A bit of light reading - NOT!

Submitted: Friday, Mar 18, 2005 at 00:30
ThreadID: 21308 Views:2491 Replies:9 FollowUps:25
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Boys and Girls

I’ve noticed over time there appears to be some confusion regarding electricity and how it all comes together. I have no wish to be regarded as a clever dick … far from it, but I thought I’d throw these little thoughts together in the hope that they may clear up some common misconceptions. Let me state here and now I am no leccie bright spark and that being the case would welcome correction where deemed necessary.

First off … consider a water tank up on a stand, with a hose to ground level. At the bottom of the hose at ground level we have pressure, right??

Now one of the primary laws of physics is that energy cannot be created or destroyed, in other words it is merely changed into another form.

So, getting back to the water tank, we have pressure at the bottom of the hose, this is referred to as potential energy, or in other words it has the potential to do work, such as in a hydroelectric scheme in a bigger picture, where the water pressure (energy) is converted to electricity (energy).

OK you say, what has that got to do with electricity?? Fair enough. Now think of a 10mm hose connected to the tank and also a 20mm hose. Which is going to carry the most water?? …. Correct. Now if you put a pressure gauge at the end of both hoses you would get the same reading .. same head of water, same pressure, get my drift?? Same as if you connected a long hose to your tyre, you’d get the same pressure at the end of the hose as at the valve.

However the 20mm hose will carry far more water, or do far more work (depending on your point of view). So, why is this? The 20mm hose offers less resistance than the 10mm, in this instance it’s less friction losses (an area separate to our discussion, but relevant in the big picture).

Now getting back to electricity, it’s the same story. A battery has potential difference between the two poles … the potential to do work. In this instance it’s called volts, but it means the same thing.

If you hook up 2 wires of greatly differing diameter to the +’ve and check one against the other, back to the –‘ve with a voltmeter you will get the same result, let’s say 12 volts for the purpose of the discussion. The voltage, or potential difference remains constant, however the capacity to do work (carry current) varies greatly.

Let’s assume a wire has a resistance of 1 ohm/metre. V(voltage)=i(amps)*R(resistance)

So i=V/R … 1 metre of wire at 1 ohm/m means 12/1=12 amps capacity
2 metres of wire means 12/2=6 amps capacity.

The longer the wire the less the capacity to do work, due to losses .. in this instance resistance, rather than friction, as in a water hose.

Now P(power or watts)=V*i. Remembering V stays constant at 12, we get an increasing value of P as i increases. If i=6 amps then P=12*6 = 72 watts, but if 1=12amps then P=12*12 = 144 watts.

Resistance in conductors (wires) reduces significantly with increase in diameter .. check the size of your battery cables. The bigger the wire, the less the resistance, the greater the capacity to do work.

This will hopefully explain why you need large diameter wires (say 6mm2) for your fridges, to ensure the losses are reduced to a minimum.

Now let’s consider 3 way fridges. The heating element is a large resistance, that’s why it gets hot, much the same as your toaster.

In this instance the value of R is significant. Let’s assume it’s rated at 160 watts, which is about average. P = V*i or i = P/V.

160/12 = 13.33 amps. But let’s say you battery has run down to 10 volts. 160/10 = 16 amps. Because the resistance is fixed and constant, the more your battery discharges, the greater the current draw. A vicious circle until the battery is stuffed.

I hope the above is of interest to some and apologise to those who find all a bit of a yawn.

Cheers
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Reply By: Member -Dodger - Friday, Mar 18, 2005 at 00:36

Friday, Mar 18, 2005 at 00:36
Thanks for that.

But I don't know the Alphabet.
I used to have a handle on life, but it broke.

Cheers Dodg.

Lifetime Member
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AnswerID: 102857

Reply By: Wok - Friday, Mar 18, 2005 at 07:30

Friday, Mar 18, 2005 at 07:30
Rosco,

Excellent effort, unfortunately IMHO there are flaws in your assumption:

1. [Q] Now let’s consider 3 way fridges. The heating element is a large resistance, that’s why it gets hot, much the same as your toaster[/Q]
==should be LOW resistance

2. [Q] 160/12 = 13.33 amps. But let’s say you battery has run down to 10 volts. 160/10 = 16 amps. Because the resistance is fixed and constant, the more your battery discharges, the greater the current draw. A vicious circle until the battery is stuffed [/Q]

Your logic is correct IF the fridge has electronics to provide constant current to the heater element. This is where I believe your assumption is wrong.

