Why has DIYNOT got a electrics forum?

[slippyr4]

This post is not a "dig" as big_spark, I just decided to further clarify one of the points he could have explained better.

The Power in an electrical circuit is constant, this cannot change unless the parameters of the circuit are changed....

...Current cannot be changed unless either the resistance, capacitance, inductance or VOLTAGE in a circuit changes.

Lowering the voltage causes the current to rise, converseley increasing the voltage will cause the currrent to fall.

That's not true. If you only change one parameter in a circuit, the voltage, then as you lower the voltage the current will be lower (because the resistance is fixed).

Now, if you redesign your circuit with the goal of maintaining constant power, then if you lower the voltage you will need to lower resistance to have the same power. And it's here that the current will increase.

That's why a 1KW electric fire bought in the UK is only a 0.5 KW electric fire if you plug it in in the USA.

-------------------------------------------------

The current discussion of whether current or power kills is really 6 of one, half a dozen of the other.

If current flows through a resistance, power is dissipated. Flow a current through your heart, you flow power through your heart. You can't separate them.

HOwever, if you take a current sufficient to stop the heart, lets say 50 mA, if you were to double the current to 100 mA, you will dissipate twice the power in the heart. But you won't stop the heart twice.

Given that, I beleive it's more accurate to say that current kills. But it's both really, cos you can't have one without the other.

 

[slippyr4]

Going back to the arguments earlier the sparks involved should be ashamed of themselves quoting resistance when talking of AC circuits, now I want you to repeat after me I M P E D A N C E spells impedance

Not at all, it was deliberate. Since our hypothetical circuit was purely resistive, the impedance was resistance. This makes it less confusing, and more accessible for those whose electrical knowledge is lacking.

 

[ban-all-sheds]

Going back to the arguments earlier the sparks involved should be ashamed of themselves quoting resistance when talking of AC circuits, now I want you to repeat after me I M P E D A N C E spells impedance

In a lot of domestic situations there is little reactance to be concerned with.

As for the load causing so much grief here (the human body), is it reactive?

 

[kendor]

Going back to the arguments earlier the sparks involved should be ashamed of themselves quoting resistance when talking of AC circuits, now I want you to repeat after me I M P E D A N C E spells impedance

Not at all, it was deliberate. Since our hypothetical circuit was purely resistive, the impedance was resistance. This makes it less confusing, and more accessible for those whose electrical knowledge is lacking.

So you are saying that the human body's conductance doesn't have reactance? Your hypothetical circuit is a waste of time if you substitute the human body as a resistor instead of reality!

 

[kendor]

Going back to the arguments earlier the sparks involved should be ashamed of themselves quoting resistance when talking of AC circuits, now I want you to repeat after me I M P E D A N C E spells impedance

In a lot of domestic situations there is little reactance to be concerned with.

As for the load causing so much grief here (the human body), is it reactive?

in so far as the conductivity changing with frequency yes. therefore it has reactance to an ac circuit.

 

[slippyr4]

So you are saying that the human body's conductance doesn't have reactance? Your hypothetical circuit is a waste of time if you substitute the human body as a resistor instead of reality!

Not at all. In fact, I've always thought that the conductivity of the body was even more complex than that- the impedance is constantly variable, and in particular dramatically changes when voltage/frequency is applied to do biological events - it's too complex to model simply.

But: the majority of discussions here have been about how current changes with voltage, and for those purposes the hypothetical circuit server ideally.

 

[Igorian]

Igorian....I posted my example as that..an example to show that power in an electrical circuit is constant if no other factors influence the circuit.

the 230V in my example is because the mains supply is rated at 230V, so it is the current drawn from the mains if the PSU was operating at maximum load of 300W. The rest of what you bang on about is irrelevent to this discussion.

In your example you have changed the parameters, consequently the outcome of that circuit has changed. Equally if you changed the input voltage of the PSU in my example, all other parameters would change. I was trying to show Oilman that it is not power that kills as this is a product of the parameters of an electrical circuit, and whilst power remains constant in any electrical circuit unless other factors change, altering the voltage, as you have pointed out, will cause a change (on reflection perhaps the PSU was a bad choice of example after being awake for 17 hours!! and not giving a more indepth explanation).

