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Motors

Motors. Physics 102 Professor Lee Carkner Lecture 20. Ring in Solenoid. If the current flows clockwise through the solenoid, the B field inside is straight down To get maximum flux, the ring should face up (same direction as solenoid)

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Motors

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  1. Motors Physics 102 Professor Lee Carkner Lecture 20

  2. Ring in Solenoid • If the current flows clockwise through the solenoid, the B field inside is straight down • To get maximum flux, the ring should face up (same direction as solenoid) • We need to find the flux through the loop before and after the current is switched off • F = BA cos q = BA • B = m0nI = (4pX10-7)(1000)(10) = 0.0126 T • A = (0.1)(0.1) = 0.01 m2

  3. Current in Ring • F = BA = (0.0126)(0.01) = 1.26 X 10-4 Wb • In 1 second the flux goes to 0 • DF = (1.26 X 10-4) - (0) = 1.26 X 10-4 • Dt = 1 • e = -N(DF/Dt) = (1)(1.26 X 10-4) = 1.26 X 10-4 V • DV = IR or I = e/R = 1.26 X 10-4/10 • I = 1.26 X 10-5 A

  4. Motional emf • If we make the loop larger or smaller, or move it in or out of a field, we will induce a potential • remember emf is a potential difference (or voltage) • How does motion in a field translate to voltage?

  5. The area of the loop increases by LDx in time Dt but Dx = vDt, so DA = LvDt DF/Dt = BDA/Dt = (BLvDt)/Dt e = BLv Motional emf - Derived DA X B field into page v L x Dx in time Dt

  6. Motional emf -- Direction • If the area decreases, the flux decreases and thus the induced B field is in the same direction as the original

  7. Generators • What is the most efficient way to use inductance to produce current? • This changing flux produces an emf in the loop • Turn the loop with an external source of work • A machine that converts work into emf is called a generator

  8. Alternating Current • Which way does the current flow? • As the loop turns the flux points in one direction and then the other • If the loop is turned quickly, the changes in direction can occur rapidly • This is called alternating current

  9. emf From a Generator • Consider a loop of wire rotating in a magnetic field with angular speedw • From Faraday’s Law: e = N(DF/Dt) • The cos q term changes with time and can be related to the angular frequency w (radians/second) • The change of F with time is thus BAw sin wt, so the emf is: e = NBAw sin wt

  10. Sinusoidal Variations emax • As the loop makes one complete rotation (wt goes from 0 to 2p radians) the emf goes from 0, to maximum +, to maximum -, and back to zero again • The current through the loop goes one way and then the other, sometimes is weak and sometimes is strong

  11. Frequency • The number of these cycles made per second is the frequency f = w/2p • 1 turn per second (f=1) means 2p radians per second (w=2p) • NBAw is the maximum emf • sin wt tells us where we are in the cycle between positive and negative maximums

  12. Power Generation • A potential difference causes current to flow • Produced (in general) in two ways: • Chemical reactions separate charges so that one terminal is + and one is - • A changing magnetic field separates charges

  13. An Alternating Current Generator

  14. Motors • If you run a generator backwards it becomes a motor • Motor converts emf to work • This reduces the emf of the loop and is called back emf • Example: A motor initially has 120 volts, but if the motor produces a back emf of 70 volts, then the total emf is 50 volts

  15. Force on Eddy Currents

  16. Eddy Currents • As the field through the loop drops, it induces a field in the same direction • If the object is not a loop, circular currents can still be induced which have the same effect • Net effect: • Metal objects moving through a magnetic field will be slowed

  17. Next Time • Read 21.7, 21.9-21.11 • Homework: Ch 21, P 14, 23, 30, 39

  18. What is the direction of current in the loop from the PAL (seen from top down)? • clockwise • counterclockwise • left • right • down

  19. A bar magnet held north pole up is dropped straight down through a face up coil of wire. What is the direction of the current in the coil as the magnet enters and leaves the coil? • clockwise, counterclockwise • counterclockwise, clockwise • clockwise, clockwise • counterclockwise, counterclockwise • no current is induced

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