Physics 2113 Jonathan Dowling Lecture 33: MON 13 APR Induction and Inductance III Fender Stratocaster Solenoid Pickup
F a r a d a y ' s E x p e r i m e n t s I n a s e r i e s o f e x p e r i m e n t s , M i c h a e l F a r a d a y i n E n g l a n d a n d J o s e p h H e n r y i n t h e U . S . w e r e a b l e t o g e n e r a t e e l e c t r i c c u r r e n t s w i t h o u t t h e u s e o f b a t t e r i e s . T h e c i r c u i t s h o w n i n t h e f i g u r e c o n s i s t s o f a w i r e l o o p c o n n e c t e d t o a s e n s i t i v e a m m e t e r ( k n o w n a s a " g a l v a n o m e t e r " ) . I f w e a p p r o a c h t h e l o o p w i t h a p e r m a n e n t m a g n e t w e s e e a c u r r e n t b e i n g r e g i s t e r e d b y t h e g a l v a n o m e t e r . 1 . A c u r r e n t a p p e a r s o n l y i f t h e r e i s r e l a t i v e m o t i o n b e t w e e n t h e m a g n e t a n d t h e l o o p . 2 . F a s t e r m o t i o n r e s u l t s i n a l a r g e r c u r r e n t . 3 . I f w e r e v e r s e t h e d i r e c t i o n o f m o t i o n o r t h e p o l a r i t y o f t h e m a g n e t , t h e c u r r e n t r e v e r s e s s i g n a n d f l o w s i n t h e o p p o s i t e d i r e c t i o n . T h e c u r r e n t g e n e r a t e d i s k n o w n a s " i n d u c e d c u r r e n t " ; t h e e m f t h a t a p p e a r s i s k n o w n a s " i n d u c e d e m f " ; t h e w h o l e e f f e c t i s c a l l e d " i n d u c t i o n . "
loop 1 loop 2
Lenz’s Law • The Loop Current Produces a B Field that Opposes the CHANGE in the bar magnet field. • Upper Drawing: B Field from Magnet is INCREASING so Loop Current is Clockwise and Produces an Opposing B Field that Tries to CANCEL the INCREASING Magnet Field • Lower Drawing: B Field from Magnet is DECREASING so Loop Current is Counterclockwise and Tries to BOOST the Decreasing Magnet Field.
30.5: Induction and Energy Transfers: • If the loop is pulled at a constant velocity v, one must apply a constant force F to the loop since an equal and opposite magnetic force acts on the loop to oppose it. The power is P=Fv. • As the loop is pulled, the portion of its area within the magnetic field, and therefore the magnetic flux, decrease. According to Faraday’s law, a current is produced in the loop. The magnitude of the flux through the loop is ΦB =BA =BLx. • Therefore, • The induced current is therefore • The net deflecting force is: • The power is therefore
Example: Eddy Currents • 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?
Another Experimental Observation • 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?? Energy is Dissipated by Resistance: P=i2R. This acts like friction!! N S
spark 12V http://www.familycar.com/Classroom/ignition.htm Start Your Engines: The Ignition Coil • 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 even work at all!? • 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
Start Your Engines: The Ignition Coil Transformer: P=iV
30.6: Induced Electric Fields, Reformulation of Faraday’s Law: Consider a particle of charge q0moving around the circular path. The work W done on it in one revolution by the induced electric field is W =Eq0, where E is the induced emf. From another point of view, the work is Here where q0E is the magnitude of the force acting on the test charge and 2pr is the distance over which that force acts. In general,
dA Changing B-Field Produces E-Field! B • We saw that a time varying magnetic FLUX creates an induced EMF in a wire, exhibited as a current. • Recall that a current flows in a conductor because of electric field. • Hence, a time varying magnetic flux must induce an ELECTRIC FIELD! • A Changing B-Field Produces an E-Field in Empty Space! To decide direction of E-field use Lenz’s law as in current loop.
Path I: Path II: Example R2 R1 • The figure shows two circular regions R1 & R2 with radii r1 = 1m & r2 = 2m. In R1, the magnetic field B1 points out of the page. In R2, the magnetic field B2 points into the page. • Both fields are uniform and are DECREASING at the SAME steady rate = 1 T/s. • Calculate the “Faraday” integral for the two paths shown. B1 I B2 II
magnetic field lines electric field lines Solenoid Example B • A long solenoid has a circular cross-section of radius R. • The magnetic field B through the solenoid is increasing at a steady rate dB/dt. • Compute the variation of the electric field as a function of the distance r from the axis of the solenoid. Next, let’s look at r > R: First, let’s look at r < R:
i i Added Complication: Changing B Field Is Produced by Changing Current i in the Loops of Solenoid! E(r) r r = R Solenoid Example Cont. B E
Summary Two versions of Faradays’ law: • A time varying magnetic flux produces an EMF: • A time varying magnetic flux produces an electric field:
30.7: Inductors and Inductance: An inductor (symbol ) can be used to produce a desired magnetic field. If we establish a current i in the windings (turns) of the solenoid which can be treated as our inductor, the current produces a magnetic flux FBthrough the central region of the inductor. The inductance of the inductor is then The SI unit of inductance is the tesla–square meter per ampere (T m2/A). We call this the henry (H), after American physicist Joseph Henry,
Inductors: Solenoids Inductors are with respect to the magnetic field what capacitors are with respect to the electric field. They “pack a lot of field in a small region”. Also, the higher the current, the higher the magnetic field they produce. CapacitanceC how much potential for a given charge: Q=CV InductanceL how much magnetic flux for a given current: Φ=Li Using Faraday’s law: Joseph Henry (1799-1878)
loop 1 loop 2 (30–17)
i (a) 50 V (b) 50 V Example • The current in a L=10H inductor is decreasing at a steady rate of i=5A/s. • If the current is as shown at some instant in time, what is the magnitude and direction of the induced EMF? • Magnitude = (10 H)(5 A/s) = 50 V • Current is decreasing • Induced EMF must be in a direction that OPPOSES this change. • So, induced EMF must be in same direction as current