Physics 122B Electricity and Magnetism

1. Physics 122B Electricity and Magnetism Lecture 23 (Knight: 33.8-33.10) Inductance and LR Circuits Martin Savage

2. Lecture 23 Announcements • Lecture HW has been posted and is due on Wednesday at 10 PM. • Midterm examination 3 is this coming Friday---questions from lecture, tutorial AND Lab. Physics 122B - Lecture 23

3. Generators The figure shows a coil with N turns rotating in a magnetic field, with the coil connected to an external circuit by slip rings that transmit current independent of rotation. The flux through the coil is: Therefore, the device produces emf and current that will vary sinusoidally, alternately positive and negative. This is called an alternating current generator, producing what we call AC voltage. Physics 122B - Lecture 23

4. Example: An AC Generator A coil with area 2.0 m2 rotates in a 0.10 T magnetic field at a frequency of 60 Hz. How many turns are needed to generate an AC emf with a peak voltage of 160 V? Physics 122B - Lecture 23

5. Transformers When a coil wound around aniron core is driven by an AC voltageV1cos wt, it produces an oscillatingmagnetic field that will induce anemf V2cos wt in a secondary coilwound on the same core. This iscalled a transformer. The input emf V1 induces acurrent I1 in the primary coil thatis proportional to 1/N1. The flux inthe iron is proportional to this, andit induces an emf V2 in the secondary coil that is proportional to N2. Therefore, V2 = V1(N2/N1). From conservation of energy, assuming no losses in the core, V1I1 = V2I2. Therefore, the currents in the primary and secondary are related by the relation I1 = I2(N2/N1). A transformer with N2>>N1 is called a step-up transformer, which boosts the secondary voltage. A transformer with N2<<N1 is called a step-down transformer, and it drops the secondary voltage. Physics 122B - Lecture 23

6. The Tesla Coil A special case of a step-up transformer is the Tesla coil. It uses no magnetic material, but has a very high N2/N1 ratio and uses high-frequency electrical current to induce very high voltages and very high frequencies in the secondary. There is a phenomenon called “the skin effect” that causes high frequency AC currents to reside mainly on the outer surfaces of conductors. Because of the skin effect, one does not feel (much) the electrical discharges from a Tesla coil. Physics 122B - Lecture 23

7. Metal Detectors Metal detectors like those used at airports can detect any metal objects, not just magnetic materials like iron. They operate by induced currents. A transmitter coil sends high frequency alternating currents that will induce current flow in conductors in its field. Because of Lenz’s Law, the induced current opposes the field from the transmitter, so that net field is reduced. A receiver coil detects the reduction in the magnetic fields from the transmitter and registers the presence of metal. Physics 122B - Lecture 23

8. (Self-) Inductance We define the inductanceL of a coil of wire producing flux Fm as: The unit of inductance is the henry: 1 henry = 1 H = 1 T m2/A = 1 Wb/A The circuit diagram symbol used to represent inductance is: Example: The inductance of a long solenoid with N turns of cross sectional area A and length l is: Physics 122B - Lecture 23

9. Example: Length of an Inductor An inductor is made by tightly winding 0.30 mm diameter wire around a 4.0 mm diameter cylinder. What length cylinder has an inductance of 10 mH? Physics 122B - Lecture 23

10. Potential Across an Inductor Physics 122B - Lecture 23

11. Potential Across an Inductor (2) Physics 122B - Lecture 23

12. Inductors and Sparks An inductor responds to an interruption in current flow by developing a very large potential (L dI/dt) across its terminals. This potential is typically dropped across the object (usually a switch) that is effecting the change in current. Therefore, breaking a closed circuit with an inductor often results in a spark across the switch contacts. Over time, this can erode and destroy the switch. Spark plugs in automobiles are driven by inductances that are electronically interrupted. Physics 122B - Lecture 23

13. Example:Large Voltage across an Inductor A 1.0 A current passes through a 10 mH inductor coil. What potential difference is induced across the coil if the current drops to zero in 5 ms? ( 1.0 A ) Physics 122B - Lecture 23

14. Question The potential at a is higher than the potential at b. Which of the following statements about the inductor current I is consistent with this observation? • I is from a to b and steady. • I is from a to b and increasing • I is from a to b and decreasing • I is from b to a and steady • I is from b to a and increasing Physics 122B - Lecture 23

15. Energy in Inductors and Magnetic Fields A magnetic field stores considerable energy. Therefore, an inductor, which operates by creating a magnetic field, stores energy. Let’s consider how much energy UL is stored in an inductor L carrying current I: A solenoid of length l , area A, and N turns has L = m0N2A/l, so: Physics 122B - Lecture 23

16. e 2 E UE = 0 2 Energy Density in the Fields This is the magnetic analog of the energy stored in an electric field Magnetic Electric Physics 122B - Lecture 23

17. The LR Circuit Physics 122B - Lecture 23

18. Example: Exponential Decayin an LR Circuit The switch shown has been in position a for a long time. It changes to position b at t=0. a. What is the current in the circuit at t = 5 ms? b. At what time has the current decayed to 1% of its original value? Physics 122B - Lecture 23

19. Plumber’s RL Analogy Valve Constriction P1 P2 Pump Flywheel The “plumber’s analogy” of an RL circuit is a pump (=battery) pumping water in a closed loop of pipe that includes a valve (=switch), a constriction (=resistor), and rotating flywheel turned by the flow. When the valve starts the flow, the flywheel begins to spin until a steady flow is achieved and the pressure difference across the flywheel (P2-P3) goes to zero. P3 Pump = Battery Valve = Switch Constriction = Resistor Flywheel = Inductor Pressure = PotentialWater Flow = Current Physics 122B - Lecture 23

20. Capacitors: Early and Late* Initially, when a switch closes there is a potential difference of 0 across an uncharged capacitor. After a long time, the capacitor reaches its maximum charge and there is no current flow through the capacitor. Therefore, at t=0 the capacitor behaves like a short circuit (R=0), and at t=∞ the capacitor behaves like at open circuit (R=∞). Example: 100 V 100 V 12.5 A 100 V 2.5 A 10 A 0 V at t=0 at t=∞ Circuit Calculate initial currents.IB = 100 V/8 W = 12.5 A Calculate final potentials. * From Lecture 16 Physics 122B - Lecture 23

21. Inductors: Early and Late Initially, when a switch closes an inductor appears to have an infinite resistance and has a maximum potential drop across it. Ultimately, the inductor reaches a steady current flow with no potential drop across it. Therefore, at t=0 the inductor behaves like at open circuit (R=∞), and at t=∞ the inductor behaves like a short circuit (R=0). This behavior is opposite that of a conductor. Example: at t=0 at t=∞ Circuit Calculate initial potentials. Calculate final currents. Physics 122B - Lecture 23

22. Question Rank the order of the time constants in these circuits. • t1=t2=t3 ; (b) t1<t2<t3 ;(c) t2<t1<t3 ; • (d) t1>t2>t3 ;(e) t2>t1>t3 ; Physics 122B - Lecture 23

23. Lecture 23 Announcements • Lecture HW has been posted and is due on Wednesday at 10 PM. • Midterm examination 3 is this coming Friday---questions from lecture, tutorial AND Lab. Physics 122B - Lecture 23