| a. |
With
the aids of a diagram, describe the bonds between the atoms in a pure semi-conductor.
Explain why the electrical conductivity of a pure semi-conductor increases
as temperature increases. |
4
marks |
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Each silicon atom has four outer shell electrons,
and each electron participates with electrons from neighbouring atoms in
forming bonds that hold the crystal together. As these electrons do not
move about, pure silicon is not a good conductor of electricity at low
temperature. |
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At high temperature,
however, the agitation of the atoms is higher due to increase in kinetic
energy. Some energetic electrons are able to escape from the lattice against
the electrostatic attraction. Conduction of electricity is then increased. |
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| b. |
By
introducing certain impurities into a pure semi-conductor, the electrical
conductivity would also increase. Explain this by describing the electricity
conduction in an n-type semi-conductor. |
4
marks |
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An n-type semi-conductor is the pure semi-conductor
doped with impurities which have five outer-shell electrons, like phosphorus. |
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Only four out of five electrons of the impure
atom fit into the crystal structure.The fifth electron does not fit in
and is relatively free to diffuse throughout the crystal. |
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As a result, the impure semi-conductor has
negative charge carriers. |
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| c. |
A
d.c. power pack converts the alternating mains electricity at 220V 50Hz
into smooth steady form at 12V d.c. Using suitable diagrams, describe the
uses of
i)
the transformer
in the d.c. power pack. Exact numerical values for the components used
are not required, but the principles in choosing suitable values for such
components are expected. |
7
marks |
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2 |
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The transformer is used to step down the voltage to a low
value. |
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The turn ratio should follow: |
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ii)
the diode bridge consisting of four diodes |
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The diode bridge is used to rectify the a.c. so that current
flows only in one direction rather than both. |
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iii)
the storage capacitor |
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The storage capacitor has a large capacitance. It can be
charged quickly. But, when discharges through the load, it performs slowly.
Thus, there is a considerable high output voltage across it throughout. |
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iv)
the filter circuit |
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The filter circuit is to eliminate the fluctuating components
of the voltage output from the storage capacitor. The inductor has a high
inductance which blocks the a.c. components and allowing the d.c. components
to flow through the load. |
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| d. |
Describe
how a highly stable d.c. voltage could be obtained by connecting a zener
diode across the output of a d.c. power pack. |
1
mark |
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An zener diode is operated in reverse bias. It has a constant
p.d. across it for whatever applied voltage greater than its breakdown
voltage. |
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