P= VI, it is a product of these two parameters, Oilmans assertion was incorrect, although I can understand why he may think that way. Change either one, as I have said and did say, and you change Power, that is correct...BUT Power in constant in electrical circuit for a given voltage.

However my example is valid as an example of how power remains constant.

Mate you change your mind so often that it's difficult to know what you are 'banging', to use your phrase, on about. First you say that Power is constant regardless of voltage and then you say the opposite and then twist it to make others look wrong. I hope you never have to lecture anyone on the subject.

 

[pickles]

What the f$%K u all going on about in here. This is supposed to be general chat, not degree level electricity with added abuse go and have a lie down for gods sake

What kind of degree did you take pickles, if you think that i=V/R is so difficult?

A law degree

I certainly have great respect for that achievement, but are you serious when you say that I=V/R is too technical?

In case the answer is yes, the favourite analogy is a hosepipe....

The pressure of water at one end of the pipe is analogous to the potential difference, measured in Volts.
The resistance of the hosepipe to water that is otherwise willing to flow inside it, is analogous to, well, er, resistance (measured in Ohms).
The flow of water along the hosepipe, when you open the tap, is analogous to current, measured in Amps.

So, if you have a long hosepipe, and a short one, and the two are identical bore and constriction, it follows that the resistance of the longer one is greater, and the effect on the flow is to reduce it.

This is basically it - the flow is determine by the pressure and the resistance. In a simple electrical conductor (e.g. a wire), the flow of current (Amps) is determined by the 'pressure' (potential difference, in Volts) between one end of the conductor and the other, and the electrical resistance of the wire.

Thanks to various scientists over the decades, we have a means of calculating the current, without measuring it, if we know the Volts and the Ohms for a given conductor.

I (current) = V (Volts) divided by R (resistance)

Lo and behold, I=V/R.

BTW, I is used for current because C got reserved for Coulombs, which is a measurement of charge (related to current, but I'm heading off track).

If you were winding us up, then you probably regard my explanation as patronising, in which case please accept my apologies - I'm just trying to help.

It isn't now you have explained it. I was slightly over egging the cake out of frustration However that this thread from a non techies point of view is getting a bit tedious due to

a) everyone takin part having had their sense of humour bypassed for the occassion

b) It being basically unintelligible to a lay person

c) It not reflecting well on electricians, because if you can't agree amongst yourselves how can any one else rely on you. This rubs salt in the part P wound, because we have to get you lot in to at least check things we have been doing as competent DIYer's for years without any problems eg cables catching fire, gruesome deaths or the whole street shorting out. My experience of domestic electrics is that there are basic rules that have to be learnt and followed to the letter for safetys sake but that it is not rocket science There are many sources of good info
in books and such like and it is perfectly possible to do work that is as safe and compliant. Some of the strangest things I have seen have undoubtedly been done by professionals

d) it actually confirms the original gripe that started the thread which I agree with that it's hard to get straight advice in the electrics forum. You tend to get nothing, snide comments, over academic advice or suggestions that you should use an electrician. You need to give a straight answer or not answer at all. It is a common fault of people with technical knowledge that they are bad at explaning themselves or helping others, I should know, and it is on regular display in electrics. the other forums here generally are not like that and some of the subjects can be just as technical

ps I am not having a pop at anyone it's just that as a group your wires are hanging out (no pun intended)

 

[Bazdaa]

Hear hear (moan moan grunt) ORDER ORDER!

P.S. I might not be an electrician, but I am a techie, and still got lost in the thread!

Bazdaa

 

[oilman]

Hear hear (moan moan grunt) ORDER ORDER!

P.S. I might not be an electrician, but I am a techie, and got lost in the thread!

Bazdaa

Whose fault's that? You started it

 

[Softus]

Personal Opinion, your entitled to it, but I stand by my comment.

Your comment was that oilman's post was total ballony (sic.). This is not true, viz:

115V will not give rise to twice the current in a circuit that 230V will. It's just that you need twice the current to achieve the same power output.

Applying the same rationale to this scenario as you used against BAS, you must have been lying. I prefer to think that you were simply mistaken. You prefer to concede neither.

Softus, Why does the increase in current occur? The Power in an electrical circuit is constant, this cannot change unless the parameters of the circuit are changed.

I'm comparing the two scenarios of two different voltages being applied to identical circuits.

