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Low volts high volts big amps little amps relationship


ohdearme
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Good morning.

I am trying to understand this so could a kind someone show me a simple circuit where I could use say a 1.5 and a 9 volt battery to explain the principle of high and low voltage against big and little amps. Not to tecky please, I have a multimeter, some leds, resistors and a few other bits and pieces. 

Regards

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P = I * U

Power is Current multiplied by Voltage.

If we multiply both sides of equation by time t:

P*t = I*t * U

and use knowledge that energy E=P*t, and that charge Q=I*t, then:

E = Q * U

But charge Q is quantized to e (elementary charge = 1.6021766 * 10^-19 C). There can be 1 electron, 2 electrons, 10 electrons, billion of electrons, etc. but can't exist "half of electron", "quarter of electron" etc.

So, we can write:

I1 * U1 = I2 * U2

I1/I2=U2/U1

and

Q1*U1=Q2 * U2

Q1/Q2=U2/U1

In other words large numbers of electrons with small voltage (and small kinetic energies) can be converted to small number of electrons with large kinetic energies, or vice versa, in e.g. transformer.

 

Edited by Sensei
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1 hour ago, ohdearme said:

Good morning.

I am trying to understand this so could a kind someone show me a simple circuit where I could use say a 1.5 and a 9 volt battery to explain the principle of high and low voltage against big and little amps. Not to tecky please, I have a multimeter, some leds, resistors and a few other bits and pieces. 

Regards

 

I am not disputing what Sensei says, but I don't think it suits  your question.

 

Let's clear up this misconception first, as it is vital to understanding electronics.

Quote

where I could use say a 1.5 and a 9 volt battery to explain the principle of high and low voltage against big and little amps

 

The value (size) of the volts and amps in a circuit are only sometimes related. But for most circuits these days they are not related.

In fact the 1.5 volt battery is more likely to be used in a situation of high amps than a 9 volt battery.

And batteries do not work (on their own) with transformers in any case.

 

Some devices need to be supplied with a minimum voltage threshold before they will work at all.

For old fashioned vacuum tubes, that could be several hundred volts.
Battery voltages of 90 volts and above were common in those days.

The introduction of semiconductor (solid state) devices reduced this to  several volts, which is when the 9 volt battery came into common use.

But 9 volt batteries are more expensive and less efficient than lower voltage cells.
Particularly they have difficulty supplying higher currents (amps), compared to 1.5 volt cells.

Often an array of 1.5 volt cells was made to supply 6, 9 or 12 volts

Subsequent advances have reduced this further so that 1.5 or 3 volt cells can be used directly for many purposes.

 

Which brings us neatly to current (amps).

The current is demanded by and controlled by the load, not the source (battery), always  assuming the source can actually supply the required current.

 

How are we doing?

Edited by studiot
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For sure you will be using ohm's law - the relationship of voltage, current, and resistance.  Voltage is usually shown as E (electromovitve force), Resistance as R, and Current as I (I don't know why)

E/R = I

I x R = E

Like a nine volt cell connected to a nine ohm resistance will allow one  amp of current to flow.

 

Edited by Jim S
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56 minutes ago, Jim S said:

Current as I (I don't know why)

Current Intensity. (But originally from the French Intensité de courant)

 

58 minutes ago, Jim S said:

Voltage is usually shown as E

I can't remember seeing E used as voltage before, with the exception of the actual electromotive force. In my experience V or U are much more common.

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All the textbooks we used expressed it that way - but that was a looong time ago. I went to electronics school in 1965 - 1968. That was in the last of the vacuum tube days.

Old Mr. Ohm's law still works though! I suspect it always will.

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3 hours ago, Jim S said:

All the textbooks we used expressed it that way - but that was a looong time ago. I went to electronics school in 1965 - 1968. That was in the last of the vacuum tube days.

Old Mr. Ohm's law still works though! I suspect it always will.

Even in the 1960s there were electronic circuits where there was absolutely no relationship between voltage and current, let alone Ohm's Law.

