Please let me know if my answers are not correct/not thorough enough.
1) Q:What happens when the number of loops in the armature of a generator is increased?
1) A:More loops= more voltage
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2) Q: Why does a piece of iron behave like a temporary magnet when placed in a magnetic field?
2) A: Iron can become magnetic when placed in a magnetic field because of its crystalline structure. It has a property called ferromagnetism. When in a magnetic field the domains align in the same direction.
(I got this from a variety of sources, should it be worded different?)
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3) Why is an antenna designed to be one-half of the wavelength of the wave it is supposed to receive?
a. voltage is directly proportional to loops.
b. the domains are not perfect, some do not realign randomly when an imposing magnetic field is removed, thus, there is a residual net magnetism.
c. Antennas can be designed for any length, given a proper feed transformer. You are probably refering to a dipole, with each side one quarter wavelength. This is popular because it matches in impedance common transmission cables. It is also popular because it drives the transmission line with maximum current at the feed point.
Again, it is not necessary to design antennas this way, there are thousands of designs not based on the quarter wavelength concept. For instance, a J-Pole, one of my favorite for my ham radios....
http://www.hamuniverse.com/jpole.html
However the antenna is designed, it has to be matched to the transmission line for maximum signal to come thru. That is the design part of the problem. And at the radio end, the impedance of the cable has to match the input impedence of the radio. Any mismatch means loss of signal. Whomever asked this question is asking a complex question, like why is a motor vehicle designed with four wheels: in fact, some are not, and some have one wheel, and some have 18.