Charges that are accelerated produce electric and magnetic fields and electromagnetic waves A changing magnetic field produces an electric field A changing electric field produces a magnetic field These fields are in phase and, at any point, they both reach their maximum value at the same time Hertz’s Experiment Hertz was the first to generate and detect electromagnetic waves in a laboratory setting An induction coil was connected to two large spheres forming a capacitor Oscillations were initiated by short voltage pulses The inductor and capacitor formed the transmitter Heinrich Rudolf Hertz 1857 – 1894 Hertz’s Experiment Several meters away from the transmitter was the receiver This consisted of a single loop of wire connected to two spheres It had its own inductance and capacitance When the resonance frequencies of the transmitter and receiver matched, energy transfer occurred between them Heinrich Rudolf Hertz 1857 – 1894 Hertz’s Results Hertz hypothesized the energy transfer was in the form of waves (now known to be electromagnetic waves) Hertz confirmed Maxwell’s theory by showing the waves existed and had all the properties of light waves (with different frequencies and wavelengths) Hertz measured the speed of the waves from the transmitter (used the waves to form an interference pattern and calculated the wavelength) The measured speed was very close to 3 x 108 m/s, the known speed of light, which provided evidence in support of Maxwell’s theory Electromagnetic Waves Produced by an Antenna When a charged particle undergoes an acceleration, it must radiate energy in the form of electromagnetic waves Electromagnetic waves are radiated by any circuit carrying alternating current An alternating voltage applied to the wires of an antenna forces the electric charge in the antenna to oscillate Electromagnetic Waves Produced by an Antenna Two rods are connected to an ac source, charges oscillate between the rods (a) As oscillations continue, the rods become less charged, the field near the charges decreases and the field produced at t = 0 moves away from the rod (b) The charges and field reverse (c) and the oscillations continue (d) Electromagnetic Waves Produced by an Antenna Because the oscillating charges in the rod produce a current, there is also a magnetic field generated As the current changes, the magnetic field spreads out from the antenna The magnetic field is perpendicular to the electric field Properties of Electromagnetic Waves Electromagnetic waves are transverse The E and B fields are perpendicular to each other and both fields are perpendicular to the direction of motion Electromagnetic waves travel at the speed of light (light is an electromagnetic wave): The ratio of the electric field to the magnetic field is equal to the speed of light Properties of Electromagnetic Waves Electromagnetic waves carry energy as they travel through space, and this energy can be transferred to objects placed in their path Energy carried by em waves is shared equally by the electric and magnetic fields Electromagnetic waves transport linear momentum as well as energy The Spectrum of EM Waves Types of electromagnetic waves are distinguished by their frequencies (wavelengths): c = ƒ ? There is no sharp division between one kind of em wave and the next – note the overlap between types of waves The Spectrum of EM Waves Radio waves are used in radio and television communication systems Microwaves (1 mm to 30 cm) are well suited for radar systems + microwave ovens are an application Infrared waves are produced by hot objects and molecules and are readily absorbed by most materials The Spectrum of EM Waves Visible light (a small range of the spectrum from 400 nm to 700 nm) – part of the spectrum detected by the human eye Ultraviolet light (400 nm to 0.6 nm): Sun is an important source of uv light, however most uv light from the sun is absorbed in the stratosphere by ozone The Spectrum of EM Waves X-rays – most common source is acceleration of high-energy electrons striking a metal target, also used as a diagnostic tool in medicine Gamma rays: emitted by radioactive nuclei, are highly penetrating and cause serious damage when absorbed by living tissue Answers to Even Numbered Problems Chapter 21: Problem 8 141 mA 235 mA Answers to Even Numbered Problems Chapter 21: Problem 14 12.6 ? 6.19 A 8.75 A Answers to Even Numbered Problems Chapter 21: Problem 40 (a) Z = R = 15 ? (b) 41 Hz (c) At resonance (d) 2.5 A Answers to Even Numbered Problems Chapter 21: Problem 64 6.0036 × 1014 Hz increases by 3.6 × 1011 Hz