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. 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 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. 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 Transformers A transformer is a device that uses induction to change the voltage level in a circuit. So long as the current is changing, its field will be changing—and so another nearby circuit will respond. Transformers use aligned sets of looped wire, called coils. The primary coil is connected to the original power source; the secondary coil is then aligned with it and responds to changes in the magnetic field caused by the primary coil’s current. 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: VS/VP = NS/NP 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 When/why would we want to do this? Consider the vast range of electrical devices you might want to use… Consider also the long-distance transmission of electrical current.. 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 Check your understanding: The primary coil below is attached as shown to a battery with a steady voltage difference V. When the switch is closed on the secondary side, the potential difference across R is… A. VN2/N1 B. VN1/N2 C. V D. zero E. insufficient information 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 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, DVC, to Q is its capacitance: C = Q/DVC An inductor is a device that stores energy in a magnetic field caused by its own current, I. The device’s response, FM, to I is its inductance: L = FM/I 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 C depends on the physical dimensions/properties of the capacitor, which is typically a set of parallel plates, modeled with no resistance: C = Q/DVC = Q/(Ed) = Q/[(Q/Ae0)d] = e0A/d L depends on the physical dimensions/properties of the inductor, which is typically a solenoid, modeled with no resistance: L = FM/I = NBA/I = N[m0NI/l]A/I = m0N2A/l 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 The potential difference across the capacitor: DVC = (±)Q/C The device’s ability to store energy: UC = (1/2)C(DVC)2 The potential difference across the inductor: DVL = –L(dI/dt) An inductor’s ability to store energy: UL = (1/2)L(I)2 5/31/17 Oregon State University PH 213, Class #26

Oregon State University PH 213, Class #26 The potential at a is higher than at b. Which of these statements about the inductor current I could be true? I is flowing from a to b and is steady. I is flowing from a to b and is increasing. I is flowing from a to b and is decreasing. STT33.6Answer: D 5/31/17 Oregon State University PH 213, Class #26