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7A Powered By Docstoc
					7A: Microphones, Filters,
Oscilloscopes, and


It stores electrons from a negative potential, and holes, or a positive charge
from a positive potential.

The unit of capacitance = farad.
A 1-farad capacitor can store one coulomb of charge at 1 volt.
A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons.

For an AC signal….

       Current flows until the capacitor is charged
       If the frequency is fast enough, the capacitor does not have enough time to
        charge, and does not impede flow of current
       If the frequency is low, the capacitor has time to completely charge. Once it is
        charged, current no longer flows.


coil of wire = electromagnet
     When current first starts flowing in the coil, the coil
      wants to build up a magnetic field.
     While the field is building, the coil inhibits the flow of
     Once the field is built, current can flow normally through
      the wire.

For an AC signal:
Consider the time it takes to build up a current and you can
see how resistance varies with frequency

The capacity of an inductor is controlled by four factors:

       The number of coils - More coils means more inductance.
       The material that the coils are wrapped around (the core)
       The cross-sectional area of the coil - More area means more inductance.
       The length of the coil - A short coil means narrower (or overlapping) coils, which
        means more inductance.

Putting iron in the core of an inductor gives it much more inductance than air or any non-
magnetic core would.

The standard unit of inductance is the henry. The equation for calculating the number of
henries in an inductor is:
                   H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)

The area and length of the coil are in meters. The term mu is the permeability of the core. Air
has a permeability of 1, while steel might have a permeability of 2,000.


Channel 1 = signal out of function generator
Channel 2 = signal out of filter

Record peak to peak amplitudes of both signals:
1) Figure out your cutoff frequencies from
measured component values
2) Measure Frequency response at:
     A couple of very low frequencies
     A couple of very high frequencies
     Several frequencies around the cutoff
So you can get a good bode plot…


New Extra Credit Question
Answer using the web pages:

10. How do capacitors work? Why is their impedance inversely proportional to

11. How do inductors work? Why is their impedance proportional to frequency?

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