Must a load consume the entire available current or only what it needs

[Boltzman Oscillation]

TL;DR Summary

If i have a 5V 20 Amp source and I have a 3000 Ohm load with a maximum power rating of 5 watts does the load have to take in all of the current?

So if I have a source of 5V and 20 amp then the maximum power I can get from the source will be:
$$ P_{max} = 5*20 = 100W$$
but my load can only take in 5 watts, it will blow up. But, does this have to happen or does my load only take what current it needs? My guess is that the load takes in what is available. For analogy, if I am filling a bottle with a hose and keep adding water after the bottle has filled then the bottle will shatter (i actually have not experimented with this so i may be wrong). This is the same for a balloon, if i add too much air then it will burst.

 

[Guineafowl]

To answer the question: only what it needs.

The 20A rating of your source is a maximum figure. Just like the maximum output power you have correctly calculated.

You have 5V across 3000ohm, so I = V/R = 5/3000 = 1/600 A.

1/600 A flowing through a 3000ohm load, so P = I2R = 8.3 mW.

“It will blow up”. I wrote this exact phrase a long time ago in school about a kettle falling into the sink. The teacher underlined it and put a big red cross beside. I think the reason was loose language. Try to think through more carefully what would happen to a 3000 ohm resistance that exceeds its max power rating.

I would not use either of those analogies. Stick to V=IR, P=IV, P=I2R and so on.

 

[Boltzman Oscillation]

To answer the question: only what it needs.

The 20A rating of your source is a maximum figure. Just like the maximum output power you have correctly calculated.

You have 5V across 3000ohm, so I = V/R = 5/3000 = 1/600 A.

1/600 A flowing through a 3000ohm load, so P = I2R = 8.3 mW.

“It will blow up”. I wrote this exact phrase a long time ago in school about a kettle falling into the sink. The teacher underlined it and put a big red cross beside. I think the reason was loose language. Try to think through more carefully what would happen to a 3000 ohm resistance that exceeds its max power rating.

I would not use either of those analogies. Stick to V=IR, P=IV, P=I2R and so on.

Ok, so the load will only take up the amount of current that Ohm's law allows:
$$I = V/R$$
I just never understand why I can't just flip Ohm's law to work for me:
$$V = IR$$
Why is the voltage used by the circuit always the given 5 V and the current limited by this?
Why can't the current be 20A and I limit the voltage? Then I would have 20 Amp going through the 3000 ohm resistance:
$$V = I*R = 3000*20 = 60000Volts$$
Which is not possible given my source which can only output 5V.
But what if I had a source that could output 60000V? I guess what my question is, what limits what? The voltage limits the current or the current limits the voltage?

 

[Guineafowl]

Why is the voltage used by the circuit always the given 5 V and the current limited by this?

The voltage isn’t ‘used’ as such. It’s provided by the source, which is constructed to provide a constant 5V, and to maintain this up to a max 20A of current, giving a max 100W power, which is the ultimate limiting factor.

Why can't the current be 20A and I limit the voltage? Then I would have 20 Amp going through the 3000 ohm resistance:

V=I∗R=3000∗20=60000Volts​

That V=IR is correct. If you had 60,000V across 3000ohm, it would drive 20A through it. Notice how I’ve used ‘across’ and ‘through’. This is how to think of volts and amps.

The last question about what limits what isn’t easy to answer, as it depends on the source and the load in question. You can get power supplies that force a constant current, some that force constant voltage, and some that do both. From what you’ve shown here, I suggest your next post is a set of definitions of volts, amps and ohms.

 

[Guineafowl]

Ok, so the load will only take up the amount of current that Ohm's law allows

Just noticed this wording, that might give a clue to your misunderstanding. Don’t think of the load taking up current like it’s some kind of unit. Current is a flow, a rate. Voltage pushes that flow. How much flow a voltage can push through a load is determined by the resistance of the load.

Take R=V/I, which you know. Ohms=volts/amps. So an ohm can be thought of as a volt PER amp.

Your 3000 ohm load can therefore be redefined as 3000 volts per amp. In other words, you’ll need 3000 volts across the load for every amp of current through the load.

Does that help?

 

[Boltzman Oscillation]

The voltage isn’t ‘used’ as such. It’s provided by the source, which is constructed to provide a constant 5V, and to maintain this up to a max 20A of current, giving a max 100W power, which is the ultimate limiting factor.That V=IR is correct. If you had 60,000V across 3000ohm, it would drive 20A through it. Notice how I’ve used ‘across’ and ‘through’. This is how to think of volts and amps.

The last question about what limits what isn’t easy to answer, as it depends on the source and the load in question. You can get power supplies that force a constant current, some that force constant voltage, and some that do both. From what you’ve shown here, I suggest your next post is a set of definitions of volts, amps and ohms.

