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Lecture 18 RLC circuits Transformer Maxwell Electromagnetic Waves

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Self-inductance Self-inductance occurs when the changing flux through a circuit arises from the circuit itself As the current increases, the magnetic flux through a loop due to this current also increases The increasing flux induces an emf that opposes the change in magnetic flux As the magnitude of the current increases, the rate of increase lessens and the induced emf decreases This opposing emf results in a gradual increase of the current

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RL Circuit When the current reaches its maximum, the rate of change and the back emf are zero The time constant, , for an RL circuit is the time required for the current in the circuit to reach 63.2% of its final value

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Energy Stored in a Magnetic Field The emf induced by an inductor prevents a battery from establishing an instantaneous current in a circuit The battery has to do work to produce a current This work can be thought of as energy stored by the inductor in its magnetic field PE L = ½ L I 2 Like ½ C V 2

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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 Δv = ΔV max sin 2ƒt Δv is the instantaneous voltage ΔV max is the maximum voltage of the generator ƒ is the frequency at which the voltage changes, in Hz

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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

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Power Revisited The average power dissipated in resistor in an AC circuit carrying a current I is

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Table 21-1, p.695

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Fig. 21-1, p.694

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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 ΔV R,rms = I rms R Also applies to the maximum values of v and i

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Capacitive Reactance and Ohm’s Law The impeding effect of a capacitor on the current in an AC circuit is called the capacitive reactance and is given by When ƒ is in Hz and C is in F, X C will be in ohms Ohm’s Law for a capacitor in an AC circuit ΔV C,rms = I rms X C

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Inductive Reactance and Ohm’s Law The effective resistance of a coil in an AC circuit is called its inductive reactance and is given by X L = 2ƒL When ƒ is in Hz and L is in H, X L will be in ohms Ohm’s Law for the inductor ΔV L,rms = I rms X L

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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

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Fig. 21-8, p.699

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Phasor Diagram for RLC Series Circuit The voltage across the resistor is on the +x axis since it is in phase with the current 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°

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Phasor Diagram, cont The phasors are added as vectors to account for the phase differences in the voltages ΔV L and ΔV C are on the same line and so the net y component is ΔV L - ΔV C

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ΔV max 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 The equations also apply to rms values

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Impedance of a Circuit The impedance, Z, can also be represented in a phasor diagram

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Fig. 21-11c, p.700

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Impedance and Ohm’s Law Ohm’s Law can be applied to the impedance ΔV max = I max Z This can be regarded as a generalized form of Ohm’s Law applied to a series AC circuit

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Summary of Circuit Elements, Impedance and Phase Angles

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Nikola Tesla 1865 – 1943 Inventor Key figure in development of AC electricity High-voltage transformers Transport of electrical power via AC transmission lines Beat Edison’s idea of DC transmission lines

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Problem Solving for AC Circuits Calculate as many unknown quantities as possible For example, find X L and X C Be careful of units – use F, H, Ω Apply Ohm’s Law to the portion of the circuit that is of interest Determine all the unknowns asked for in the problem

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Power in an AC Circuit No power losses are associated with pure capacitors and pure inductors in an AC circuit In a capacitor, during one-half 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

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Power in an AC Circuit, cont The average power delivered by the generator is converted to internal energy in the resistor P av = I rms ΔV R = I rms ΔV rms cos cos is called the power factor of the circuit Phase shifts can be used to maximize power outputs

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Resonance in an AC Circuit Resonance occurs at the frequency, ƒ o, where the current has its maximum value Demo1, Demo2Demo1 Demo2 To achieve maximum current, the impedance must have a minimum value This occurs when X L = X C Then,

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Fig. 21-14, p.704

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Resonance, cont 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 Metal Detector The portal is an inductor, and the frequency is set to a condition with no metal present When metal is present, it changes the effective inductance, which changes the current The change in current is detected and an alarm sounds

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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 N 1 turns

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Transformers, 2 The other side, called the secondary, is connected to a resistor and has N 2 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 The rate of change of the flux is the same for both coils

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Fig. 21-15, p.705

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Transformers, 3 The voltages are related by When N 2 > N 1, the transformer is referred to as a step up transformer When N 2 < N 1, the transformer is referred to as a step down transformer V=-N ΔØ/Δt

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Transformer, final The power input into the primary equals the power output at the secondary I 1 ΔV 1 = I 2 ΔV 2 You don’t get something for nothing This assumes an ideal transformer In real transformers, power efficiencies typically range from 90% to 99%

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Electrical Power Transmission When transmitting electric power over long distances, it is most economical to use high voltage and low current Minimizes I 2 R power losses In practice, voltage is stepped up to about 230 000 V at the generating station and stepped down to 20 000 V at the distribution station and finally to 120 V at the customer’s utility pole

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p.707

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James Clerk Maxwell 1831 – 1879 Electricity and magnetism were originally thought to be unrelated in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena

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More of Maxwell’s Contributions Electromagnetic theory of light Kinetic theory of gases Nature of Saturn’s rings Color vision Electromagnetic field interpretation Led to Maxwell’s Equations

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Maxwell’s Starting Points Electric field lines originate on positive charges and terminate on negative charges Magnetic field lines always form closed loops – they do not begin or end anywhere A varying magnetic field induces an emf and hence an electric field (Faraday’s Law) Magnetic fields are generated by moving charges or currents (Ampère’s Law)

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Maxwell’s Predictions Maxwell used these starting points and a corresponding mathematical framework to prove that electric and magnetic fields play symmetric roles in nature He hypothesized that a changing electric field would produce a magnetic field Maxwell calculated the speed of light to be 3x10 8 m/s He concluded that visible light and all other electromagnetic waves consist of fluctuating electric and magnetic fields, with each varying field inducing the other

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Hertz’s Confirmation of Maxwell’s Predictions 1857 – 1894 First to generate and detect electromagnetic waves in a laboratory setting Showed radio waves could be reflected, refracted and diffracted The unit Hz is named for him

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Hertz’s Basic 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

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LC Circuit, cont 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 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

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Hertz’s Experimental Apparatus An induction coil is connected to two large spheres forming a capacitor Oscillations are initiated by short voltage pulses The inductor and capacitor form the transmitter

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Hertz’s Experiment Several meters away from the transmitter is 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

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Hertz’s Conclusions Hertz hypothesized the energy transfer was in the form of waves These are now known to be electromagnetic waves Hertz confirmed Maxwell’s theory by showing the waves existed and had all the properties of light waves They had different frequencies and wavelengths

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Hertz’s Measure of the Speed of the Waves Hertz measured the speed of the waves from the transmitter He used the waves to form an interference pattern and calculated the wavelength From v = f λ, v was found v was very close to 3 x 10 8 m/s, the known speed of light This provided evidence in support of Maxwell’s theory

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