The power consumption is a variable. From your analogy of the water tank, the lower battery voltage is equivalent to a lower head in the watertank[as the waterlevel drops]. Hence there is less water pressure = less flowrate

Hence......I=10[V] X R
and..........P[new] = 10 X 10 X R
c.f............P[original] = 12 X 12 X R

The current drops as the supply voltage drops = power consumption drops.

cheers
AnswerID: 102864

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 08:52

Friday, Mar 18, 2005 at 08:52
G'day Wok

Can't agree mate.

1. If it was low resistance, it wouldn't get hot. If it offered no resistance the current would slip straight through, without doing any work, much the same as battery cables.

2. I don't reckon electronics need be involved. The power output remains constant, as it is merely a resistance of a fixed value. Hence if R is constant, decreasing V equates to increasing i.

The power consumption won't drop, the current draw increases.

Good healthy debate hey??

Cheers
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Follow Up By: Jimmy - Friday, Mar 18, 2005 at 10:40

Friday, Mar 18, 2005 at 10:40
Wok is correct

The lower the resistance the higher the current

I=V/R
Current = volts divided by resistace

current flow = heat

Drop a shifter across your battery terminals (extremely low resistance almost a dead short) and see how hot it gets!

My 90a/h AGM batteries have a short circuit current of 900amps!

Agree with point 2, power draw will remain constant therefore current must increase as volts drop

P=VA
Power= volts x amps

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FollowupID: 360531

Follow Up By: Wok - Friday, Mar 18, 2005 at 19:49

Friday, Mar 18, 2005 at 19:49
Ok guys,

On the point of constant power draw whereby the current increases..

@12v>>I = 160/12 = 13.33amps
@10V>>I = 160/10 = 16.00amps
@ 5V>>I = 160/5 = 32.00 amps
@ 0.1V>>I = 160/0.1 = 1600.00 amps
@0V>>I= 160/0 = HEAPS OF AMPS!!????????????[infintely large current]

In other words, you have heaps of current with NOTHING CONNECTED TO THE FRIDGE.

or

The fridge should be working with no battery.

As you can see.......assumption of constant power holds no electrons.

cheers.......................e-WOK
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FollowupID: 360596

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 23:39

Friday, Mar 18, 2005 at 23:39
Wok me old son

Your assumption is quite correct .... the power source (battery) will very quickly run out of available electrons ... when it's flat there are no more left to play with. Seems to me it bears out my point of view.

Cheers cob
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FollowupID: 360629

Reply By: Ray Bates - Friday, Mar 18, 2005 at 08:27

Friday, Mar 18, 2005 at 08:27
A lot of people get confused with pressure and volume. Pressure is measured in Kps. Volume is measured by Lts /Hr But I have an 18mm2 cable running from my car to the DCB on the drawbar of my caravan. Bugger these 6 & 8mm cables suggested and I can get 30amps X 13.5 volts at the discharge end of the cable
AnswerID: 102870

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 08:58

Friday, Mar 18, 2005 at 08:58
Good idea with the cable Ray.

Pressure is kPa (kilopascals). Flow (current) is ltr/hr.

Cheers
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FollowupID: 360521

Follow Up By: Ray Bates - Friday, Mar 18, 2005 at 09:22

Friday, Mar 18, 2005 at 09:22
Hi Rosco, The Cable that I used is OFC power cable 4g available from Altronics (I do not work for them) cat. no W 4200(red) W 4202 (black) and is very flexable and fits well in a 175amp Anderson plug
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FollowupID: 360524

Reply By: Member - Blue (VIC) - Friday, Mar 18, 2005 at 08:53

Friday, Mar 18, 2005 at 08:53
I Made it even simpler...

I put really fat wires in and then my fridge worked good...
AnswerID: 102874

Reply By: old-plodder - Friday, Mar 18, 2005 at 08:59

Friday, Mar 18, 2005 at 08:59
Any one who used to have a 6 volt system in a car is very aware of this.
Good simple explanation.
Yes, there are other factors. But some us need it explaining simply first!
AnswerID: 102876

Reply By: theshadows - Friday, Mar 18, 2005 at 09:58

Friday, Mar 18, 2005 at 09:58
not5 bad but you got to explain potential {amperage} as the potential difference from the bottom to the top of the hose .

and of course explain that the water actually flows up the hose not down it.

shadow
AnswerID: 102887

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 16:49

Friday, Mar 18, 2005 at 16:49
You've lost me mate.

Cheers
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FollowupID: 360565

Follow Up By: theshadows - Friday, Mar 18, 2005 at 18:55

Friday, Mar 18, 2005 at 18:55
the electrons flow from the - terminal to the + terminal . so in your terms...water flows up qed.

shadow
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FollowupID: 360589

Reply By: TLC - Friday, Mar 18, 2005 at 10:45

Friday, Mar 18, 2005 at 10:45
G'Day Rosco,
I gotta aggree with WOK .
The reason the heating element in a 3-way fridge gets hot is because it's an extremely low resistance. (less than 1 ohm) and small in diameter.

