Fundamentals of electronics - Electricity

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Electricity is a generic name given to a whole series of quantities and laws that we will shortly investigate. Every quantity we introduce shall have a letter which identifies it within the formulae and the circuits it appears in. The most important physical quantity is the current (I) which is measured in Ampere, generated by the flow of electrons through a conductor. The latter, like any physical element, is composed of atoms, an atom being a particle with a nucleus which has a certain charge, conventionally identified as "positive" and a certain number of electrons (with a negative charge) which orbit around the nucleus (clearly things are in fact a lot more complicated, but this being a sound engineering course and not a quantum mechanics course, we feel justified in simplifying things so as to aid us in our dissertations, so long as we don't digress too far from reality of course). The electrons are kept in position by the nucleus whose opposite polarity attracts them. In fact two elements of opposite polarity attract each other, whilst two elements with the same polarity repulse each other. The force with which these two poles attract each other varies depending upon the kind of atom (in other words, depending upon the kind of material we are considering): with metals the attractive force is very weak, whereas with insulating materials the attractive force is far more difficult to overcome. As we shall see, this is the reason why metals are excellent conductors whilst insulators are not. Let's consider a bronze cable, an excellent conductor, to which we'll apply, at each end of the cable, respectively a negative and a positive charge.

Fundamentals of electronics - Generation of a current

Generation of a current

Electrons belonging to the bronze atoms within the conductor, being of negative polarity, shall be attracted by the positive charge and repulsed by the negative charge applied at the cable's ends. Seeing that the attractive force is very weak in conductor materials, the electrons are "stolen away" from the nucleus, thus generating a flow of charges (q). The measuring unit for the amount of charge is the Coulomb (C). At this point we know enough to state that a current is the quantity of charge that travels through a conductor within a unit of time (1 second). Current is measured in Ampere[5 ].

Let's now give a better definition of the charges we have applied to the conductor. A local accumulation of positive or negative charges is called potential. By applying two different charges to the opposite far ends of a conductor, a difference in potential is generated, which is called voltage (V) and is measured in Volt. By applying a voltage to the far ends of a conductor, a flow of charge is generated, whose value depends upon the applied voltage and the characteristics of the conductor.

When the voltage (or the current) is constant in time, we name it continuous voltage (or direct current) whereas if it varies in time we call it alternate voltage (or alternate current). A classic example of alternate current is a normal household electric socket, in which we have an alternate voltage applied a sinusoidal rate with a frequency of 50 Hz and an amplitude of 220 Volt.

[5 ] As with all physics formulae, pay attention to not confuse the quantity unit symbol (I for current) with the symbol for its value (A for Ampere which gives the amount of current). We'll say that a current I measures 5 Ampere, for example.

By definition 1 Ampere is the current generated by a charge of 6.26x1018 electrons which passes through a conductor in 1 second. Translated into a formula: Q= I x t (Q=charge, measured in Coulombs, I=current, measured in Ampere, t=time, measured in seconds).


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