2 Physics 102 recently Basic principles of magnetism Lecture 10 – magnetic fields & forcesLecture 11 – magnetic dipoles & current loopsLecture 12 – currents & magnetic fieldsConnection between electricity & magnetismLecture 13 – motional EMF & Lenz’ lawLecture 14 – Faraday’s law of inductionLecture 15 – electromagnetic waves
3 Today we will... Learn how electric fields are created from... Motion in magnetic fields (“motional EMF”)Changing magnetic fieldsLearn Lenz’ law: principle unifying electricity and magnetismApply these concepts:MagnetoreceptionElectrical generators & hybrid cars
4 CheckPoint 1: Moving bar A conducting bar moves in a uniform external B field at speed vBextv––––Magnetic force pushes – electrons to top, leaves + charge at bottom of barSeparated + and – charge induces E field & ΔVMoving bar acts like a battery! Motional EMFAt equilibrium, forces must sum to zero
5 Magnetoreception in sharks Sharks can sense changes in magnetic fieldsAmpullae of LorenziniShark do not have magnetic organelles like magnetotactic bacteria, but they do have “ampullae of Lorenzini”, which sense E fieldModel of magnetoreception in sharks: motional EMF from moving in B field generates E field detected by ampullae
6 Motional EMFBar slides with speed v on a conducting track in a uniform B fieldIBext++v+LI+IRbulb++IDEMO: 1329Can moving bar drive current around the circuit?+ charges in moving bar experience force downElectrical current induced clockwise!(Recall that e– actually move, opposite current)
7 ACT: CheckPoint 2.1The conducting bar moves to the right in the opposite B fieldBextvWhich way does the current flow?ClockwiseCounterclockwiseThe current is zero
8 ACT: Two metal barsCircuit now has two metal bars moving right at the same speed vBextvvWhich way does the current flow?ClockwiseCounterclockwiseThe current is zero
9 Motional EMF and forceWhere does the energy come from to generate electricity?BextFbarIBextvLMoving bar carries current, so B field exerts a force FbarNote: Fbar is NOT FB which drives current around loopFbar opposes v, so bar deceleratesTo maintain constant v, you must provide external force Fext opposing FbarFext does the work to generate electrical energy
10 Electrical generators Motional EMF is the basis for modern electrical generationInstead of sliding bar, use spinning loop in B fieldExternal torque (from turbine, gas engine, hand crank) spins loopDEMO
11 ACT: CheckPoint 2.2The B field is now reversed and points into the page.BextvTo keep the bar moving at the same speed, the hand must supply:A force to the rightA force to the leftNo force, the bar slides by inertia
12 Changing B field Now loop is fixed, but B field changes Bext If Bext increases, current I flows clockwiseIf Bext decreases, current I flows counterclockwiseIf Bext is constant, no current flowsWhat is changing here and in previous cases? Magnetic flux !
13 Area inside loop filled with B field Magnetic fluxFlux “counts” number of B field lines passing through a loopAngle between normal vector and B fieldUnit: Wb (“Weber”)1 Wb = 1 T∙m2B field in loopArea inside loop filled with B fieldAngle φ affects how many B field lines pass through loopBnormalDEMO: 1350 (hula hoop flux)BnormalφTop viewSide view
14 CheckPoint 3.1 Compare the flux through loops a and b B a b normalnormalBabA. Fa > Fb B. Fa < Fb C. Fa = Fb
15 Magnetic flux practice A solenoid generating a B field is placed inside a conducting loop. What happens to the flux through the loop when...BsolBsolLoopLoopITop viewSide viewThe area of the solenoid increases?The current in the solenoid increases?The area of the loop increases?
16 Induced EMF ε opposes change in flux Lenz’s lawInduced EMF ε opposes change in flux BextBindIf increases:ε generates new B field opposite external B fieldIf decreases:ε generates new B field along external B fieldIf is constant:ε is zeroDEMO: 543 – light bulb with induction magnetDEMO: 371 – Helmholtz coil & magnetDEMO: 1142 – credit card demoSide viewBextBindOne principle explains all the previous examples!Top view
17 Lenz’s law: changing loop area ε opposes change in flux EX 1IvBindA & increasesε generates Bind opposite BextIEX 2vBextBindA & decreasesε generates Bind along BextSame answers as before!EX 3vvA & remains constantε is zero
18 ACT: Lenz’ law: changing B field A loop is placed in a uniform, increasing B fieldBextIn which direction does the induced B field from the loop point?Into the pageOut of the pageThere is no induced B field
19 ACT: moving loopsThree loops are moving in a region containing a uniform B field. The field is zero everywhere outside.ABCIn which loop does current flow counterclockwise at the instant shown?Loop A B. Loop B C. Loop Cε opposes change in flux
20 ACT: Solenoid & loopA solenoid is driven by an increasing current. A loop of wire is placed around it. In which direction does current in the loop flow?BsolBsolClockwiseCounterclockwiseThe current is zeroITop viewSide view
21 Induction cannonA solenoid is driven by an increasing current. A loop of wire is placed around it.Current loop and solenoid behave like magnetic dipolesOpposite currents = opposite polaritiesLike poles repel, so loop shoots up!BsolRecall Lect. 11DEMO: 157IDEMO
22 Summary of today’s lecture Electric fields are created fromMotion in magnetic fields (“motional EMF”)Changing magnetic fieldsLenz’ Law: EMF ε opposes change in flux ε does NOT oppose ε opposes change in Lenz’ law gives direction of EMFFaraday’s law gives us magnitude of EMF (next lecture!)
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