# Chapters 34,36: Electromagnetic Induction. PHY2061: Chapter

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1 Chapters 34,36: Electromagnetic Induction PHY2061: Chapter

2 Electromagnetic Induction Magnetic flux Induced emf Faraday s Law Lenz s Law Motional emf Magnetic energy Inductance RL circuits Generators and transformers Topics PHY2061: Chapter

3 Reading Quiz 1 Magnetic flux through a wire loop depends on: 1) thickness of the wire 2) resistivity of the wire 3) geometrical layout of the wire 4) material that the wire is made of 5) none of the above Φ B = B d A A Flux depends only on geometrical properties PHY2061: Chapter

4 Reading Quiz 2 An induced emf produced in a motionless circuit is due to 1) a static (steady) magnetic field 2) a changing magnetic field Faraday s law 3) a strong magnetic field 4) the Earth s magnetic field 5) a zero magnetic field PHY2061: Chapter

5 Reading Quiz 3 Motional emf relates to an induced emf in a conductor which is: 1) long 2) sad 3) stationary 4) insulated 5) moving Potential difference proportional to velocity PHY2061: Chapter

6 Faraday s law says that Reading Quiz 4 a) an emf is induced in a loop when it moves through an electric field b) the induced emf produces a current whose magnetic field opposes the original change c) the induced emf is proportional to the rate of change of magnetic flux Faraday s law PHY2061: Chapter

7 A generator is a device that: Reading Quiz 5 a) transforms mechanical into electrical energy b) transforms electrical into mechanical energy c) transforms low voltage to high voltage PHY2061: Chapter

8 Electromagnetic Induction Faraday discovered that a changing magnetic flux leads to a voltage in a wire loop Induced voltage (emf) causes a current to flow!! Symmetry: electricity electric current magnetic field magnetic field electric current magnetism We can express this symmetry directly in terms of fields Changing E field B field ( displacement current ) Changing B field E field (Faraday s law) These & other relations expressed in Maxwell s 4 equations (Other 2 are Gauss law for E fields and B fields) Summarizes all of electromagnetism See Chapter 32 PHY2061: Chapter

9 Experimental Observation of Induction This effect can be quantified by Faraday s Law PHY2061: Chapter

10 Magnetic Flux Define magnetic flux Φ B Φ B = B A= BAcosθ θ is angle between B and the normal to the plane Flux units are T-m 2 = webers When B field is not constant or area is not flat Integrate over area Φ B = B d A A PHY2061: Chapter

11 Φ B = B A= BAcosθ 1 Φ = 0 Φ = BA Φ B = BA B B 2 PHY2061: Chapter

12 Faraday s Law of Induction dφ E = N dt B induced emf rate of change of flux with time number of loops The faster the change, the larger the induced emf Flux change caused by changing B, area, or orientation The induced emf is a voltage PHY2061: Chapter

13 Rotating coil Faraday s Law & Flux Change Φ B = BAcosωt dφ E = N B = NωBAsinωt dt φ B is maximum when coil faces up E is maximum when coil faces sideways Stretched coil B constant, θ constant Area shrinks Flux decreases PHY2061: Chapter

14 Faraday s Law of Induction dφ E = N dt B induced emf rate of change of flux with time number of loops Minus sign from Lenz s Law: Induced current produces a magnetic field which opposes the original change in flux PHY2061: Chapter

15 Comment on Lenz s Law Why does the induced current oppose the change in flux? Consider the alternative If the induced current reinforced the change, then the change would get bigger, which would then induce a larger current, and then the change would get even bigger, and so on... This leads to a clear violation of conservation of energy!! PHY2061: Chapter

16 Direction of Induced Current Bar magnet moves through coil Current induced in coil A S N v Reverse pole Induced current changes sign B N S v v Coil moves past fixed bar magnet Current induced in coil as in (A) C S N Bar magnet stationary inside coil D S N No current induced in coil PHY2061: Chapter