Using your example 160watt elements at 12 volts draws 13 amps (P=VI)

Transposing Ohms Law (V=IR) to R =V/I we get 12volt divided by 13 amps = less than 1 ohm.

cheers

TLC
AnswerID: 102889

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 15:17

Friday, Mar 18, 2005 at 15:17
TLC

Ya got me there .. have to agree, though it sorta doesn't make sense.

Leave it with me for a while.

Cheers
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FollowupID: 360558

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 16:39

Friday, Mar 18, 2005 at 16:39
TLC

Thought a bit more about it. We are missing a few vital points.

In a circuit without any load, there is obviously no voltage drop.

In this instance the electrical energy is converted to heat energy. To do that there must be a restriction on the flow of electrons, hence the element must be high resistance. The vital point is that there is not 12V left. All the volts and all the amps are used up in the conversion.

12V and 13.3A into the high resistance, bugger all volts or amps out, but a swag of heat.

How does that sound??

Cheers
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FollowupID: 360563

Follow Up By: Mike Harding - Friday, Mar 18, 2005 at 18:09

Friday, Mar 18, 2005 at 18:09
My Finch 3 way fridge takes about 7 amps when running on 12V.

As R = V / I - the resistance of the Finch heater is 12 / 7 = 1.7 ohms.

As power = V * I my Finch consumes 12 * 7 = 84 watts
(Power can also be calculated as W = (I * I) * R - which is I squared R)

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

If my battery voltage drops to 11V then as I = V / R my Finch will now consume 11 / 1.7 = 6.5 amps.

It's power consumption will still be V * I so it will be consuming 11 * 6.5 = 71.5 watts.

So as the applied voltage falls the current forced through the load resistance also falls and as power is the product of current and voltage the power _must_ also fall hence my Finch does not work as well on 11 volts as it does on 12 volts.

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

Next week boys and girls: how to make your own directional Electro-Magnetic Pulse generator to kill speed cameras :) (Think I'm joking? :)

Mike Harding
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FollowupID: 360576

Follow Up By: Wok - Friday, Mar 18, 2005 at 19:56

Friday, Mar 18, 2005 at 19:56
Mike,

Haven't you heard?..........they are developing an all-valve speed camera!!

:) :) LOL

cheers
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FollowupID: 360597

Follow Up By: Mike Harding - Friday, Mar 18, 2005 at 20:44

Friday, Mar 18, 2005 at 20:44
>Haven't you heard?......they are developing an all-valve speed camera!!

In _that_ case we'll just use the shaped charge and forget the coils.

Mike Harding

PS. Remember: reduced component count is always good :)
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FollowupID: 360602

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 23:35

Friday, Mar 18, 2005 at 23:35
Mike

Can't go with it mate.
Fridge requires, by your reckoning, 84 watts. That figure won't vary, as it's a resistance of fixed value, so reduction in voltage means greater amperage required to maintain fixed power output.

As I suggested further down this thread, we're losing sight of the fact that electrical energy is transformed to heat energy, hence all the little volt and amp jobbies are going in one end but not coming out the other... they are either going up the chimney as hot air (lot of that around hey??), or doing the job for which they were intended ... keeping the grog icy.

Cheers
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FollowupID: 360628

Follow Up By: Wok - Saturday, Mar 19, 2005 at 05:16

Saturday, Mar 19, 2005 at 05:16
Mike,

An interesting variation, I was working towards a Photon torpedo, figured that would interfere with the hole migration between the the cathode & anodes....thus creating an avalanche effect with consequent thermal runaway.................................The component count isn't the problem.......the 1kV battery is proving a challenge

:)
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FollowupID: 360643

Follow Up By: Mike Harding - Saturday, Mar 19, 2005 at 08:16

Saturday, Mar 19, 2005 at 08:16
Hello Rosco

>Fridge requires, by your reckoning, 84 watts.

The key word here is "requires" - certainly it _wants_ 84W but as your mother told you; "people who want don't always get" :)

>That figure won't vary,
Not correct. The power dissipated in the fridge will be the _product_ of the current and voltage applied to the fridge.

>as it's a resistance of fixed value,
Correct

>so reduction in voltage means greater amperage required
>to maintain fixed power output.

