1. 5/23/16 1Oregon State University PH 213, Class #25

2. The magnet is falling downward away from the stationary loop. What is the direction of the induced current (if any) in the loop, as viewed from above? 1.No induced current 2.Yes, clockwise 3.Yes, counterclockwise 4.No, clockwise 5/23/16 2Oregon State University PH 213, Class #25

3. 5/23/16 Oregon State University PH 213, Class #253 A rectangular loop of wire was falling freely before it passes through a uniform magnetic field directed into the page. With the loop in the position shown… What is the direction of the induced current, I, in the loop? And what is the direction of the net magnetic force, F mag, on the loop? A. I is clockwise; F mag is upward. B. I is counter-clockwise; F mag is upward. C. I is clockwise; F mag is downward. D. I is counter-clockwise; F mag is downward. E. None of the above.

4. 5/23/16 Oregon State University PH 213, Class #254 Applications of Induction: Magnetic Braking (using eddy currents)

5. 5/23/16 Oregon State University PH 213, Class #255 Applications of Induction: Electric Generator (a “motor in reverse”) Reminder: If the coil isn’t turned (by some external work) so that the flux is changing, no voltage is induced in the coil. Induction works only when the flux is changing.

6. 5/23/16 Oregon State University PH 213, Class #256 Compare: Here’s the schematic for an electric motor; its operation is basically the opposite of a generator. Note: If we want the generator (previous page) to generate Direct (not Alternating) Current, we use the commutator device shown here.

7. The primary coil of a transformer is connected to a battery, a resistor, and a switch. The secondary coil is connected to an ammeter. When the switch is thrown closed, the ammeter shows… 1. zero current. 2. a nonzero current for a short instant. 3.a steady current. (And then what happens when you open the switch again?) 5/23/16 7Oregon State University PH 213, Class #25

8. 5/23/16 Oregon State University PH 213, Class #258 Question: Suppose you double the number of turns in the secondary coil, then repeat the experiment. How would the result differ? Transformers A transformer is a device that uses induction to change the voltage level in a circuit. When/why would we want to do this? Consider long-distance transmission of electrical current..

9. 5/23/16 Oregon State University PH 213, Class #259 How do transformers raise or lower voltage levels? They use aligned sets of looped wire, called coils—much like the previous examples (only with many more loops). The primary coil is connected to the original power source; the secondary coil is then aligned with it and responds to changes in the primary coil’s current. To operate properly, however, a transformer must use ac (alternating current). Why? As it happens, the math works out rather simply: The ratio of the voltage induced across the secondary coil to the voltage across the primary coil is the same as the ratio of the number of loops in the two coils:  V S /  V P = N S /N P

10. When the switch is closed, the potential difference across R is 1. VN2 /N1. 2. VN1/N2. 3. V. 4. zero. 5. insufficient information 5/23/16 10Oregon State University PH 213, Class #25

11. 5/23/16 Oregon State University PH 213, Class #2511 Inductors A capacitor is a device that stores energy in an electric field caused by its own collection of charge, Q. The device’s response,  V C, to Q is its capacitance: C = Q/  V C An inductor is a device that stores energy in a magnetic field caused by its own current, I. The device’s response,  M, to I is its inductance: L =  M /I

12. 5/23/16 Oregon State University PH 213, Class #2512 C depends on the physical dimensions/properties of the capacitor, which is typically a set of parallel plates, modeled with no resistance: C = Q/  V C = Q/(Ed) = Q/[(Q/A  0 )d] =  0 A/d L depends on the physical dimensions/properties of the inductor, which is typically a solenoid, modeled with no resistance: L =  M /I = NBA/I = N[  0 NI/l]A/I =  0 N 2 A/l

13. 5/23/16 Oregon State University PH 213, Class #2513 The potential difference across the capacitor:  V C = (±)Q/C The device’s ability to store energy: U C = (1/2)C(  V C ) 2 The energy density in the electric field: u E = (1/2)  0 E 2 The potential difference across the inductor:  V L = –L(dI/dt) An inductor’s ability to store energy: U L = (1/2)L(I) 2 The energy density in the magnetic field: u B = (1/2)B 2 /  0

14. The potential at a is higher than at b. Which of these statements about the inductor current I could be true? A. I is flowing from a to b and is steady. A. I is flowing from a to b and is increasing. A. I is flowing from a to b and is decreasing. 5/23/16 14Oregon State University PH 213, Class #25