17 ConcepTest: Lenz s Law If a North pole moves towards the loop from above the page, in what direction is the induced current? (a) clockwise (b) counter-clockwise (c) no induced current Must counter flux change in downward direction with upward B field PHY2061: Chapter

18 ConcepTest: Induced Currents A wire loop is being pulled through a uniform magnetic field. What is the direction of the induced current? (a) clockwise (b) counter-clockwise (c) no induced current No change in flux, no induced current x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x PHY2061: Chapter

19 ConcepTest: Induced Currents x x x x x x x x x x x x x x x x x x x x x x x x x Counter None None (max flux, x x x x x x x x x x x x x x x x x x x x so x not x x changing) x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x 1 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x In each of the 3 cases above, what is the direction of the induced current? (Magnetic field is into the page and has no boundaries) 2 3 (a) clockwise (b) counter-clockwise (c) no induced current PHY2061: Chapter

20 ConcepTest: Lenz s Law If a coil is shrinking in a B field pointing into the page, in what direction is the induced current? (a) clockwise (b) counter-clockwise (c) no induced current Downward flux is decreasing, so need to create downward B field PHY2061: Chapter

21 Induced currents 1 A circular loop in the plane of the paper lies in a 3.0 T magnetic field pointing into the paper. The loop s diameter changes from 100 cm to 60 cm in 0.5 s What is the magnitude of the average induced emf? What is the direction of the induced current? If the coil resistance is 0.05Ω, what is the average induced current? V ( ) dφ π B = = 3.0 = Volts dt 0.5 Direction = clockwise (Lenz s law) Current = / 0.05 = 60.3 A PHY2061: Chapter

22 ConcepTest: Induced Currents A wire loop is pulled away from a current-carrying wire. What is the direction of the induced current in the loop? (a) clockwise (b) counter-clockwise (c) no induced current Downward flux through loop decreases, so need to create downward field I PHY2061: Chapter

23 ConcepTest: Induced Currents A wire loop is moved in the direction of the current. What is the direction of the induced current in the loop? (a) clockwise (b) counter-clockwise (c) no induced current Flux does not change when moved along wire I PHY2061: Chapter

24 ConcepTest: Lenz s Law If the B field pointing out of the page suddenly drops to zero, in what direction is the induced current? (a) clockwise (b) counter-clockwise (c) no induced current Upward flux through loop decreases, so need to create upward field If a coil is rotated as shown, in a B field pointing to the left, in what direction is the induced current? (a) clockwise (b) counter-clockwise (c) no induced current Flux into loop is increasing, so need to create field out of loop PHY2061: Chapter

25 ConcepTest: Induced Currents Wire #1 (length L) forms a one-turn loop, and a bar magnet is dropped through. Wire #2 (length 2L) forms a two-turn loop, and the same magnet is dropped through. Compare the magnitude of the induced currents in these two cases. (a) I 1 = 2 I 2 (b) I 2 = 2 I 1 (c) I 1 = I 2 0 (d) I 1 = I 2 = 0 Voltage doubles, but R also doubles, leaving current the same (e) Depends on the strength of the magnetic field PHY2061: Chapter

26 Motional EMF Consider a conducting rod moving on metal rails in a uniform magnetic field: ε dφ B d( BA) d( BLx) dx = = = = BL dt dt dt dt Current will flow counter-clockwise in this circuit. Why? x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x v L ε = BLv PHY2061: Chapter

27 Force and Motional EMF Pull conducting rod out of B field Current is clockwise. Why? ε i = = BLv R R Current within B field causes force F 2 2 = ilb = B Lv R Force opposes pull (RHR) Also follows from Lenz s law We must pull with this force to maintain constant velocity PHY2061: Chapter

28 Power and Motional EMF Force required to pull loop: Power required to pull loop: Energy dissipation through resistance 2 2 = ilb = B L v R Same as pulling power! So power is dissipated as heat Kinetic energy is constant, so energy has to go somewhere Rod heats up as you pull it F P 2 BLv P = i R = R = R = Fv= B L v R B L v R PHY2061: Chapter

