Question

Due February 23 (Fri), 9:15 AM Chapter 21-4 Farady's Law of Induction 1. (10 points) A fixed 12-crn-diameter wire coil is perpendicular to a magnetic field 0.48 T pointing up. In 0.1 the field is changed to 0.20 T pointing down. What is the average induced emf in the coll? 2. (10 points) What is the direction of the induced current in the circular loop due to the current shown in each part of Fig. 1? Explain why 001- I constant I decreasing increasing - 一。 I decreasing Figure 1: Problem 2. Chapter 21-5 Generators 3. (10 points) The generator of a car idling at 1100 rpm produces 12.7 V. What will the output be at a rotation speed of 2000 rpm, assuming nothing else changes? Chapter 21-6 Back EMF and Torque 4 (10 points) The back emf in a motor is 72 V when operating at 1800 rpm. What would be the back emf at 1000 rpm if the magnetic field is unchanged? Chapter 21-7 Transformers 5. (10 points) A transformer is designed to change 110 V into 15,000 V, and there are 140 turns in the primary coil. How many turns are in the secondary coil? 6. (10 points) The output voltage of a 125-W transformer is 12 V, and the input current is 25 A. (a) Is this a step-up or a step-down transformer? (b) By what factor is the voltage multiplied? Chapter 22-2 EM Waves 7. (10 points) If the electrie ild in an EM wave has a peak magnitude of 0.93x10-2 Vim, what is the peak mag- nitude of the magnetic field strength? 8. (10 points) How long should it take the voices of astronauts on the Moon to reach the Earth? Explain in detail. Chapter 22-5 Energy in EM Wave 9. (20 points) The E field in an EM wave has a peak of 415 mV/m. What is the average rate at which this wave carries energy across unit area per unit time? Chapter 22-7 Radio, TV 10. (10 points) Estimate the wavelength for a 2.0-GHz cell phone transmitter.

Answer 1

1. Area of the wire coil = πd^2/4 =3.14*0.0144/4 =0.0113 m^2

Now change in magnetic flux = change in magnetic field*Area

= [0.48-(-0.20)]*0.0113 weber

= 0.007686 weber

Thus required emf induced = change un flux/time

= 0.007686/0.15 volt

= 0.0512 volt

=