Peoples Physics Concepts

15.1. Capacitors http://www.ck12.org

Simulation

Capacitor Lab (PhET Simulation)

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1. Design a parallel plate capacitor with a capacitance of 100 mF. You can select any area, plate separation, and
   dielectric substance that you wish.


2. Show, by means of a sketch illustrating the charge distribution, that two identical parallel-plate capacitors
   wired in parallel act exactly the same as a single capacitor with twice the area.


3. A certain capacitor can store 5 C of charge if you apply a voltage of 10 V.
   a. How many volts would you have to apply to store 50 C of charge in the same capacitor?
   b. Why is it harder to store more charge?


4. A capacitor is charged and then unhooked from the battery. A dielectric is then inserted using an insulating
   glove.
   a. Does the electric field between plates increase or decrease? Why?
   b. did it take negative work, no work or positive work when the dielectric was inserted? (.e. did it get
   ’sucked in’ or did you have to ’push it in’ or neither) Explain.
   The capacitor is now reattached to the battery
   a. Does the voltage of the capacitor increase, decrease or stay the same? Explain
   b. The dielectric is now removed from the middle with battery attached. What happens?

Answers to Selected Problems

1..
2..
3. a. 100 V b. Because as charges build up they repel each other from the plate and a greater voltage is needed
to create a stronger electric field forcing charge to flow
4. a. decrease b. negative work (’sucked in’) c. Goes back up to what it was before inserted dielectric d. V
is constant, since capacitance goes down, charge must go down. There’s probably a discharge across the
capacitor.