 

In my experience Ohm's Law is a very bad place to start as it is usually coupled with untenable and unneccessary explanations about hosepipes, electrons and other confusing matters which leaves all too many beginners floundering.

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There never was and could never be an example of electricity flowing in a circuit that doesn't obey Ohm's law. It's basic - like gravity.

In ac circuits it gets more complicated because of capacitive and inductive reactance, but Ohm's law always applies. Always will too.

Beginners may flounder, but the basic laws of nature don't change. 

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6 minutes ago, Jim S said:

There never was and could never be an example of electricity flowing in a circuit that doesn't obey Ohm's law. It's basic - like gravity.

In ac circuits it gets more complicated because of capacitive and inductive reactance, but Ohm's law always applies. Always will too.

Beginners may flounder, but the basic laws of nature don't change. 

So let us see your explanation of the working of an SN7400 using Ohm's Law.

Or perhaps you could treat us to the application of OL to a 9 volt PP3 battery?

Edited by studiot
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SN7400 is a chip consisting of very many circuits, each of which could be drawn out by the designer and each of which would obey ohm's law. To think otherwise is just silly.

The original question was regarding a simple circuit using a battery and some resistors. This WILL involve ohm's law  - or else they won't learn anything worthwhile. 

Anyone wanting to learn basic electronics will absolutely be working with ohm's law,  otherwise they are just fooling around with stuff without really understanding what's happening. I made my living with this stuff - and not by not understanding it at a basic level. 

 

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10 hours ago, Jim S said:

SN7400 is a chip consisting of very many circuits, each of which could be drawn out by the designer and each of which would obey ohm's law. To think otherwise is just silly.

As I'm sure you are aware, the SN7400 was one of the original digital logic gate chips and as such the inputs and outputs definitely do not obey Ohm's law.

There are many circuit components which do not obey Ohm's law here are some recent examples at SF

 

 

10 hours ago, Jim S said:

Anyone wanting to learn basic electronics will absolutely be working with ohm's law,  otherwise they are just fooling around with stuff without really understanding what's happening. I made my living with this stuff - and not by not understanding it at a basic level. 

No one has said that Ohm's Law doesn't have it's place. But it can be (and IMHO usually is) introduced too early into the study of electrical phenomena.

 

10 hours ago, Jim S said:

The original question was regarding a simple circuit using a battery and some resistors. This WILL involve ohm's law  - or else they won't learn anything worthwhile. 

 

Yes indeed so why don't you help the OP with some useful circuits, from your lifetime of electronics experience, as requested instead of squabbling with me?

I am going to start that now carrying on where ohdearme left off last time someone tried to push Ohm's Law onto him.

 

 


Hello again ohdearme, I note you have some LEDs. Plus some resistors.

So to start let use these us look into current limiting resistors. Current limiting resistors are essential components in many, if not most, circuits.

If they are connected the right way round, standard LEDs will show a light output with a current from 0.01 to 0.1 amps.

So if you connect your 9 volt battery to an LED through a 1000 ohm resistor and then 100 ohm resistor you should see a difference in brightness.

basiccirc1.jpg.69a09a39336900d5f84f445e2a72e860.jpg

 

You should try both a 1.5 volt and a 9v volt battery and try both the resistor and the LED each way round and note what happens in each case.

If you can try also two and/ or three 1.5 volt batteries to see what happens.

Here is a thread to help you get several 1.5 volt batteries stacked up correctly.

 

If you can get hold of a bunch of cheap (on/off) switches and some old fashioned filament light bulbs it would be good for another time.

I realise that folks looking into electrical matters are interested in how their toothbrush/vacuum cleaner/cooker/heater/doorbell/radio/cooker/loudspeaker....  works and most importantly, how to safely connect and use them and this will be aimed at that, if you want to continue?

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1 hour ago, studiot said:


Hello again ohdearme, I note you have some LEDs. Plus some resistors.

So to start let use these us look into current limiting resistors. Current limiting resistors are essential components in many, if not most, circuits.

