# Active Figure 32.3 (SLIDESHOW MODE ONLY)

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1 RL Circuit, Analysis An RL circuit contains an inductor and a resistor When the switch is closed (at time t = 0), the current begins to increase At the same time, a back emf is induced in the inductor that opposes the original increasing current

2 Active Figure 32.3 (SLIDESHOW MODE ONLY)

3 RL Circuit, Analysis, cont. Applying Kirchhoff s loop rule to the previous circuit gives d I ε I R L = 0 dt Looking at the current, we find I ε = R ( Rt L 1 e )

4 RL Circuit, Analysis, Final The inductor affects the current exponentially The current does not instantly increase to its final equilibrium value If there is no inductor, the exponential term goes to zero and the current would instantaneously reach its maximum value as expected

5 RL Circuit, Time Constant The expression for the current can also be expressed in terms of the time constant, τ, of the circuit I ε = R ( t τ 1 e ) where τ = L / R Physically, τ is the time required for the current to reach 63.2% of its maximum value

6 RL Circuit, Current-Time Graph, (1) The equilibrium value of the current is ε /R and is reached as t approaches infinity The current initially increases very rapidly The current then gradually approaches the equilibrium value

7 RL Circuit, Current-Time Graph, (2) The time rate of change of the current is a maximum at t = 0 It falls off exponentially as t approaches infinity In general, d I dt = ε e L t τ

8 Active Figure 32.6 (SLIDESHOW MODE ONLY)

9 Energy in a Magnetic Field In a circuit with an inductor, the battery must supply more energy than in a circuit without an inductor Part of the energy supplied by the battery appears as internal energy in the resistor The remaining energy is stored in the magnetic field of the inductor

10 Energy in a Magnetic Field, cont. Looking at this energy (in terms of rate) Iε = I R+ LI d dt 2 I Iε is the rate at which energy is being supplied by the battery I 2 R is the rate at which the energy is being delivered to the resistor Therefore, LI di/dt must be the rate at which the energy is being stored in the magnetic field

11 Energy in a Magnetic Field, final Let U denote the energy stored in the inductor at any time The rate at which the energy is stored is du dt = L I d I dt To find the total energy, integrate and 0 I I U = L d I

12 Energy Density of a Magnetic Field Given U = ½ L I B B U = μonal = A 2 μon 2μo Since Al is the volume of the solenoid, the magnetic energy density, u B is u B U = = Al B 2μ 2 o This applies to any region in which a magnetic field exists (not just the solenoid) l

13 Example: The Coaxial Cable Calculate L for the cable The total flux is b μo I μo Il b Φ B = BdA= l dr= ln a 2πr 2π a Therefore, L is ΦB μol b L = = ln I 2π a The total energy is U 1 μ l I 2 4π b a 2 2 o = LI = ln

14 LC Circuits A capacitor is connected to an inductor in an LC circuit Assume the capacitor is initially charged and then the switch is closed Assume no resistance and no energy losses to radiation

15 Oscillations in an LC Circuit Under the previous conditions, the current in the circuit and the charge on the capacitor oscillate between maximum positive and negative values With zero resistance, no energy is transformed into internal energy Ideally, the oscillations in the circuit persist indefinitely The idealizations are no resistance and no radiation

16 Oscillations in an LC Circuit, 2 The capacitor is fully charged The energy U in the circuit is stored in the electric field of the capacitor The energy is equal to Q 2 max / 2C The current in the circuit is zero No energy is stored in the inductor The switch is closed

17 Oscillations in an LC Circuit, 3 The current is equal to the rate at which the charge changes on the capacitor As the capacitor discharges, the energy stored in the electric field decreases Since there is now a current, some energy is stored in the magnetic field of the inductor Energy is transferred from the electric field to the magnetic field

18 Oscillations in an LC Circuit, 4 The capacitor becomes fully discharged It stores no energy All of the energy is stored in the magnetic field of the inductor The current reaches its maximum value The current now decreases in magnitude, recharging the capacitor with its plates having opposite their initial polarity

19 Oscillations in an LC Circuit, final Eventually the capacitor becomes fully charged and the cycle repeats The energy continues to oscillate between the inductor and the capacitor The total energy stored in the LC circuit remains constant in time and equals 2 Q 1 U = U 2 2 I C + UL = + L C 2

20 LC Circuit Analogy to Spring- Mass System, 1 The potential energy ½kx 2 stored in the spring is analogous to the electric potential energy ½C(ΔV max ) 2 stored in the capacitor All the energy is stored in the capacitor at t = 0 This is analogous to the spring stretched to its amplitude

21 LC Circuit Analogy to Spring- Mass System, 2 The kinetic energy (½ mv 2 ) of the spring is analogous to the magnetic energy (½ L I 2 ) stored in the inductor At t = ¼ T, all the energy is stored as magnetic energy in the inductor The maximum current occurs in the circuit This is analogous to the mass at equilibrium

22 LC Circuit Analogy to Spring- Mass System, 3 At t = ½ T, the energy in the circuit is completely stored in the capacitor The polarity of the capacitor is reversed This is analogous to the spring stretched to -A

23 LC Circuit Analogy to Spring- Mass System, 4 At t = ¾ T, the energy is again stored in the magnetic field of the inductor This is analogous to the mass again reaching the equilibrium position

