Physics 102: Lecture 10. Faraday s Law. Changing Magnetic Fields create Electric Fields. Physics 102: Lecture 10, Slide 1

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1 Physics 102: Lecture 10 Faraday s Law Changing Magnetic Fields create Electric Fields Physics 102: Lecture 10, Slide 1

2 Last Two Lectures Magnetic fields Forces on moing charges and currents Torques on current loops Magnetic field created by Long straight wire Solenoid Physics 102: Lecture 10, Slide 2

3 Today Changing magnetic flux creates a oltage Faraday s and Lenz s Laws Generators Transformers Electromagnetic Waes Physics 102: Lecture 10, Slide 3

4 Motional EMF A metal bar slides with elocity on a track in a uniform B field I q + F q Moing + charges in bar experience force down (RHR1) Electrical current drien clockwise! Moing bar acts like a battery (i.e. generates EMF)!! Physics 102: Lecture 10, Slide 4 (Recall that e- actually moe, opposite current)

5 Faraday s Law of Induction: induced EMF = rate of change of magnetic flux The principle that unifies electricity and magnetism Key to many things in E&M Generating electricity Electric guitar pickups Credit card readers ε = Φ Δt = Φ f Φ i t f t i Physics 102: Lecture 10, Slide 5

6 First a definition: Magnetic Flux Counts number of field lines through loop. B Uniform magnetic field, B, passes through a plane surface of area A. f B Magnetic flux F = B A (Units Tm 2 = Wb) Magnetic flux F B A cos(f) f is angle between normal and B Physics 102: Lecture 10, Slide 6

7 CheckPoint 1.2 n n B a b Compare the flux through loops a and b. 1) F a >F b 2) F a < F b Physics 102: Lecture 10, Slide 7

8 Faraday s Law of Induction: induced EMF = rate of change of magnetic flux Physics 102: Lecture 10, Slide 8 ε = Φ Δt = Φ f Φ i t f t i Since F= B A cos(f), 3 things can change F 1. Area of loop (coered by flux) 2. Magnetic field B 3. Angle f between normal and B

9 ACT: Change Area W C L A B Which loop has the greatest induced EMF at the instant shown aboe? A moes right B moes down C moes down Physics 102: Lecture 10, Slide 9

10 Faraday: Change Area W W t L t=0 F i = BLW F = B A cos(f) t F f = BL(W+t) EMF Magnitude: ε = Φ Δt = Φ f Φ i t 0 BL(w + t) BLw = t 0 = BL Physics 102: Lecture 10, Slide 10 What about the sign of the EMF?

11 Lenz s Law (EMF direction) I B ind Flux is increasing Induced current is clockwise Current loop generates induced B field from RHR2, into page, opposite external B field! What happens if the elocity is reersed? Physics 102: Lecture 10, Slide 11

12 Lenz s Law (EMF direction) I B ind Flux is decreasing Induced current is counterclockwise Current loop generates induced B field from RHR2, out of the page, along external B field! Induced EMF opposes change in flux Physics 102: Lecture 10, Slide 12

13 Lenz s Law (EMF Direction) Induced emf opposes change in flux ε = Φ Δt = Φ f Φ i t f t i If flux increases: New EMF makes new field opposite to original field If flux decreases: New EMF makes new field in same direction as original field Physics 102: Lecture 10, Slide 13 EMF does NOT oppose B field, or flux! EMF opposes the CHANGE in flux

14 Motional EMF also creates forces Magnitude of current I = e/r = BL/R I q + Direction of Current F bar Clockwise (+ charges go down thru bar, up thru bulb) F q B field generates force on current-carrying bar F bar = ILB sin(q), to left (RHR1) F bar opposes! Careful! There are two forces: F bar = force on bar from induced current F q = force on + charges in bar driing induced current Physics 102: Lecture 10, Slide 14

15 CheckPoint 1.1 The field is reersed to point out of the page instead of in To keep the bar moing at the same speed, the force supplied by the hand will hae to: Increase Stay the Same Decrease Physics 102: Lecture 10, Slide 15

16 Faraday s Law of Induction: induced EMF = rate of change of magnetic flux ε = Φ Δt = Φ f Φ i t f t i Physics 102: Lecture 10, Slide 16 Since F= B A cos(f), 3 things can change F 1. Area of loop (coered by flux) 2. Magnetic field B 3. Angle f between normal and B

17 ACT: Induction cannon (Demo) A solenoid is drien by an increasing current. A loop of wire is placed around it As current increases in the solenoid, what direction will induced current be in ring? A. Same as solenoid B. Opposite of solenoid C. No current Physics 102: Lecture 10, Slide 17

18 Induction cannon (Demo) A solenoid is drien by an increasing current. A loop of wire is placed around it Recall: current loop and solenoid behae like bar magnets Like poles repel! Loop shoots up Physics 102: Lecture 10, Slide 18

19 N S ACT: Change B (Demo) Which way is the magnet moing if it is inducing a current in the loop as shown? A. Up B. Down Physics 102: Lecture 10, Slide 19

20 Magnetic Flux Example A solenoid (B=m o ni) is inside a conducting loop. happens to the flux through the loop when you What Increase area of solenoid Increase area of loop Increase current in solenoid F B A cos(f) Physics 102: Lecture 10, Slide 20

21 Faraday s and Lenz s Law Faraday: Induced emf = rate of change of magnetic flux ε = Φ Δt = Φ f Φ i t f t i Lenz: Induced emf opposes change in flux Physics 102: Lecture 10, Slide 21 Since F= B A cos(f), 3 things can change F Next lecture 1. Area of loop 2. Magnetic field B 3. Angle f between normal and B