If they are connected the right way round, standard LEDs will show a light output with a current from 0.01 to 0.1 amps.

So if you connect your 9 volt battery to an LED through a 1000 ohm resistor and then 100 ohm resistor you should see a difference in brightness.

Studiot, when the last time you were playing with LEDs (3mm or 5mm).. ?

They have max recommended current 25mA...30mA.. And you want to give them 90...100 mA? With 9 V?

e.g.

https://www.sparkfun.com/products/9590

"1.8-2.2VDC forward drop

Max current: 20mA

Suggested using current: 16-18mA

Luminous Intensity: 150-200mcd"

 

Edited by Sensei
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Sensei you are right, LEDs have become more efficient.

This only highlights further the need for current limiting resistors.

 

Also I think your equations for power are a more useful  starting point than Ohm's Law.

People are more interested in the energy / power and efficiency of electrical devices than materials properties such as conductivity/resistivity, to start with.

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16 hours ago, studiot said:

Even in the 1960s there were electronic circuits where there was absolutely no relationship between voltage and current, let alone Ohm's Law.

 

In my experience Ohm's Law is a very bad place to start as it is usually coupled with untenable and unneccessary explanations about hosepipes, electrons and other confusing matters which leaves all too many beginners floundering.

Are you referring to the fact that it is possible for R to vary with the current, or are you referring to possible circuits where V = IR does not hold?

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Remember me everyone, I am the one that started this thread and I seem to have started a forum punch up in the process.  You are quite right Jim,  original question was regarding a simple
circuit using a battery and some resistors, it seems to have gone off in all directions, making me smile this is. 

Thank you for the circuit diagram, I am not a scientist, my education if that is what you call it was a short electronics  course then books. I have put this circuit together before but I do not think it demonstrates how say the same voltage input can give high or lower amps. Has anyone got one?

By the way from a non scientist, I think Ohms law matters in this question,  I am prepared to stand corrected. Are you all having a good Easter?  Carry on bickering.

Was it you  Studiot who said that I should  get hold of a bunch of cheap (on/off) switches and some old fashioned filament light bulbs? I thought that filament light bulbs changed their
resistance (impedance?) according to the voltage that was put through them, in which case would it be useful in my question? Now it is ME who is changing the subject.

Best

Alan

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2 hours ago, J.C.MacSwell said:
18 hours ago, studiot said:

Even in the 1960s there were electronic circuits where there was absolutely no relationship between voltage and current, let alone Ohm's Law.

 

In my experience Ohm's Law is a very bad place to start as it is usually coupled with untenable and unneccessary explanations about hosepipes, electrons and other confusing matters which leaves all too many beginners floundering.

Are you referring to the fact that it is possible for R to vary with the current, or are you referring to possible circuits where V = IR does not hold?

 

I assume your question refers to the underlined statement?

 

Let me first observe that even with the simple devices the OP is starting with, he can have voltage without current.
Having current without voltage is more difficult, but possible.

As to the 1960s, that is when the quad NAND gate integrated circuit was introduced.
This houses four identical 3 terminal circuits that can sink/source an output current up to a specified maximum at the voltage logic level for binary 1 or 0, when presented with suitable logic level inputs.

Simplified the rules are

To pull an input low you must apply between 0.0 and +0.8 volts sinking current of aout 1.5milli amps to zero.
To pull the input high you must apply a voltage between +2.0 and +5.0 supplying a current of 40micro amps to the positive rail.

The output can source or sink ten current times these values at an output voltage of +2.4 volts.

This is nothing like Ohm's Law.

 

But it was the start of modern digital electronics.

 

Ohdearme, I see you replied whilst I was composing the above.

 

The switches can be used with LEDs and current limiting resistors to play with study digital electronics easily.
I also use them to look at series and parallel circuits and other things. You can get 10 simple switches for £1 on Ebay.

The (tiny torch bulbs) filament lamps do not need the limiting resistors that is the only reason I suggested them.

The important thing about Ohm's Law is knowing when you can use it and when you can't, which would come out in the projects I was offering.

