Ohm's Law?

[Tim Mackey]

I think I will find I am lost

[Brian Parker]

Timbo,

Quite often the best position to be in.

[Former Member]

[This posting has been removed]

[Rob Cope]

Thanks Brian,

I've used capactiors loads, but inductors have always been a bit... well... scary... all those volts on disconnecting and rarity of experience.

Regards, Rob 

[Tony Richardson]

Gotta go with Eric on this one, way too technical for me,  my simple mantra ( motor windings kv being constant ) is more volts = less prop or more volts + ruducing gearbox = same or larger prop, check your amp draw with watt/amp meter of your choice and do not try and push 60 amps through a 40amp ESC and we should be able to go OHM happy at the end of the day.

[Kelly]

Yawn 

Intellectual anorak attack .

[Ian Mathieson]

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.

[Chris Bott - Moderator]

Well said Ian I believe that is what most of us are thinking, but maybe not in such a clear manner.

[Tim Mackey]

Yes thanks Ian for further backing up my original piece... I believe the editorial team have some sort of response planned, but quite what I await to discover

Since this thread strayed way from the original topic, and flew into an area of unknown mathematics for a simple soul like me, I have not recently particiapated, it remains just to say that I always try and write my pieces ina simple way, as I believe the majority of readers and indeed modellers have neither the technical knowledge or desire to obtain it t complicated levels.

I simply ( or so I thought ) explained to the ed ( with prior discussion I hasten to add ) that increasing from a 4 cell to 5 cell would not be the holy grail that he thought it might be, and that doing so could  "potentially" ( dreadful pun intended ) worsen his problems - if capacity and ability to deliver same was not increased as the higher voltage would ....in the main...... slightly INCREASE the current drawn.  Mr Annells complicated what was simple explanation by introducing a further formula of power equations which were incorrect in that the power in said circuit WILL by necessity have to increase as the voltage and subsequebnt current will do so. I never said that the power consumption would remain the same.

  Ah well.....some  folks eh

[Ian Mathieson]

Many thanks for your support, Chris. Glad to know that my modest diatribe was clear and intelligible - its all to easy to lose clarity when writing this sort of thing, so that it makes sense to the person who wrote it but confuses others.

[Ian Mathieson]

Ahh, Timbo - sanity is restored at last!

[Ivor Tucker]

I am double double confused.com and just want to fly my plane. I have been contemplating going electric but all this mumbo jumbo has convinced me. Leave it alone.

[Brian Parker]

Ian,

Are you inferring that the contributors, other than yourself,  to this thread are insane?

[William Pead 2]

My professor of mechanical engineering said that ANYBODY who believes in an intangible fluid that flows along solid pipes is necessarily one sandwich short of a picnic.

[Andy Gates]

Ivan

 Please don't skip electro flight because of a thread like this one.

The simplest answer is to measure the current draw with an inline meter which can be sources at quite a reasonable cost and will save any damage to components.

Although it does sometimes seem very confusing, once a basic grasp on the requirements is achieved then all will be well and some great flying can be had.

[Peter Beeney]

Is this Real, or what?

Gentlemen, now I’m confused again.   At great personal risk, can we go back to the impedance bit, the expression impedance = resistance + reactance. I would say that we can satisfactorily conclude that this expression relates to an AC circuit, containing resistance, capacitance, inductance, and is also influenced by the applied frequency.

 I think the reciprocal can be shown to be admittance = conductance + susceptance, but not by using the product over sum formula. The impedance expression is, I think, a complex quality, the resistance part is a real number, with the reactance part containing a j-operator, which makes it an imaginary number. So we cannot multiply the resistance by the reactance. The admittance expression is also likewise a complex number, with the same j-operator in the susceptance part. So, likewise we cannot multiply the conductance by the susceptance.  Also both the resistance part and the reactance part can be shown, (and conductance) + (susceptance), at least theoretically, to be equal to zero, so that again gives us problems.

I would consider too, that the product over sum only holds good for two parallel resistors, after that it simply becomes 1 / 1/r1 + 1/r2 + 1/r3…... 1/rN

I’m still hoping the drift here is again also imaginary, Ohm’s Law must still pop up in here somewhere, surely?  Or is that Hall’s Effect? Or even shades of Aeromodellers Nightmare perhaps?

Perplexed Pete.  

PS. Please don’t let this sort of rubbish put anyone off electric flight. This is all throwaway stuff! However, perhaps the more you know about the basic general principles, the easier it becomes to sort out the many problems.  

