Acar is moving towards a student at constant speed. the student notices that the sound the car makes gets louder and louder as the car approaches. the student suggests that the car sounds louder as it approaches because of the doppler effect that he learned about in science class this week. how would you respond to the above statement made by the student? a) the student has stated a valid conclusion for his observation. b) the student has drawn a poor conclusion because the car does not get louder. c) the student has drawn a poor conclusion because the doppler effect is about frequency and not amplitude. d) the student has drawn a poor conclusion because the doppler effect is about amplitude and not frequency.

Search coils and credit cards. one practical way to measure magnetic field strength uses a small, closely wound coil called a search coil. the coil is initially held with its plane perpendicular to a magnetic field. the coil is then either quickly rotated a quarter-turn about a diameter or quickly pulled out of the field. (a) derive the equation relating the total charge q that flows through a search coil to the magnetic-field magnitude b. the search coil has n turns, each with area a, and the flux through the coil is decreased from its initial maximum value to zero in a time ∆t. the resistance of the coil is r, and the total charge is q = i∆t, where i is the average current induced by the change in flux. (b) in a credit card reader, the magnetic strip on the back of a credit card is rapidly “swiped” past a coil within the reader. explain, using the same ideas that underlie the operation of a search coil, how the reader can decode the information stored in the pattern of magnetization on the strip. (c) is it necessary that the credit card be “swiped” through the reader at exactly the right speed? why or why not?