Chapter 30 Inductance

Transcription

1 Chapter 30 Inductance - Mutual Inductance - Self-Inductance and Inductors - Magnetic-Field Energy - The R- Circuit - The -C Circuit - The -R-C Series Circuit

2 . Mutual Inductance - A changing current in coil causes B and a changing magnetic flux through coil that induces emf in coil. ε N dφ B Magnetic flux through coil : N B M Φ i Mutual inductance of two coils: M ε dφ B N M M M N Φ i B M is a constant that depends on geometry of the coils M.

3 - If a magnetic material is present, M will depend on magnetic properties. If relative permeability (K m ) is not constant (M not proportional to B) Φ B not proportional to i (exception). ε ε M M Mutual inductance: emf opposes the flux change M N Φ i B NΦ i B - Only a time-varying current induces an emf. Units of inductance: Henry Weber/A V s/a J/A Ex. 30.

4 . Self Inductance and Inductors - When a current is present in a circuit, it sets up B that causes a magnetic flux that changes when the current changes emf is induced. enz s law: a self-induced emf opposes the change in current that caused it Induced emf makes fficult variations in current. NΦ i B Self-inductance dφ N B ε Self-induced emf

5 Inductors as Circuit Elements Inductors oppose variations in the current through a circuit. -In DC-circuit, helps to maintain a steady current (despite fluctuations in applied emf). In AC circuit, helps to suppress fast variations in current. - Reminder of Kirchhoff s loop rule: the sum of potential fferences around any closed loop is zero because E produced by charges stributed around circuit is conservative E c. -The magnetically induced electric field within the coils of an inductor is nonconservative (E n ). - If R 0 in inductor s coils very small E required to move charge through coils total E through coils E c + E n 0. Since E c 0 E c -E n accumulation of charge on inductor s terminals and surfaces of its conductors to produce that field. E n dl

6 - E n 0 only within the inductor. b a b a E E n c dl dl E c E + n 0 (at each point within the inductor s coil) - Self-induced emf opposes changes in current. Potential fference between terminals of an inductor: Vab Va Vb V ab is associated with conservative, electrostatic forces, despite the fact that E associated with the magnetic induction is non-conservative Kirchhoff s loop rule can be used. - If magnetic flux is concentrated in region with a magnetic material µ 0 in eqs. must be replaced by µ K m µ 0.

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8 3. Magnetic-Field Energy - Establishing a current in an inductor requires an input of energy. An inductor carrying a current has energy stored in it. Energy Stored in an Inductor Rate of transfer of energy into : P Vabi i Energy supplied to inductor during : du P i Total energy U supplied while the current increases from zero to I: U I i 0 I Energy stored in an inductor - Energy flows into an ideal (R 0) inductor when current in inductor increases. The energy is not ssipated, but stored in and released when current decreases.

9 Magnetic Energy Density r A N π µ 0 0 ) ( r I N A r U V U u π µ π 0 I r A N I U π µ -The energy in an inductor is stored in the magnetic field within the coil, just as the energy of a capacitor is stored in the electric field between its plates. Ex: toroidal solenoid (B confined to a finite region of space within its core). V (πr) A Energy per unit volume: u U/V magnetic energy density

10 µ µ 0 B u B u ( ) 0 µ π B r I N r NI B π µ 0 Magnetic energy density in vacuum Magnetic energy density in a material

11 4. The R- Circuit - An inductor in a circuit makes it fficult for rapid changes in current to occur due to induced emf. Current-Growth in an R- Circuit At t 0 Switch closed. v ab I R v bc ε i R 0 ε ir ε R i

12 initial final ε 0 ε R I ( t 0 i 0 V ab 0) ε I R ( t f / 0) i ε R ( ) R i ' i' ( ε ) R t 0 0 i ln ( ε R) ε R R ' R Current in R- circuit: t ε i e R ( R ) t

13 ε e ( R )t t 0 i 0, / ε / t i ε/r, / 0 Time constant for an R- circuit: τ R At t τ, the current has risen to (-/e) (63 % ) of its final value. Power supplied by the source: ε i i R Power ssipated by R + i Power stored in inductor

14 Current-Decay in an R- Circuit i I 0 e ( R )t t 0 I 0 Τ /R for current to decrease to /e (37 % of I 0 ). Total energy in circuit: 0 i R + i No energy supplied by a source (no battery present)

15 5. The -C Circuit - In -C circuit, the charge on the capacitor and current through inductor vary sinusoidally with time. Energy is transferred between magnetic energy in inductor (U B ) and electric energy in capacitor (U E ). As in simple harmonic motion, total energy remains constant.

16 -C Circuit t 0 C charged Q C V m C scharges through inductor. Because of induced emf in, the current does not change instantaneously. I starts at 0 until it reaches I m. During C scharge, the potential in C induced emf in. When potential in C 0 induced emf 0 maximum I m. During the scharge of C, the growing current in leads to magnetic field energy stored in C (in its electric field) becomes stored in (in magnetic field). After C fully scharged, some i persists (cannot change instantaneously), C charges with contrary polarity to initial state. As current decreases B decreases induced emf in same rection as current that slows decrease in current. At some point, B 0, i 0 and C fully charged with V m (-Q on left plate, contrary to initial state). If no energy loses, the charges in C oscillate back and forth infinitely electrical oscillation. Energy is transferred from capacitor E to inductor B.

17 Electrical Oscillations in an -C Circuit Shown is + i dq/ (rate of change of q in left plate). If C scharges dq/<0 counter clockwise i is negative. Kirchhoff s loop rule: d q x + q C 0 Acos( ω t + ϕ) -C circuit q C Analogy to eq. for harmonic oscillator: 0 d x k + x m 0 q Q cos t ( ω +ϕ) dq i ω Qsin( ωt + ϕ) ω C Angular frequency of oscillation ω π f If at t 0 Q max in C, i 0 φ 0 If at t 0, q0 φ ±π/ rad

18 Energy in an -C Circuit Analogy with harmonic oscillator (mass attached to spring): E total 0.5 k A KE + U elas 0.5 m v x k x (A oscillation amplitude) v x ± k m A x Mechanical oscillations - For -C Circuit: Q C i + q C Total energy initially stored in C energy stored in + energy stored in C (at given t). i ± C Q q Electrical oscillations

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20 6. The -R-C Series Circuit Because of R, the electromagnetic energy of system is ssipated and converted to other forms of energy (e.g. internal energy of circuit materials). [Analogous to friction in mechanical system]. - Energy loses in R i R U B in when C completely scharged < U E Q /C - Small R circuit still oscillates but with damped harmonic motion circuit underdamped. - arge R no oscillations (e out) critically damped. - Very large R circuit overdamped C charge approaches 0 slowly.

21 Analyzing an -R-C Circuit ir q C 0 (i dq/) d q R dq + + q C 0 For R < 4/C: (underdamped) q Ae ( R / ) t cos C R 4 t + ϕ A, φ are constants ω ' C R 4 underdamped -R-C series circuit

22 Inductors in series: eq + Inductors in parallel: / eq / + /