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Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. A Ca2+ cation and an O2- anion are separated by a distance of 2.4 Å. Calculate the resultant dipole moment. (Charge on an electron = 1.6 × 10-19 C)

  2. Consider a capacitor in a computer power supply, possessing a capacitance of 2200 μF. If a voltage of 10 V is applied to this capacitor, what will the charge on the positive plate be? (2 sig figs)

  3. In which of the cases below does A have a higher static dielectric constant than B, assuming that both A and B are dielectrics? (note that more than one answer may be correct)

    a A has a higher permittivity than B.
    b A is a non-polar gas; B is a polar gas.
    c Two capacitors, X and Y, have identical geometry. Capacitor X contains a sample of A and has a capacitance of 200 nF. Capacitor Y contains a sample of B and has a capacitance of 0.6 μF.
    d A is a sample of water at a temperature of 20 °C; B is a sample of water at a temperature of 80 °C.
    e An electric field of strength 1000 V m-1 passes through both A and B. The field strength is reduced less on passing through A than it is on passing through B.

  4. Under what conditions is the refractive index related to the dielectric constant by κ ≈ n2 ?

    a Any dielectric at low frequency, non-polar dielectrics at all frequencies
    b Any dielectric at optical frequency, non-polar dielectrics at all frequencies
    c Any dielectric at low frequency, polar dielectrics at all frequencies
    d Any dielectric at optical frequency, polar dielectrics at all frequencies

  5. A polar liquid is subjected to an alternating current at 50 Hz. The current frequency is then increased to just above the relaxation frequency of the orientational mode of polarisation. Which of these best describes the behaviour of the dielectric constant as the frequency is increased?

    a It remains roughly constant.
    b It gradually decreases.
    c It remains roughly constant until the relaxation frequency of the orientational mechanism is reached, at which point it drops sharply.
    d It gradually decreases until the relaxation frequency of the orientational mechanism is reached, at which point it drops sharply.

  6. And which best describes the behaviour of the dielectric loss as the frequency is increased?

    a It gradually decreases.
    b It gradually increases.
    c It shows a dip around the relaxation frequency of the orientational mechanism.
    d It shows a peak around the relaxation frequency of the orientational mechanism.

  7. You need to make a capacitor that will operate at low electric field strengths and store a large quantity of charge. Energy efficiency does not need to be high (i.e. loss can be tolerated). Which of the following are you most likely to place between the capacitor plates?

    a Nothing - I would use an empty capacitor.
    b A ceramic with a non-centrosymmetric structure.
    c A polymer with polar chains and an amorphous structure.
    d A polymer with polar chains and a crystalline structure.


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