29 Example Pull a 30cm x 30cm conducting loop of aluminum through a 2T B field at 30cm/sec. Assume it is 1cm thick. EMF Circumference = 120cm = 1.2m, cross sectional area = 10-4 m 2 R = ρl/a = 2.75 x 10-8 * 1.2 / 10-4 = 3.3 x 10-4 Ω Current Force ε = BLv = = 0.18V i F 4 = ε / R = 0.18/ = 545A = ilb = = 327 N 74 lbs! Power P 2 = i R =98W About C per sec (from specific heat, density) PHY2061: Chapter

30 Electric Generators Rotate a loop of wire in a uniform magnetic field: changing θ changing flux induced emf Φ B = B A cos θ = B A cos(ωt) Rotation: θ = ωt PHY2061: Chapter

31 Electric Generators Flux is changing in a sinusoidal manner ε Leads to an alternating emf (AC generator) dφ dcos( ωt) B = N = NBA = NBAω sin( ωt) dt dt This is how electricity is generated Water or steam (mechanical power) turns the blades of a turbine which rotates a loop Mechanical power converted to electrical power PHY2061: Chapter

32 ConcepTest: Generators A generator has a coil of wire rotating in a magnetic field. If the B field stays constant and the area of the coil remains constant, but the rotation rate increases, how is the maximum output voltage of the generator affected? (a) Increases (b) Decreases (c) Stays the same (d) Varies sinusoidally ε = NBAω sin( ωt) PHY2061: Chapter

33 Induction in Stationary Circuit Switch closed (or opened) Current induced in coil B (directions as shown) Steady state current in coil A No current induced in coil B B A PHY2061: Chapter

34 Inductance Inductance in a coil of wire defined by Can also be written From Faraday s law Li = NΦ ε B dφb di = N = L dt dt L = NΦ i B This is a more useful way to understand inductance Inductors play an important role in circuits when current is changing! PHY2061: Chapter

35 Self - Inductance Consider a single isolated coil: Current (red) starts to flow clockwise due to the battery But the buildup of current leads to changing flux in loop Induced emf (green) opposes the change This is a self-induced emf (also called back emf) ε dφ di = N = L dt dt L is the self-inductance units = Henry (H) 12V induced emf PHY2061: Chapter

36 Total flux (length l) B= μ 0 in Inductance of Solenoid NΦ = nl BA = n Ali ε B μ 0 ( )( ) 2 dφb 2 di di = N = μ0n Al = L dt dt dt L 0 2 = μ n Al To make large inductance: Lots of windings Big area Long PHY2061: Chapter

37 LR Circuits Inductance and resistor in series with battery of EMF V Start with no initial current in circuit Close switch at t = 0 Current is initially 0 (initial increase causes voltage drop across inductor) Find i(t) Resistor: ΔV = Ri Inductor: ΔV = L di/dt V R L V Ri Ldi / dt = 0 PHY2061: Chapter

38 Differential equation is General solution: (Check and see!) Analysis of LR Circuit K = V/R (necessary to make i = 0 at t = 0) i V = R ( tr / L 1 e ) di / dt + i R / L = V / R i = V / R+ Ke ( ) tr / L Rise from 0 with time constant τ = L / R Final current (maximum) PHY2061: Chapter

39 Current vs Time in RL Circuit (Initially Zero Current in Inductor) ( ) ( / ) max 1 = tr L it i e PHY2061: Chapter

40 L-R Circuits (2) Switch off battery: Find i(t) if current starts at i 0 0 = Ldi / dt + / 0 tr L Ri i= i e Exponential fall to 0 with time constant τ = L / R Initial current (maximum) PHY2061: Chapter

