Hi Guys,
I've only just found this Forum thread as a result of receiving the latest e-mail from the Ed. about the September Prize Draw. I'd already written to said Ed. with a reply to Kevin Annells' statements in 'All Write' but I thought it worth posting here for completeness even though the thread has digressed an almost infinite distance from the original topic. I wrote:
I have read the letter titled Ohm’s Law in the All Write section of the October edition of RCM&E and wish to state that Kevin Annells is decidedly incorrect in what he states:
He gives an extract from the August edition, quoting “Ohm’s Law states that current consumption will increase with a higher voltage when applied to a circuit of the same resistance” and then states that this is incorrect. He then supports his claim by giving an example based on maintaining a fixed power level, and not a fixed resistance, and as part of his claim, he states that Ohm’s Law describes that Current = Power/Voltage.
I feel compelled to put the record straight:
Mr Ohm formulated his law to state only that when a voltage difference V volts (or potential difference, to give it it’s original and correct name) is applied to an electrical conductor of Conductance G mhos, a current of I amps will flow such that I = G x V. Mr Ohm made no statement about Power in the circuit; this has since evolved by the process of manipulation and substitution in Mr Ohm’s equation using the basic definition that the power P is given by the product I x V, hence P = I x V.
In practice, it is harder to measure conductance than it is to measure resistance. As Resistance = 1/Conductance (or vice-versa) Ohm’s Law was re-written for convenient day-to-day use as I = 1/R x V which simplifies to I = V/R. Note the unit of resistance is the Ohm and the unit of Conductance is the Mho which is a re-arrangement of the word Ohm resulting from the fact that one is the reciprocal of the other.
In the original statement quoted above, the resistance is fixed, and the voltage is increased. If we use numerical values, and say, make R = 10 ohms and the voltage initially 20 volts, then I = 20/10 = 2 Amps.
If we now increase the voltage to say, 30 volts, and keep the resistance fixed as stated in the original article, I = 30/10 = 3 Amps.
Clearly, increasing the voltage results in a larger current, which is precisely what the original article stated.
Any argument against this, based on a fixed Power level is entirely invalid and incorrect and demonstrates a complete lack of understanding of the use of Ohm’s Law in a simple circuit. Sorry, Mr Annells, I don’t mean to be rude, but I do wish to put the record straight for the benefit of other readers.
Yours etc.