By H. Cotton D.Sc. (auth.)

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Extra resources for Basic Electrotechnology

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Which is required to send the current through that portion of the circuit included between those points. f. d. V. The symbol for the unit is V. Resistance The SI unit of resistance is called the ohm and is defined as follows: The ohm is the resistance between two points of a conductor when a constant difference of potential of one volt, applied between these points, produces in this conductor a current of one ampere, this conductor not being the source of any electromotive force. The symbol for the entity is R or r and for the unit n.

It is obvious that the total resistance must be equal to the sum of the separate resistances ; the proof is as follows. f. This must be equal to the sum of the individual voltage-drops V= V1 + V2 + V3 + V4 This is not a law of universal application since, in general, it only holds when the current is constant in magnitude, that is does not change from instant to instant. We are only concerned with steady currents and so the equation applies. Applying Ohm's law to the complete circuit, calling its resistance R, we have V =JR.

D. When resistance has to be overcome in order that motion may take place a force must be applied and work done. Thus, if water is to flow along a horizontal pipe which offers resistance to the flow it is necessary to apply a force, in this case the water pressure. -2* 34 BASIC BLBCTROTBCHNOLOGY when electrons are made to flow along a wire some kind of resistance is offered by the wire; for example electrons collide with the metal atoms and with one another, this impedance to motion being equivalent to a resistance.