41 Current vs Time in RL Circuit (For Initial Current i max in Inductor) ( / it = ie trl ) () 0 PHY2061: Chapter

42 Exponential Behavior τ = L/R is the characteristic time of any RL circuit Only t / τ is meaningful t = τ Current falls to 1/e = 37% of maximum value Current rises to 63% of maximum value t =2τ Current falls to 1/e 2 = 13.5% of maximum value Current rises to 86.5% of maximum value t =3τ Current falls to 1/e 3 = 5% of maximum value Current rises to 95% of maximum value t =5τ Current falls to 1/e 5 = 0.7% of maximum value Current rises to 99.3% of maximum value PHY2061: Chapter

43 ConcepTest: Generators and Motors A current begins to flow in a wire loop placed in a magnetic field as shown. What does the loop do? (a) moves to the right (b) moves up (c) rotates around horizontal axis (d) rotates around vertical axis (e) moves out of the page B This is how a motor works!! PHY2061: Chapter

44 Electric Motors Current is supplied from an external source of emf (battery or power supply) Forces act to rotate the wire loop A motor is essentially a generator operated in reverse! PHY2061: Chapter

45 Motor Forces act to rotate the loop towards the vertical. When loop is vertical, current switches sign and the forces reverse, in order to keep the loop in rotation. This is why alternating current is necessary for a motor to operate. PHY2061: Chapter

46 Motors Generators Electrical mechanical energy Mechanical electrical energy PHY2061: Chapter 34,36 46

47 Energy Stored in Magnetic Field Just like electric fields, magnetic fields store energy u E = 1 ε 2 0 E 2 Electric field u B = 2 B 2μ 0 Magnetic field Let s see how this works PHY2061: Chapter

48 Energy of an Inductor How much energy is stored in an inductor when a current is flowing through it? Start with loop rule ε = ir + di L dt Multiply by I to get power equation di Li dt Pin = P heat + P store 2 ε i = i R + R ( ) ( ) P L = rate at which energy is being stored in inductor Energy stored in inductor 2 Similar to capacitor: L U U L C di d PL = Li = Li dt dt 2 = = 1 2 Li 2 q 2C ( 1 2 ) PHY2061: Chapter

49 Energy in Magnetic Field (2) Apply to solenoid (constant B field) L ( μ ) U = Li = n la i Use formula for B field: B = U L = 2 B 2μ 0 la Calculate energy density: μ ni 0 U L ub = V = Al V l r N turns u B = 2 B 2μ 0 B field ue = 1 ε 2 0 E 2 E field This is generally true even if B is not constant PHY2061: Chapter

50 Energy Calculation Examples Calculate u B for earth field, B = 5 x 10-5 T u B 2 0 ( ) 2 B = = 2μ 7 2 4π J/m Calculate u B for neutron star, B = 10 8 T u B 2 0 ( 8 10 ) 2 B = = 4 10 J/m 2μ 7 2 4π Calculate u B for magnetar, B = T u B 2 0 ( ) 2 B = = 4 10 J/m 2μ 7 2 4π 10 u B ρ = 4 10 kg/m c 27 3 Use E = mc 2 PHY2061: Chapter

51 Original magnetar discovery Web Sites More recent magnetar discovery (Feb. 2005) Online articles on magnetars html Articles on neutron stars (second one has videos_ PHY2061: Chapter

52 Gigajoule Magnet at CERN CMS experiment at CERN p-p collisions at world s highest energy in 2007 Hope to discover new particles, find the origin of mass and new fundamental forces Compact Muon Solenoid PHY2061: Chapter

53 Human PHY2061: Chapter

54 CMS Experiment Magnet Large central solenoid magnet to study particle production B = 4T, R = 3.15 m, L = 12.5 m U B = 2.6 x 10 9 J = 2.6 gigajoules!! 2 2 B 4 UB = la = π 2μ 7 2 4π 10 0 ( )( 12.5) PHY2061: Chapter

55 Home page CMS Articles and Pictures Pictures of detector orinfo.html Interesting article on solenoid, with pictures hure/magnetbrochure.pdf Other documents & pictures about CMS nts.html PHY2061: Chapter

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