All i can say is that the amp is the measure of the rate of which charge (electrons in most cases) move through a space. Ohms is the resistance of the material that tries to stop the electrons from moving through the space through perhaps collision, and coulombs forces. Voltage is what I have always struggled with. For now I always considered voltage as a comparison. Stating the voltage at one point is gibberish, you need two points because voltage is the change of energy across those two points.

 

[Guineafowl]

What you’ve said there is enough to understand Ohm’s Law and your power supply example, so that’s good.

Think of voltage as electrical pressure - in fact, it used to be referred to in such terms.

Your power supply is regulated to supply 5V of pressure. Put that pressure across a resistor and the resultant flow rate will be determined by the resistance. If more than 20A is drawn, the pressure will drop (voltage sag), hence the rating.

Another power supply might be constant current, like an LED driver. It might be set up to drive 5A through whatever load it’s connected to, and it will do this by varying the voltage. Again, it will hit limits.

 

[sophiecentaur]

Think of voltage as electrical pressure - in fact, it used to be referred to in such terms.

And 'we' grew out of that idea because it is a poor and needless analogy. Voltage is Potential Difference which, by its name, involves Energy and not Force (or Force per unit area). Bearing in mind that anyone who intends to learn EE will need to get the 'proper' story eventually, then why not start off with the actual definition if you are trying to help them? I bit of effort at the start will take them 'potentially' the whole way.

 

[Guineafowl]

And 'we' grew out of that idea because it is a poor and needless analogy. Voltage is Potential Difference which, by its name, involves Energy and not Force (or Force per unit area). Bearing in mind that anyone who intends to learn EE will need to get the 'proper' story eventually, then why not start off with the actual definition if you are trying to help them? I bit of effort at the start will take them 'potentially' the whole way.

The OP understands voltage as potential difference (post #6), but still doesn’t ‘get it’, as far as I can see. So the actual definition you’re citing is not helping in this case.

Those last three words are key, I think. In this case. Not everyone thinks the same, and some prefer physical analogies. Analogies do not have to be exactly correct (otherwise they wouldn't really be an analogy, just a description). I maintain the opinion that they make perfectly good stepping stones.

I very much doubt that the use of analogies will result in a generation of EEs, standing in front of electrical panels, totally baffled because they can’t stop thinking about hosepipes. We have, for a century, been teaching a model of the atom to youngsters that we know to be wrong. Still makes pretty good physicists, doesn’t it?

You might say P=IV, V=IR, Q=IT and so on tell you everything you need to know, but that’s YOU. Neurodiversity is just becoming mainstream now, and is informing better teaching techniques.

 

[hutchphd]

But the fact that the flow of current is inviscid for the analogy to hold can rapidly lead to trouble. That being said, I remember my learning (at age 10 or so) was much facilitated by the analog.
Shall we agree that care must be taken...

 

[Guineafowl]

But the fact that the flow of current is inviscid for the analogy to hold can rapidly lead to trouble. That being said, I remember my learning (at age 10 or so) was much facilitated by the analog.
Shall we agree that care must be taken...

Care must be taken with any explanation, particularly analogies because at some point they always fall apart. If they didn’t, as I said, they would most likely be an exact description, not an analogy. Just like (here’s an analogy!) how telling the truth is much easier than lies; the truth always makes sense and you can’t be tripped up.

Why does the flow need to be inviscid for the analogy to work? Pressure drop across the resistor releases energy.

 

[hutchphd]

As I recall, the resistance to flow of viscous water in a pipe is not inverse with area (I think area² but don't hold me to it) ...so you can get the wrong idea about resister geometry for one thing.

 

[anorlunda]

It's still problematical. The student learns what? "gets it" what? How the analogy works, or how electricity works?

My problem with the analogies is when we come to energy and power. Students taught with the flow analogy, come to PF thinking that energy is like filling a bucket of water. A "bucket full" of electrons is a certain amount of energy.

You can do useful work by filling a bucket of water on the top of the hill. No need for a closed circuit.

When we use an analogy to teach, the lesson should always end with the warning.
NOW STOP! GO NO FURTHER WITH THIS ANALOGY. If that is not said, or not heeded, we risk doing more harm than good.

 

[sophiecentaur]

We have, for a century, been teaching a model of the atom to youngsters that we know to be wrong.

Who are "we"? Which model and how it is actually 'wrong'? It is a limited model and that is (should be) taught in any introductory Physics course.
Many so-called engineers do, in fact stand and stare at dials on equipment yet they do not know what they actually mean and couldn't deal with a problem if it arose. A real Electrical Engineer uses the right models and can solve problems and design equipment that will work. I have 'engineers' come to my house to fix things and they usually manage it. But they can only follow a script which involves replacing parts, according to the statistical likelihood of failure. (That goes for most motor mechanics too).
I would hope that PF members are after something a bit more than that.
Also, assuming you tell a student something by means of a home brewed analogy, what does that student take away with him / her? How do they know that any extrapolation they make is valid? What book can they go to in order to see how well they have 'got it'? The only sources they can access to support 'analogies', these days will be on-line and you really take a risk in that direction. PF's rules are that ideas that are not very mainstream need peer review. There aren't many sources that apply this gold standard.