You would indeed use it to calculate the size of current limiting resistors

 

 

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Thanks Studiot. I was questioning to find out, and assuming you knew something I did not. So is V=IR simply not applicable (won't get you anywhere) or actually wrong in principal? I always assumed it was always correct, just that the resistance might vary in a way that it made it not useful.

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19 hours ago, studiot said:

 

In my experience Ohm's Law is a very bad place to start as it is usually coupled with untenable and unneccessary explanations about hosepipes, electrons and other confusing matters which leaves all too many beginners floundering.

That's been the way in everything I've ever read on it and has been the bane of my proper understanding of it. You bringing it up has made me think i ought to try again from another angle. Is there any literature that tackles it differently?

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20 hours ago, studiot said:

In my experience Ohm's Law is a very bad place to start as it is usually coupled with untenable and unneccessary explanations about hosepipes, electrons and other confusing matters which leaves all too many beginners floundering.

Good point. Maybe this is one of those cases where the analogy only makes sense to people who already understand the concepts!

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1 hour ago, J.C.MacSwell said:

So is V=IR simply not applicable (won't get you anywhere) or actually wrong in principal? I always assumed it was always correct, just that the resistance might vary in a way that it made it not useful.

Voltmeter (especially analogous) uses Ohm's law for measurement of voltage drop in test circuit. It has built-in resistor that has constant resistance e.g. 10 Mega Ohms (it should be even mentioned on the box, and on device). If you plug it in the middle of test circuit, it creates additional branch in circuit, and through it there is flowing very very little current (because of using ultra high resistance). Even if you want to measure voltage drop on e.g. diode, transistor or other semiconductor.

How to convert electric current to human readable output? Pass current through electromagnet which is in the middle of permanent magnet. The larger current, the stronger electromagnet. It is free to spin in one, or two directions, and analogous needle can show force needed to move it on the scale.

 

 

Edited by Sensei
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2 minutes ago, Strange said:

Good point. Maybe this is one of those cases where the analogy only makes sense to people who already understand the concepts!

It's probably the worst one I've come across. I think it's pants...that and the trampoline :) 

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58 minutes ago, J.C.MacSwell said:

Thanks Studiot. I was questioning to find out, and assuming you knew something I did not. So is V=IR simply not applicable (won't get you anywhere) or actually wrong in principal? I always assumed it was always correct, just that the resistance might vary in a way that it made it not useful.

 

Yes exactly V = IR is the problem.

 

If you plot a graph of voltage against current (or better the other way round) the above relationship is a straight line through the origin, so beloved of schoolboy Physics

"Please Miss I got a straight line through the origin for my practical"

These are known as transfer curves and, like the rest of more advanced Physics life is never that simple.

The simplest step up is of course "what if it's affine?"  That is V = IR + C, but still a straight line?

In fact the ratio R = V/I is the slope of this graph at any point and the inverse ratio (called the conductance) is perhaps better used and we have that current is some function of applied voltage,  I = f(V).

We can then do calculus on this.

JimS should remember something called transconductance, appropriate to valve (tube) circuitry and FET circuitry these days.

For a junction transistor, the function f(V) is too complicated to write as an equation so electronic manufacturers publish graphs showing families curves for their products.

 

Next up the transfer curve ladder comes what statisticians call the Gompertz curve or the S or Z shaped curve which allows switching activity to be created in a circuit.

Then we have devices with negative resistance regions such as tunnel diodes (available in the 1960s) that have more that one solution for the equation.

This allows the creation of oscillator circuits vibrating back and fore between the available states represented by alternative solutions.

It can also lead to circuits with chaotic behaviour.

 

I mentioned oscillators, which introduces other variables - time or frequency.
Temperature is another one.

 

Ohm's law is an idealisation, just as the Gas Laws, but we abstract ideal transfer curves for all the more complicated situations.

Ohm's Law then becomes just the simplest possible idealisation in the form V = IR.
But in the form R = V/I it can be used to define resistance as the limit of ratio of the voltage to the current at a point.

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