Your Comments. I don't understand how to find current given the velocity and magnetic field. I only understand how to find external force

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Alisha Copeland
6 years ago
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1 Your Comments CONFUSED! Especially with the direction of eerything The rotating loop checkpoint question is incredibly difficult to isualize. All of this is pretty confusing, but 'm especially confused about how to find what direction the current flows through a moing conductor connected by 2 rails and a resistor. This whole prelecture is really confusing. ots of new topics, ery quick explanation for eerything. really hope this stuff gets cleared up in lecture. did not understand the generator example at all. The right hand rule in the beginning did not seem to apply because it looked like, B, and F were all in the same plane. Also, how does the E-field help us determine the direction of current? don't understand how to find current gien the elocity and magnetic field. only understand how to find external force Ya know what really grinds my gears? This nonsense about the direction of current being the flow of positie charges. get that when this was all discoered, they didn't really know what was going on at the atomic leel, but come on, we can fix that now. n chemistry, when we discoer things that contradict what we preiously thought, we fix it. Why cant physicists be the same? ike eery time hear about positie charges flowing in a conductor, a little part of me on the inside wants to scream "NOOOOO!"

2 Physics 212 ecture 16 Today s Concept: Motional EMF Emotional Electricity & Magnetism ecture 16, Slide 2

3 Some Confusion? What part of Don t you understand? Physics 212 ecture 16, Slide 3

4 The Big dea When a conductor moes through a region containing a magnetic field: Magnetic forces may be exerted on the charge carriers in the conductor F q These forces produce a charge separation in the conductor B B X X X X X X X X X F + - F X X X X X X X X X + This charge distribution creates an electric field in the conductor The equilibrium distribution is reached when the forces from the electric and magnetic fields cancel - qb qe E B + - E F B + F E + - E The equilibrium electric field produces a potential difference (emf) in the conductor V E V B Electricity & Magnetism ecture 16, Slide 4

5 CheckPoint 1 Two identical conducting bars (shown in end iew) are moing through a ertical magnetic field. Bar (a) is moing ertically and bar (b) is moing horizontally. Which of the following is true? A. A A motional emf exists in the bar for case (a), but not (b) B. B A motional emf exists in the bar for case (b), but not (a) C. C A motional emf exists in the bar for both cases (a) and (b) D D. A motional emf exists in the bar for neither case (a) nor case (b) Rotate picture by 90 o X X X X X X X X X X a + - B b F + - F X X X X X X X X X Bar a No force on charges No charge separation No E field No emf Bar b Opposite forces on charges Charge separation E B emf E B F a 0 F b qb Electricity & Magnetism ecture 16, Slide 5

CheckPoint 2a A conducting bar (green) rests on two frictionless wires connected by

A B Changing Area C D The entire apparatus is placed in a uniform magnetic field

Rotate picture by 90 o The A motion of the green bar creates a current through the

6 CheckPoint 2a A conducting bar (green) rests on two frictionless wires connected by a resistor as shown. A B Changing Area C D The entire apparatus is placed in a uniform magnetic field pointing into the screen, and the bar is gien an initial elocity to the right. Rotate picture by 90 o The A motion of the green bar creates a current through the bar A. B going up B. going down Bar Opposite forces on charges Charge separation E 0 B emf E 0 B R Equialent circuit V Electricity & Magnetism ecture 16, Slide 6

right. The A current through this bar results in a force on the bar A. B down C B. D up C. E right D. left E.

7 CheckPoint 2b A conducting bar (green) rests on two frictionless wires connected by a resistor as shown. A B Changing Area C The D entire apparatus is placed in a uniform magnetic field pointing into the screen, and the bar is gien an initial elocity to the right. The A current through this bar results in a force on the bar A. B down C B. D up C. E right D. left E. into the screen F B B F B R P F Current up through bar F points to left F B R B 2 R Energy External agent must exert force F to the right to maintain constant This energy is dissipated in the resistor! Electricity & Magnetism ecture 16, Slide 7

8 F Changing Area Electricity & Magnetism ecture 16, Slide 8

9 CheckPoint 5 A wire loop traels to the right at a constant elocity. Which plot best represents the induced current in the loop as it traels from left of the region of magnetic field, through the magnetic field, and then entirely out on the right side? B = 0 B = 5 T Out of Screen et s step through this one Electricity & Magnetism ecture 16, Slide 9

10 A wire loop traels to the right at a constant elocity. Which plot best represents the induced current in the loop as it traels from left of the region of magnetic field, through the magnetic field, and then entirely out on the right side? - + Only leading side has charge separation emf B (cw current) t A wire loop traels to the right at a constant elocity. Which plot best represents the induced current in the loop as it traels from left of the region of magnetic field, through the magnetic field, and then entirely out on the right side? t eading and trailing sides hae charge separation emf B B 0 (no current) A wire loop traels to the right at a constant elocity. Which plot best represents the induced current in the loop as it traels from left of the region of magnetic field, through the magnetic field, and then entirely out on the right side? Only trailing side has charge separation emf B (ccw current) Electricity & Magnetism ecture 16, Slide 10 t