Correct. _BUT_ that would also require the value of the load resistance to fall in order for a reduced voltage to force a bigger current through it. The current which flows in a resistor is determined by two things: the value of the resistor and the voltage applied to it. The value of the heater resistance in my fridge will _always_ be 1.7 ohms. So when the fridge is in my vehicle with the engine running it is being supplied with 14V so it's current draw will be 14/1.7 = 8.2A and 8.2 * 14 = 115W. But with a dead battery at 10V it will be 10/1.7 = 5.9A and 5.9 * 10 = 59W - so the fridge won't work very well at 10V, which is exactly what we observe because my white wine warms up :)

The nub of all this is Ohms Law - I=V/R, R=V/I and V=IR.

As you can see from the above equation; if you want more current you _MUST_ either reduce the resistor value or increase the voltage.

Mike Harding

mike_harding@fastmail.fm
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FollowupID: 360646

Follow Up By: Mike Harding - Saturday, Mar 19, 2005 at 08:20

Saturday, Mar 19, 2005 at 08:20
Wok

http://www.airpower.maxwell.af.mil/airchronicles/kopp/apjemp.html

Forget the photon route - _far_ too much energy needed :)

Mike Harding
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FollowupID: 360647

Follow Up By: Rosco - Bris. - Saturday, Mar 19, 2005 at 15:17

Saturday, Mar 19, 2005 at 15:17
Mike

I can certainly see you line of reasoning. My best suggestion is to refer back to 3rd sentence, 1st paragraph of my original posting.

Great input to an excellent discussion.

Thanks cobber
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FollowupID: 360675

Follow Up By: Mike Harding - Saturday, Mar 19, 2005 at 18:56

Saturday, Mar 19, 2005 at 18:56
Hi again Rosco

>I can certainly see you line of reasoning. My best suggestion is
>to refer back to 3rd sentence, 1st paragraph of my original posting.

No problem. It's not a competition but, hopefully, it is about learning - I guess that's why most of us post/read the various newsgroups and forums on the net? None of us knows it all - be a boring world if we did :)

Mike Harding
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FollowupID: 360692

Reply By: D-Jack - Friday, Mar 18, 2005 at 11:45

Friday, Mar 18, 2005 at 11:45
You may be all wrong

What if we are all just brains sitting in jars thinking that this is all happening, and the only facts we think we know are we are thinking? Or what you are having is a long dream about life where in fact one day you will wake up and things will not be as they dreamed? Or what if you constantly have a headache, but get used to it so that on the odd occasion when the headache stops you actually feel pain?

D-Jack

P.s. can you please repeat that Rosco from Brissie!!
AnswerID: 102901

Follow Up By: Member - Blue (VIC) - Friday, Mar 18, 2005 at 13:13

Friday, Mar 18, 2005 at 13:13
My dreams appear to be lacking in imagination... a fat cable & a fridge... WooHoo! Huh hum, I mean "yeah"(very softly...) The least I could've done was fill it... Well I guess if I am just a brain in a jar, I'm really not that bad off...
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FollowupID: 360540

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 15:19

Friday, Mar 18, 2005 at 15:19
Can I have some too please cobber ....;-))
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FollowupID: 360559

Follow Up By: Member - Bradley- Friday, Mar 18, 2005 at 17:45

Friday, Mar 18, 2005 at 17:45
hello morpheus, have you reached zion yet ?
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FollowupID: 360571

Reply By: Willem - Friday, Mar 18, 2005 at 16:49

Friday, Mar 18, 2005 at 16:49
I have read some of this thread but it reminded me of maths class at school and I still have nightmares from all those years ago.

Frankly if the fridge makes things cold then it is working well. If it doesn't, I'll get a new one. How it works, who cares?,.........hahahahaha
AnswerID: 102929

Follow Up By: Rosco - Bris. - Friday, Mar 18, 2005 at 16:53

Friday, Mar 18, 2005 at 16:53
Willie

That doesn't surprise me. I'll betcha when you went to school they taught you the earth was flat.

Hahahahahhahahahahaha

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Follow Up By: Willem - Friday, Mar 18, 2005 at 17:01

Friday, Mar 18, 2005 at 17:01
It isn't????????????????????????????
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FollowupID: 360568

Follow Up By: gottabjoaken - Friday, Mar 18, 2005 at 17:32

Friday, Mar 18, 2005 at 17:32
You wouldn't want it to be, Willem.

If it was, you wouldn't need the 4x4

Ken
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Follow Up By: Ruth from Birdsville Caravan Park - Friday, Mar 18, 2005 at 18:03

Friday, Mar 18, 2005 at 18:03
Wow what a good debate this was - I'm with you, Willem - in my case if it doesn't work then I'll buy Ian another raffle ticket and he'll win another Engel! Nice of the boys to have a discussion and not start hitting each other over the head, wasn't it?
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FollowupID: 360575

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