Neurodiversity is just becoming mainstream now, and is informing better teaching techniques.

Education has suffered from inputs of all types and qualities. No one will know if those techniques are actually better until at least one whole generation of students has passed through the system and if the new system is undisturbed for at least one teaching cohort. Politicians and educationists would never allow that - at least they never have.

At least we can rely on the fact that there will always bright students who get on well, despite how they are taught. Getting on well in your education depends on hard grind. It isn't surprising that kids in top private schools do well because the culture between the parents ensures they do many times the average hours of homework. (It happens even on Christmas Day for some kids I know). Pay tens of thousands of pounds for your offsprings' education every year and you will take a special interest in getting returns from your investment.

Shall we agree that care must be taken...

Totally - but who can apply the right caveats?

 

[Guineafowl]

This is an interesting discussion; I’m prepared to open a new thread about it if you want.

It's still problematical. The student learns what? "gets it" what? How the analogy works, or how electricity works?

They get a stepping stone to how electricity works. They don’t take the stone with them (wow, another analogy!). I once found it helpful to think of volts pushing amps through ohms. It made V=IR etc tie in nicely. Others may find the same.

My problem with the analogies is when we come to energy and power. Students taught with the flow analogy, come to PF thinking that energy is like filling a bucket of water. A "bucket full" of electrons is a certain amount of energy.

A bucket full of electrons (a coulomb?) with have a certain amount of energy, defined by the voltage, n’est-ce pas?

Who are "we"? Which model and how it is actually 'wrong'? It is a limited model and that is (should be) taught in any introductory Physics course.

All teachers that I have known. ‘We’ as a society. I mean the planetary model of an atom, with lumpy electrons orbiting round the nucleus. You call it a limited model, but I say it’s an analogy (with the solar system) that works, and since you disagree with analogies, you’re refusing to call it an analogy.

Many so-called engineers do, in fact stand and stare at dials on equipment yet they do not know what they actually mean and couldn't deal with a problem if it arose.

Because of analogies? That’s what I’m defending here. I’d say that was a broader problem of teaching to the exam rather than the career.

Education has suffered from inputs of all types and qualities.

Surely you wouldn’t object to new research informing teaching methods? Dyslexic kids used to be called ‘thick’ and put in remedial classes, whereas it’s now been found that they think differently, and have tremendous abilities in other areas. Ditto autistic children.

 

[anorlunda]

A bucket full of electrons (a coulomb?) with have a certain amount of energy, defined by the voltage, n’est-ce pas?

Yes but that's the wrong analogy for circuits. P=VI. All the current in the world delivers no power without voltage. Also, in circuit analysis, we forbid pile ups of charge at any node in the circuit. See the Insights article. Mixed metaphors and mixed analogies are a terrible way to teach.

https://www.physicsforums.com/insights/circuit-analysis-assumptions/

 

[Guineafowl]

Yes but that's the wrong analogy for circuits. P=VI. All the current in the world delivers no power without voltage. Also, in circuit analysis, we forbid pile ups of charge at any node in the circuit. See the Insights article. Mixed metaphors and mixed analogies are a terrible way to teach.

https://www.physicsforums.com/insights/circuit-analysis-assumptions/

Mixing metaphors is like trying to weave a web of lies - you get tripped up, I agree.

I don’t find the buckets of charge thing helpful, either. Strictly speaking, though, it’s still right - as long as you define V as joules per coulomb, all the current in the world does indeed deliver no power without voltage. All the buckets are full of electrons, but there’s no energy per bucket.

 

[anorlunda]

I don’t find the buckets of charge thing helpful, either. Strictly speaking, though, it’s still right - as long as you define V as joules per coulomb, all the current in the world does indeed deliver no power without voltage. All the buckets are full of electrons, but there’s no energy per bucket.

It's right in the wrong context. This whole thread is about circuit analysis, not electrostatics.
You are muddying the water.

 

[Guineafowl]

My problem with the analogies is when we come to energy and power. Students taught with the flow analogy, come to PF thinking that energy is like filling a bucket of water. A "bucket full" of electrons is a certain amount of energy.

It's right in the wrong context. This whole thread is about circuit analysis, not electrostatics.
You are muddying the water.

Hey! I was answering the point at the top that you raised, about energy and power, and the bucket analogy.

And I did offer to start a new thread.

 

[anorlunda]

I'm afraid this thread has drifted off topic, partly my fault. The OP's question was answered long ago.

Thread closed.