24 Time Functions of an LC Circuit In an LC circuit, charge can be expressed as a function of time Q = Q max cos (ωt + φ) This is for an ideal LC circuit The angular frequency, ω, of the circuit depends on the inductance and the capacitance It is the natural frequency of oscillation of the circuit ω = 1 LC

25 Time Functions of an LC Circuit, 2 The current can be expressed as a function of time I = dq = ωqmax sin( ωt + φ ) dt The total energy can be expressed as a function of time 2 Qmax U = UC + UL = cos ωt + LImax sin ωt 2c 2

26 Charge and Current in an LC Circuit The charge on the capacitor oscillates between Q max and -Q max The current in the inductor oscillates between I max and -I max Q and I are 90 o out of phase with each other So when Q is a maximum, I is zero, etc.

27 Energy in an LC Circuit Graphs The energy continually oscillates between the energy stored in the electric and magnetic fields When the total energy is stored in one field, the energy stored in the other field is zero

28 Notes About Real LC Circuits In actual circuits, there is always some resistance Therefore, there is some energy transformed to internal energy Radiation is also inevitable in this type of circuit The total energy in the circuit continuously decreases as a result of these processes

29 The RLC Circuit A circuit containing a resistor, an inductor and a capacitor is called an RLC Circuit Assume the resistor represents the total resistance of the circuit

30 RLC Circuit, Analysis The total energy is not constant, since there is a transformation to internal energy in the resistor at the rate of du/dt = -I 2 R Radiation losses are still ignored The circuit s operation can be expressed as L dq 2 + R dq + Q = 0 2 dt dt C

31 RLC Circuit Compared to Damped Oscillators The RLC circuit is analogous to a damped harmonic oscillator When R = 0 The circuit reduces to an LC circuit and is equivalent to no damping in a mechanical oscillator

32 RLC Circuit Compared to Damped Oscillators, cont. When R is small: The RLC circuit is analogous to light damping in a mechanical oscillator Q = Q max e -Rt/2L cos ω d t ω d is the angular frequency of oscillation for the circuit and ω d 1 R = LC 2L 1 2 2

33 RLC Circuit Compared to Damped Oscillators, final When R is very large, the oscillations damp out very rapidly There is a critical value of R above which no oscillations occur R C = 4 L/ C If R = R C, the circuit is said to be critically damped When R > R C, the circuit is said to be overdamped

34 Damped RLC Circuit, Graph The maximum value of Q decreases after each oscillation R < R C This is analogous to the amplitude of a damped springmass system

35 Overdamped RLC Circuit, Graph The oscillations damp out very rapidly Values of R are greater than R C

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37 Mutual Inductance The magnetic flux through the area enclosed by a circuit often varies with time because of time-varying currents in nearby circuits This process is known as mutual induction because it depends on the interaction of two circuits

38 Mutual Inductance, 2 The current in coil 1 sets up a magnetic field Some of the magnetic field lines pass through coil 2 Coil 1 has a current I 1 and N 1 turns Coil 2 has N 2 turns

39 Mutual Inductance, 3 The mutual inductance M 12 of coil 2 with respect to coil 1 is N2Φ12 M 12 I 1 Mutual inductance depends on the geometry of both circuits and on their orientation with respect to each other

40 Induced emf in Mutual Inductance If current I 1 varies with time, the emf induced by coil 1 in coil 2 is dφ d I ε = N = M dt dt If the current is in coil 2, there is a mutual inductance M 21 If current 2 varies with time, the emf induced by coil 2 in coil 1 is d I 2 ε1= M21 dt

41 Mutual Inductance, Final In mutual induction, the emf induced in one coil is always proportional to the rate at which the current in the other coil is changing The mutual inductance in one coil is equal to the mutual inductance in the other coil M 12 = M 21 = M The induced emf s can be expressed as d I ε = M and ε = M dt d I dt

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52 Quick Quiz 32.2a The circuit in the figure below consists of a resistor, an inductor, and an ideal battery with no internal resistance. At the instant just after the switch is closed, across which circuit element is the voltage equal to the emf of the battery? (a) the resistor (b) the inductor (c) both the inductor and resistor

53 Quick Quiz 32.2b After a very long time, across which circuit element is the voltage equal to the emf of the battery? (a) the resistor (b) the inductor (c) both the inductor and resistor.

54 Quick Quiz 32.2b Answer: (a). After a long time, the current has reached its final value, and the inductor has no further effect on the circuit.

55 Quick Quiz 32.3 The circuit in the figure below includes a power source that provides a sinusoidal voltage. Thus, the magnetic field in the inductor is constantly changing. The inductor is a simple air-core solenoid. The switch in the circuit is closed and the lightbulb glows steadily. An iron rod is inserted into the interior of the solenoid, which increases the magnitude of the magnetic field in the solenoid. As this happens, the brightness of the lightbulb: (a) increases (b) decreases (c) is unaffected

56 Quick Quiz 32.3 Answer: (b). When the iron rod is inserted into the solenoid, the inductance of the coil increases. As a result, more potential difference appears across the coil than before. Consequently, less potential difference appears across the bulb, so the bulb is dimmer.

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