## Alternative Homework Assignment: Tuning a Radio

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# Alternative Homework Assignment: Tuning a Radio

Part II:

a) Now consider changing the station. In order to do this you must change the resonant frequency. Change the capicitance of your circuit so that it tunes in a different station and check to see if the impedence at adjacent stations' frequencies is large enough to supress these stations' signals.

b) Would the effect have been different if you had changed the inductance of the circuit rather than the capacitance? If so, how?

c) If a variable capacitor is used to change the station, what range of capacitance must this capacitor have in order to reach the entire A.M. dial? How about the F.M. dial?

 Part III: a) Once a particular broadcast has been seperated from the other radio sources, the radio must read the information that is contained in the signal and translate this into a series of electrical impulses that are to sent to the speaker in order to changed to sound. The exact method for doing this is beyond the scope of this problem, but we can look at how the speaker changes these impulses into sound. The components of a loudspeaker are shown disassembled in the figure to the right. A pair of permanent magnets create a magnetic field that exerts forces on the current in the voice coil. The voice coil is attached to the speaker cone and the magnetic forces cause the cone to vibrate in a way that matches the vibrations of current in the voice coil. What is the net force on a current loop in a uniform magnetic field? b) The magnetic force on the voice coil arises because the magnetic field at the coil is not uniform. Suppose a voice coil in a loudspeaker has 40 turns of wire and a diameter of 1.8 cm, and the current in the coil is 0.8 A. Assume the magnetic field at each point of the wire of the coil has a constant magnitude of 0.2 T and is directed at an angle of 60º outward from the normal to the plane of the coil. Let the axis of the coil be the y direction. The current in the coil is in the direction shown in the figure below (counterclockwise as viewed from a point above the coil on the y-axis). Calculate the magnitude and direction of the resultant magnetic force on the coil. c) Explain how this system converts electric impulses to sound waves.
 Work supported in part by NSF grant DUE-9455561

These problem written and collected by K. Vick, E. Redish, and P. Cooney. These problems may be freely used in classrooms. They may be copied and cited in published work if the Activity-Based Physics (ABP) Alternative Homework Assignments (AHA's) Problem sight is mentioned and the URL given.

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Comments and questions may be directed to E. F. Redish