Look:

2000 @ 230V = 8.69A
2000 @ 115V = 17.39A

Current cannot be changed unless either the resistance, capacitance, inductance or VOLTAGE in a circuit changes.

Yes, quite so. So, if you get this, why do you not understand the arithmetic when the power changes? If I plug a 240V power drill, say, into a 220V supply, the current will be I1. If I take the same drill and plug it into a 110V supply, then the current through it will be I1/2 - half, not double. And the power halves too. Or could it be that you think the drill will run at the same speed? Is that what you think Big_Spark?

Lowering the voltage causes the current to rise, converseley increasing the voltage will cause the currrent to fall.

This is absolutely the opposite of what would happen - how do you not get this?

Read Oilmans statement again, it is electrically incorrect. The Potential Difference (Voltage) of a circuit is critical to the amount of energy (Q) required to do the same amount of work in the same amount of time. So if you want to achieve the same with less PD, then you cause an increase in the current, they are intrinsically linked together.

I'm not arguing about what's needed to maintain power output - that isn't central to my point at all. If you think that's central to oilman's point, then by all means argue it with him - we're two different people you know...

Still waiting for your reply to the FULL LOAD CURRENT question - you've gone uncharacteristically silent on that one!

No I have not, I have answered that question several times in various posts. The FULL LOAD CURRENT of a circuit is the maximum current that the circuit requires to operate or is capable of producing.

No - you haven't answered it. Firstly, the term "full load current" in your sentence is no different in meaning to the term "current", so I continue to question your use of it. Why are you persistently muddling something very simple with unnecessary words?

In the examples I gave where a person becomes part of the circuit, the FULL LOAD CURRENT is the maximum current that is capable of flowing with respect to the combined resistances in that circuit, ie, the normal circuit resistance AND the human body resistance as the human body is a limiting factor in this by virtue of the high resistance limiting the maximum current that would flow in the undamaged or unbridged circuit.

I don't disagree with that, but this is different to what you first wrote, viz:

..if you complete a circuit by making the connection ACROSS A BROKEN NEUTRAL, then the shock you recieve will be the current flowing in the circuit at that time..you will effectively become part of the circuit rather than a seperate circuit..(as with a Phase to neutral or Phase to earth shock)

No. It. Won't. Be. The. Same. It. Will. Be. Different.

You have never acknowledged this point to me. However, you later contradicted it as follows:

So, if the circuit has a 30 Ohm resistance on it, when the circuit is undamaged, the FULL LOAD CURRENT is 230V/30 = 7.66A (1761.8W)

However in the examples earlier, the Human now forms a bridge across the Neutral and completes and becomes part of the circuit. Now the TOTAL resistance in that circuit is 30 + 1500 (Average Human dry body resistance) = 1530 Ohms, so 230/1530 = 0.15A (34.5W). This is the FULL LOAD current of that circuit whilst the human forms the bridge, it is no different to putting a normal resistor of 1500 Ohms into the circuit.

Clearly - this is simple arithmetic, and you have written it correctly, therefore I concur.

Do you understand now Softus?

Do I understand Ohm's Law? Yes - I always did.

Do I understand you? No - nobody does at the moment.

 

[empip]

Blimey, after all that Tim didn't get the last word..

 

[Softus]

Blimey, after all that Tim didn't get the last word..

Come again pipme?

 

[empip]

Have done Mr C... Pip in reverse.

 

[ban-all-sheds]

Here we go again.

First, the incorrect statement.

Power is a constant regardless of voltage

Then people pointing out that it's wrong.

The word ballony comes to mind..........or is this just another of your unintended/unexplained collections of words.

Power is also not a constant, regardless of voltage and can be derived as P = V^2/R. Dunno where you guys get all this stuff from

Then B_S not agreeing, and persisting:

115V will not give rise to twice the current in a circuit that 230V will. It's just that you need twice the current to achieve the same power output.

Lowering the voltage causes the current to rise, converseley increasing the voltage will cause the currrent to fall. Read Oilmans statement again, it is electrically incorrect.

With some examples thrown in that don't help at all.

Do I understand you? No - nobody does at the moment.

Don't worry - I expect he'll be back at sometime to tell us we're all stupid, or obtuse, and that what he really meant was.......