11 CheckPoint 5 A wire loop traels to the right at a constant elocity. Which plot best represents the induced current in the loop as it traels from left of the region of magnetic field, through the magnetic field, and then entirely out on the right side? B = 0 B = 5 T Out of Screen Electricity & Magnetism ecture 16, Slide 11

12 Changing B Field A conducting rectangular loop moes with elocity toward an infinite straight wire carrying current as shown. What is the direction of the induced current in the loop? A. clockwise B. counter-clockwise C. there is no induced current in the loop B B B B Electricity & Magnetism ecture 16, Slide 12

13 Generator: Changing Orientation A) Top and Bottom legs only B) Front and Back legs only C) All legs D) None of the legs On which legs of the loop is charge separated? B Parallel to top and bottom legs Perpendicular to front and back legs Electricity & Magnetism ecture 16, Slide 13

14 Generator: Changing Orientation w A) q 0 B) q 45 o C) q 90 o D) emf is same at all angles At what angle q is emf the largest? B argest for q 0 ( perp to B) F w 2E 2 2 B 2( ) Bcosq AB cos( t) q 2 Electricity & Magnetism ecture 16, Slide 14

At this time, what is the direction of the induced (positie) current in segment ab? A. from b to a B.

15 Changing Orientation A rectangular loop rotates in a region containing a constant magnetic field as shown. The side iew of the loop is shown at a particular time during the rotation. At this time, what is the direction of the induced (positie) current in segment ab? A. from b to a B. from a to b C. there is no induced current in the loop at this time B + - X B B Electricity & Magnetism ecture 16, Slide 15

16 Putting it Together Change Area of loop Change magnetic field through loop Faraday s aw B da - Change orientation of loop relatie to B d dt We will study this law in detail next time! Electricity & Magnetism ecture 16, Slide 16

17 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x Conceptual Analysis: ong straight current creates magnetic field in region of the loop. Vertical sides deelop emf due to motion through B field Net emf produces current Strategic Analysis: Calculate B field due to wire. Calculate motional emf for each segment Use net emf and Ohm s law to get current Electricity & Magnetism ecture 16, Slide 17

18 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x What is the direction of the B field produced by the wire in the region of the loop? A) nto the page B) Out of the page C) eft D) Right E) Up Electricity & Magnetism ecture 16, Slide 18

19 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x B into page What is the emf induced on the left segment? A) Top is positie B) Top is negatie C) Zero B Electricity & Magnetism ecture 16, Slide 19

20 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x B into page What is the emf induced on the top segment? A) left is positie B) left is negatie C) Zero B perpendicular to wire Electricity & Magnetism ecture 16, Slide 20

21 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x B into page What is the emf induced on the right segment? A) top is positie B) top is negatie C) Zero B Electricity & Magnetism ecture 16, Slide 21

22 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x B into page Which expression represents the emf induced in the left wire? A) B) left left o 2x o 2x h qb qe B o 2x E B o 2x Eh h Bh C) left o 2 ( + x) Electricity & Magnetism ecture 16, Slide 22

23 Example Problem A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. What is the induced current in the loop when it is a distance x 0.7 m from the wire? h x B into page A) B) Which expression represents the emf induced in the right wire? C) right right right o 2 ( + x) o h 2x o 2 ( h + x) h qb qe o B 2 ( + x) E B Eh o 2 ( + Bh h x) Electricity & Magnetism ecture 16, Slide 23

24 Example Problem A rectangular loop (h 0.3m 1.2 m) with total A rectangular loop (h=0.3m =1.2 m) with total resistance of of 5W 5W is is moing away from a a long long straight straight wire carrying total current 8 8 amps. What What is is the the induced induced current in current the loop in when the loop it is when a distance it a x distance 0.7 m from x=0.7 m from the wire? the wire? h x B into page Which expression represents the total emf in the loop? A) loop o o h + 2 x 2 ( + x) h loop loop R B) C) loop loop o o h - 2 x 2 ( + 0 x) h o loop 2R 1 h x x Electricity & Magnetism ecture 16, Slide 24

25 Follow-Up A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. h x B into page What is the direction of the induced current? A) Clockwise left > right B) Counterclockwise Clockwise current Electricity & Magnetism ecture 16, Slide 25

26 Follow-Up A rectangular loop (h 0.3m 1.2 m) with total resistance of 5W is moing away from a long straight wire carrying total current 8 amps. h B into page x What is the direction of the force exerted by the magnetic field on the loop? A) B) UP DOWN C) EFT D) RGHT E) F 0 B into page Total force from B Points to the left! Electricity & Magnetism ecture 16, Slide 26

Physics 212. Motional EMF

Physics 212. Motional EMF Physics 212 ecture 16 Motional EMF Conductors moing in field nduced emf!! Physics 212 ecture 16, Slide 1 The ig dea When a conductor moes through a region containg a magnetic field: Magnetic forces may