Physics 102 Lecture 11 Generators and Transformers Physics

Physics 102: Lecture 11 Generators and Transformers Physics 102: Lecture 11, Slide 1

Review: Magnetic Flux B = B A cos(f) al rm no f A Units is T m 2 = Wb f is angle between normal and B Physics 102: Lecture 11, Slide 2

Review: Induction • Faraday’s Law – Magnitude of induced EMF given by: • Lenz’s Law – If the magnetic flux ( B) through a loop changes, an EMF will be created in the loop to oppose the change in flux – EMF current (V=IR) additional B-field. • Flux decreasing => B-field in same direction as original • Flux increasing => B-field in opposite direction of original Physics 102: Lecture 11, Slide 3

Physics 102: Lecture 11, Slide 4

Review: Rotation Variables v, , f, T • Velocity (v): w – How fast a point moves. – Units: usually m/s r • Angular Frequency ( ): – How fast something rotates. – Units: radians / sec v v =v/r • Frequency ( f ): – How fast something rotates. – Units: rotations / sec = Hz f = / 2 • Period (T): – How much time one full rotation takes. – Units: usually seconds Physics 102: Lecture 11, Slide 5 T = 1 / f = 2 /

Generators and EMF is voltage! = B A cos( ) =B A cos( t) w eloop = - / t = B A sin( ) = B A sin( t) eloop = B A sin( t) = BA cos( t) flux is maximum when emf is 0 And vice versa Physics 102: Lecture 11, Slide 6 1 • v 2 w. AB x q v r e t -w. AB

ACT: Generators and EMF e = A B sin( ) • • • x q x x 1 2 3 At which time does the loop have the greatest emf (greatest / t)? Physics 102: Lecture 11, Slide 7

Physics 102: Lecture 11, Slide 8

Comparison: Flux vs. EMF Flux is maximum • – Most lines thru loop EMF is minimum – Just before: lines enter from left – Just after: lines enter from left – No change! x Flux is minimum – Zero lines thru loop EMF is maximum • x Physics 102: Lecture 11, Slide 9 – Just before: lines enter from top. – Just after: lines enter from bottom. – Big change!

Preflights 11. 1, 11. 2, 11. 3 • w Flux is ______at moment shown. v v x q = 30 r When q=30°, the EMF around the loop is: = 30 increasing decreasing not changing Physics 102: Lecture 11, Slide 10 e q

Generators and Torque e = A B sin( ) Voltage! Connect loop to resistance R use I=V/R: I = A B sin( ) / R Recall: t = A B I sin( ) = A 2 B 2 sin 2( )/R • w v x r Torque, due to current and B field, tries to slow spinning loop down. Must supply external torque to keep it spinning at constant w Physics 102: Lecture 11, Slide 11 q v

Physics 102: Lecture 11, Slide 12

Generator A generator consists of a square coil of wire with 40 turns, each side is 0. 2 meters long, and it is spinning with angular velocity = 2. 5 radians/second in a uniform magnetic field B=0. 15 T. Determine the direction of the induced current at instant shown. Calculate the maximum emf and torque if the resistive load is 4 W. e = NA B w sin(q) = = t = NI A B sin(q) = = = Physics 102: Lecture 11, Slide 13 w • v v x q

Power Transmission, Preflight 11. 5 A generator produces 1. 2 Giga watts of power, which it transmits to a town 7 miles away through power lines with a total resistance 0. 01 ohms. How much power is lost in the lines if the energy is transmitted at 120 Volts? P = Power delivered by generator through lines Find I? P = I 2 R Power lost in lines Find Plost? Large current is the problem. Since P=IV, use high voltage and low current to deliver power. If V = 12, 000 Volts, loose 0. 0001 Giga Watts! Physics 102: Lecture 11, Slide 14

Physics 102: Lecture 11, Slide 16

Transformers Key to efficient power distribution Increasing current in primary creates an increase in flux through primary and secondary. e iron ~ Vp Vs Same / t NP (primary) Energy conservation! Physics 102: Lecture 11, Slide 17 I p V p = I s. V s NS (secondary) R

Preflight 11. 6 The good news is you are going on a trip to France. The bad news is that in France the outlets have 240 volts. You remember from P 102 that you need a transformer, so you wrap 100 turns around the primary. How many turns should you wrap around the secondary if you need 120 volts out to run your hair dryer? iron 1) 50 2) 100 3) 200 e ~ Vp NP (primary) Physics 102: Lecture 11, Slide 18 Vs NS (secondary) R

ACT: Transformers depend on a change in flux so they only work for alternating currents! A 12 Volt battery is connected to a transformer that has a 100 turn primary coil, and 200 turn secondary coil. What is the voltage across the secondary after the battery has been connected for a long time? 1) Vs = 0 2) Vs = 6 Physics 102: Lecture 11, Slide 19 3) Vs = 12 iron Vp NP (primary) 4) Vs = 24 Vs NS (secondary) R

Physics 102: Lecture 11, Slide 20

Questions to Think About • In a transformer the side with the most turns always has the larger peak voltage. (T/F) • In a transformer the side with the most turns always has the larger peak current. (T/F) • In a transformer the side with the most turns always dissipates the most power. (T/F) • Which of the following changes will increase the peak voltage delivered by a generator – Increase the speed it is spinning. – Increase the area of the loop. – Increase the strength of the magnetic field. Physics 102: Lecture 11, Slide 21

Physics 102: Lecture 11, Slide 22