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19.5 Capacitors and dielectrics  (Page 6/13)

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The two hydrogen atoms in the water molecule subtend an angle of one hundred and four point five degrees with oxygen at the center. This is a schematic arrangement of hydrogen and oxygen atoms in the water molecule. The molecule is polarized, with the electrons attracted more to the nucleus of the oxygen atom than toward the nuclei of the hydrogen atoms.
Artist’s conception of a water molecule. There is an inherent separation of charge, and so water is a polar molecule. Electrons in the molecule are attracted to the oxygen nucleus and leave an excess of positive charge near the two hydrogen nuclei. (Note that the schematic on the right is a rough illustration of the distribution of electrons in the water molecule. It does not show the actual numbers of protons and electrons involved in the structure.)

Phet explorations: capacitor lab

Explore how a capacitor works! Change the size of the plates and add a dielectric to see the effect on capacitance. Change the voltage and see charges built up on the plates. Observe the electric field in the capacitor. Measure the voltage and the electric field.

Capacitor Lab

Test prep for ap courses

Two parallel plate capacitors are otherwise identical, except the second one has twice the distance between the plates of the first. If placed in otherwise identical circuits, how much charge will the second plate have on it compared to the first?

  1. four times as much
  2. twice as much
  3. the same
  4. half as much

(d)

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In a very simple circuit consisting of a battery and a capacitor with an adjustable distance between the plates, how does the voltage vary as the distance is altered?

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A parallel plate capacitor with adjustable-size square plates is placed in a circuit. How does the charge on the capacitor change as the length of the sides of the plates is increased?

  1. it grows proportional to length 2
  2. it grows proportional to length
  3. it shrinks proportional to length
  4. it shrinks proportional to length 2

(a)

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Design an experiment to test the relative permittivities of various materials, and briefly describe some basic features of the results.

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A student was changing one of the dimensions of a square parallel plate capacitor and measuring the resultant charge in a circuit with a battery. However, the student forgot which dimension was being varied, and didn’t write it or any units down. Given the table, which dimension was it?

Dimension 1.00 1.10 1.20 1.30
Charge(µC) 0.50 0.61 0.71 0.86
  1. The distance between the plates
  2. The area
  3. The length of a side
  4. Both the area and the length of a side

(c)

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In an experiment in which a circular parallel plate capacitor in a circuit with a battery has the radius and plate separation grow at the same relative rate, what will happen to the total charge on the capacitor?

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Section summary

  • A capacitor is a device used to store charge.
  • The amount of charge Q size 12{Q} {} a capacitor can store depends on two major factors—the voltage applied and the capacitor’s physical characteristics, such as its size.
  • The capacitance C size 12{C} {} is the amount of charge stored per volt , or
    C = Q V . size 12{C=Q/V} {}
  • The capacitance of a parallel plate capacitor is C = ε 0 A d size 12{C=e rSub { size 8{0} } A/d} {} , when the plates are separated by air or free space. ε 0 is called the permittivity of free space.
  • A parallel plate capacitor with a dielectric between its plates has a capacitance given by
    C = κε 0 A d , size 12{C=e rSub { size 8{0} } A/d} {}
    where κ is the dielectric constant of the material.
  • The maximum electric field strength above which an insulating material begins to break down and conduct is called dielectric strength.
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Practice Key Terms 6

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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