Physics 2102 Gabriela Gonz lez An electrical current produces a magnetic field B 0in NiA 0 B z 2 z 3 1 A magnetic field can create a en electrical current too If we define magnetic flux similar to definition of electric flux but for an open surface with an edge B n dA Then a time varying magnetic FLUX creates an induced EMF and thus an electrical current if the edge is a wire Take note of the MINUS sign The induced EMF acts in such a way that it OPPOSES the change in magnetic flux Lenz s Law When the N pole approaches the loop the flux into the loop downwards increases The loop can oppose this change if a current were to flow clockwise hence creating a magnetic flux upwards So the induced EMF is in a direction that makes a current flow clockwise If the N pole moves AWAY the flux downwards DECREASES so the loop has a counter clockwise current 2 n A closed loop of wire encloses an area of 1 m2 in which in a uniform magnetic field exists at 300 to the PLANE of the loop The magnetic field is DECREASING at a rate of 1T s The resistance of the wire is 10 What is the induced current 300 Is it clockwise or counterclockwise 3 loops are shown I B 0 everywhere except in II III the circular region where B is uniform pointing out of the page and is increasing at a steady rate Rank the 3 loops in order of increasing induced EMF Just look at the rate of change of a III II I b III I II are same c ALL SAME ENCLOSED flux III encloses no flux and it does not change I and II enclose same flux and it changes at same rate 3 A square loop of wire of side L is rotated at a uniform frequency f in the presence of a uniform magnetic field B as shown Describe the EMF induced in the loop L B B BL2 2 f sin 2 ft An infinitely long wire carries a constant current i as shown A square loop of side L is moving towards the wire with a constant velocity v What is the EMF induced in the loop when it is a distance R from the loop R v x L L Choose a strip of width dx located as shown Flux thru this strip 4 R v L x What is the DIRECTION of the induced current Magnetic field due to wire points INTO page and gets stronger as you get closer to wire So flux into page is INCREASING Hence current induced must be counter clockwise to oppose this increase in flux MagLev train relies on Faraday s Law currents induced in nonmagnetic rail tracks repel the moving magnets creating the induction result levitation Guitar pickups also use Faraday s Law a vibrating string modulates the flux through a coil hence creating an electrical signal at the same frequency 5 A non magnetic e g copper aluminum ring is placed near a solenoid What happens if There is a steady current in the solenoid The current in the solenoid is suddenly changed The ring has a cut in it The ring is extremely cold Drop a non magnetic pendulum copper or aluminum through an inhomogeneous magnetic field What do you observe Why Think about energy conservation Pendulum had kinetic energy What happened to it Isn t energy conserved N S 6 The gap between the spark plug in a combustion engine needs an electric field of 107 V m in order to ignite the air fuel mixture For a typical spark plug gap one needs to generate a potential difference 104 V But the typical EMF of a car battery is 12 V So how does a spark plug work spark 12V Breaking the circuit changes the current through primary coil Result LARGE change in flux thru secondary large induced EMF The ignition coil is a double layer solenoid Primary small number of turns 12 V Secondary MANY turns spark plug http www familycar com Classroom ignition htm 7
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