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Homework Help: Max and min frequencies of a capacitor

  1. Jun 5, 2005 #1
    A variable capacitor with a range from 10 to 365 p is used with a coil to form a variable frequency LC circuit t0 tune the input to a radio.

    a. What ratio of max and min frequencies may be obtained with such capcitor.

    I used that [tex]\omega=\frac 1{\sqrt(LC)}[/tex] to get the ratio of max to min frequencies.

    b. If this circuit is to obtain frequencies from 0.54 MHz to 1.60 mHz, the rato computed is too large. By adding a capacitor in parallel, this range may be adjused. What should the capacitance of this added be?

    I said that [tex]C_{eq} = C_{old}+C_{new}[/tex]

    I have to compute what [tex] C_{new} [/tex]. From part 1, I know my [tex]C_{old} [/tex]. But, how do I find [tex]C_{eq}[/tex] first? I am thinking that I have to compute that using the frequencies, but how do I get L?
    Last edited by a moderator: Jun 5, 2005
  2. jcsd
  3. Jun 5, 2005 #2
    You're not doing anything to the coil, are you? Why would L change from what it was before adding the second capacitor?
  4. Jun 5, 2005 #3
    I thought that the L cancelled out when I was doing the ratio before. So, would I use L from part A? Do I solve for L?
  5. Jun 5, 2005 #4
    Use the earlier inductance and the frequencies the circuit is supposed to be able to deal with. Solve for C_eq and solve for C_new.
  6. Jun 5, 2005 #5


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    You are given [itex]\omega_1, ~\omega _2 [/itex]. You know [itex]C_{old}[/itex]. The unknowns are [itex]L,~C_{eq},~C_{new}[/itex].

    You have three equations in 3 unknowns...
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