Digital Electronics: Magnetic Flux Density, AC vs DC, Transistors, Logic Gates, Memory and Modulation Techniques

Digital Electronics 1
DIGITAL ELECTRONICS
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Digital Electronics 2
1(a) In your own words and with the aid of a diagram describe the concept of magnetic
flux density.
[2 marks]
Also known as the magnetic induction, Magnetic flux density is the force or amount of flux
acting in a wire placed in a perpendicular direction per unit area. Its symbol is, B. Also, it is
the product of magnetic permeability and the magnetic field strength. A certain amount of
force gets subjected to electric charges moving in a magnetic field, hence, the equation
representing the magnetic flux density is (Nasiri et al., 2012)
F = qv × B, where q =electric charge,
v = charge velocity,
× = vector product.
B = magnetic flux density at the charge position
The diagram below describes the magnetic flux density

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1(b) Two bar magnets of 50mm height and 50mm width at the pole faces are fixed in
position, with their respective N and S poles in close proximity. If the total flux is 0.1
webers, calculate the flux density between the two pole faces. Assume that fringing
effects are negligible; that is, all of the flux passes between the pole faces.
[2 marks]
B= total flux/area (Likharev, 2012)
Total flux= 0.1Wb
Area= 0.05*0.05
Hence, 0.1/(0.05*0.05)
= 40T
1(c) Why is more economical to distribute electricity over long distances using AC
rather than DC form? Please examine:
[3 marks]
In the nineteenth century, war broke out between the inventors of Alternating current and
direct current. There were a number of reasons on both sides of the divide as to which one
was the best, and it emerged that the alternating current, proved economical when
transmitting power over longer distances due to the following reasons
DC lacks an efficient technology which can easily cope up with the high voltage and its
stepdown to a lower voltage. On the other hand, the AC has a transformer which enabled the
stepdown. Since the DC lacks the incorporation of a transformer that could easily help in
converting the voltages, it implies that separate electrical lines are to be erected for
appliances which utilized different voltages. These task necessities for more wires and poles
which in the end is hazardous and wastes a lot of money hence non-economical (Della
Giovampaola & Engheta, 2014).

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Besides, to convert the DC power from one form to the other, one requires a huge
motor-generator set or spinning rotary converter, which, is very expensive in terms of the
acquisition, maintenance and efficiency (Tolpygo, 2016).
1(d) Using a figure, describe the magnetic field effects that are created by a current
flowing through a long straight conductor.
The current flowing in a straight conductor produces a magnetic effect which forms the
overall magnetic effect around the conductor. The magnetic field generated is in a
perpendicular direction to the current. The diagram below illustrates this

Digital Electronics 5
In your own words and with the aid of a figure, describe the right-hand rule.
This is a rule which technically applies for the generators by indicating the direction of the
current which is induced upon attaching a current carrying conductor in a circuit as shown
below. This rule is used to determine the direction of current in the windings of the generator
(Gabrys, 2011)
Why is it important that data cables are not run alongside power cables?
The data and power cables should not be run in parallel for the following reasons

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The data cables will easily pick up the AC frequency from the power cables through
inductive coupling. This leads to unbalancing and hence, creates noise on the receiving end.
This noise is known as the common mode and is usually rejected by the circuits (Fiori et al.,
2014).
The power cables will generate electromagnetic fields which create electromagnetic
interference to the data cables. This kind of interference is known as the crosstalk interference
and eliminating it is difficult, hence, it results in data errors at the receiving end.
One of the most common computer Ethernet cables is Cat5e UTP (i.e., unshielded
twisted pair). The lack of shielding reduces the cost and allows the cable to be more
flexible. However, research on what twisting the cable means and describe why it is
performed.
[3 marks]
Twisting the cable is an act of winding or intertwining two insulated conductors within the
same circuit for the purposes of eliminating the crosstalk interference. The major reason for
cable twisting is to limit the noise picked from external interferences or rather the
electromagnetic and crosstalk interference, which, in turn, helps in enhancing the
performance of the system by maintaining the signal levels constant (Wang et al., 2012).
2(a) Several different diode configurations are illustrated in Figure 2.1, where the
measured voltage is displayed at the anode and cathode terminals of the diode. Describe
whether the configurations (i)–(vi) are forward or reverse biased, and in each case
determine if the current I can flow in the direction that is indicated. The diode is a
silicon diode that has a forward-biased junction potential of 0.7V.