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Homework Help: Electrical Energy and Capacitance

  1. Mar 9, 2004 #1
    2 problems. I was just wondering if someone can help me start off the problem. The rest, I'll try to do because I don't know how to setup the problem.

    55) A virus rests on the bottom plate of oppositely charged parallel plates in the vacuum chamber of an electron microscope. The electric field strength between the plates is 2.00 x 10^5 N/C and the bottom plate is negative. If the virus has a mass of 1.00 x 10^-15kg and suddenly acquires a charge of -1.60 x 10^-19C, what are its velocity and position 75.0ms later? Do not disregard gravity.

    Answer: v=1.67 m/s upward
    y=6.24cm above bottom plate

    59) The immediate cause of many deaths in ventricular fibrillation, uncoordinated quivering of the heart as opposed to proper beating. An electric shock to the chest can cause momentary paralysis of the heart muscle after which the heart will sometimes start organized beating again. A defibrillator is a device that applies a strong electric shock to the chest over a time of a few milliseconds. The device contains a capacitor of several microfarads, charged to several thousand volts. Electrodes called paddles, about 8cm across and coated with conducting paste, are held against the chest on both sides of the heart. Their handles are insulated to prevent injury to the operator, who calls, "Clear!" and pushes a button on one paddle to discharge the capacitor through the patient's chest. Assume that an energy of 300W·s is to be delivered from a 30.0µF capacitor. To what potential difference must it be charged?

    Answer: 4.47kV
  2. jcsd
  3. Mar 10, 2004 #2
    For question 55, you need to find the net force acting on the virus.
    You know that the force from the electrical field will cause the virus to go up, since it is negatively charged it will be attracted to the positive plate. It will also be acted upon by gravity pulling it down.
    Net Force = (E field Force) - (Force of Gravity)
    F = qE - mg

    Once you have this force, you know that F=ma. Manipulate this to solve for a:
    a=F/m will give you the acceleration of the virus.

    Now use kinematic equations to solve for the final velocity and final position after 75.0ms.
    Vf=Vi + a(t) for final velocity
    Yf=Yi + Vi(t) + .5a(t)^2

    Try it and let us know what you get.

    I will try to get to your second question later.
  4. Mar 10, 2004 #3

    Doc Al

    User Avatar

    Staff: Mentor

    You need to know the energy stored in a charged capacitor. This can be expressed as function of the capacitance and the voltage. Look it up.
  5. Mar 11, 2004 #4

    F = (2x10^5 N/C)(1.6x10^-19) - (9.8m/s^2)(1 x 10^-15) = 2.22x10^-14N

    a = 2.22x10^-14N/(1x10^-15kg) = 22.2m/s^2

    Vf=(22.2m/s^2)(.075s) = 1.67 m/s upward

    Yf= .5(22.2m/s^2)(.075s)^2= .0624m = 6.24cm

    Thanks for the help! I appreciate it.
  6. Mar 11, 2004 #5
    Good job, looks like you nailed 55.

    How about 59?
  7. Mar 11, 2004 #6


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    Homework Helper

    Doc Al already told you. There is a formula that you can look up. It literally gives you exactly the energy stored in a cap given the capacitance and voltage on the cap. You should assume, given the way the question was stated, that all of the energy is used.
  8. Mar 12, 2004 #7
    Right...thanks...except my question was directed towards ScoutFCM who originally started this post. Scout posted his/her solution for #55, so I wanted to know if he/she got #59 as well.
  9. Mar 14, 2004 #8


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    Homework Helper

    Oh, sorry about that. I need to start paying more attention.
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