[Ivor Tucker]

Andy, dlete Ivan insert Ivor. You will never understand all this unless you CONCENTRATE.

 Pete, I thought i had it right, confirmed by the last few words of your PS. "to sort out the many problems". It all makes good reading though and very educational.

[Andy Gates]

Appologies for getting your name wrong Ivor.

Do you have to understand the electronics in your car, the molecular make up of the petrol you fill your car with, all the metalurgy in the engine, how the ABS works, the friction coefficient of the tyres on all the different road surfaces just to drive your car?

Of course you don't because someone else has worked it all out and providing you drive within the limits of the car and driver then all will be well.

 Same with model flying, some background may be advantageous but it is not a requirement to be able to fly.

Having done A level physics and part of a BTech in electronics I do understand and can see what is being said but I feel no need to interject on all the theory to be able to fly model planes. That is what I was trying to say, enjoy it for what it is.

[Peter Beeney]

 Ivor,

Perhaps you should take the reciprocal of the last few words and say “The more I know about this caper, the fewer problems I shall have.” Although it’s best to probably not bank on that too much! At least, not in the beginning anyway.

I don’t think it’s very educational, it’s not really relevant to any thing very much.

I believe there are threads on this forum aimed at beginners, etc. I’m sure someone can point you in the right direction.

Good Luck!

Pete.

[Tony Richardson]

Ivor don't give up on electric models just yet, as Andy and Pete state you do not have to know all this technical stuff to enjoy electric flight, I know  nowt of which they speak and do just fine.   Nigel hawes has a great column in the mag and does contribute to some of these threads, he makes it easy for the rest of us to enjoy electric flight by suggesting motor controller set-ups for many different models so hang in there and remember the watt meter is your friend.

[Ivor Tucker]

Andy, Peter and particularly Tony.

Thank you for your wise and very helpfull words. This is the first forum I have participated in and I now see the great value and learning one can acheive.

[Chris the gas]

electricity is only smoke traped in wires you only need to know when all the smoke has leaked out it stops working

[Chris Bott - Moderator]

Most of the smoke is kept in the speed controller, and some is in the motor, trust me, I've seen it come out of both. Smoke release is mostly caused by too big a prop and not knowing. The big secret is to use a meter of some sort, that way you know for certain the following things:- 1, The setup is carrying LESS current than the motor, ESC and battery are rated for. (at full throttle) 2, The setup is NOT pulling the battery voltage below a safe level for the battery. Say - 3V per cell for a LiPo (at full throttle) 3, Full throttle power is sufficient to fly the model. These days with highly efficient motors. 100W per lb is pretty sprightly. This is where it gets difficult to recommend a setup for a particular plane. Experienced electric flyers will either use a setup tried and tested previously, or have a good idea what to try, but then experiment and try different props to get the readings right. Sometimes even having to buy a second motor to get it right.

[Phil Taylor 3]

Dear Editor,I refer to the All Write e-mail from Kevin Annells in the October 2008 edition of RCM&E.When I was at college, I understood Ohm's Law to state that Current = Voltage divided by Resistance, Power = Voltage multiplied by Current or Power = Current squared multiplied by Resistance.Using the example given by Mr. Annells where there is 6 Watts power derived from 6 Volts and 1 Amp, the resistance of the circuit is 6 Ohms.In a circuit "of the same resistance", if the voltage is increased to 12 Volts, the Current increases to 2 Amps giving a Power of 24 Watts. (This may give a clue as to why 4s cells are used in preference to 3s cells for large models).I conclude therefore that the statement in "Give it a Whirl" was in fact quite correct.I think Ohm's Law still stands, and any attempt by Mr. Annells to reduce it to "Ohm's Theory" is to be ignored.Best Regards,Phil Taylor.

[Robert Carey]

I read with interest all the theories and counter theories relating to Ohms law.

At the end of the day can this information be utilized to  provide guidance in making a selection of motor and battery combination. Many specifications given for electric motors ( Chinese origin ) only indicate the max current the motor can withstand. No other information is provided other than dimensional sizes.

If power equates from voltage multiplied by current then can it be assumed that if i select a motor with say a maximum rated current of 10 Amps i can expect 100 watts of power if say 10 volts is applied and 150 watts if say 15 volts is applied. If this is true and efficiency needs to be included in the formula then selecting a motor based on max current may be an easier way to calculate a motor /battery combination.

Then comes the question on prop size versus current draw.

Regards

Rob Carey