How correct are these concepts about circuits?

[mohamed el teir]

first of all sorry for bad drawing, is all of the following correct :
* number 1 : the current will flow normally
* number 2 : the voltage source will not give current but Vb-Va = V of the voltage source (10 for example)
* number 3 : no current will flow
* number 4 : the voltage source will be considered as a wire Va-Vb=0

 

[berkeman]

View attachment 90859

first of all sorry for bad drawing, is all of the following correct :
* number 1 : the current will flow normally
* number 2 : the voltage source will not give current but Vb-Va = V of the voltage source (10 for example)
* number 3 : no current will flow
* number 4 : the voltage source will be considered as a wire Va-Vb=0

Looks correct to me.

(keep in mind that shorting out a voltage source like in #4 is generally a bad thing...)

 

[Chandra Prayaga]

View attachment 90859

first of all sorry for bad drawing, is all of the following correct :
* number 1 : the current will flow normally
* number 2 : the voltage source will not give current but Vb-Va = V of the voltage source (10 for example)
* number 3 : no current will flow
* number 4 : the voltage source will be considered as a wire Va-Vb=0

I am not sure what statement number 4 means. If the wire is an ideal wire, with zero resistance, that is the end of the voltage source, and if the wire has sufficient resistance not to damage the voltage source, then statement number 4 is wrong.

 

[phinds]

I am not sure what statement number 4 means. If the wire is an ideal wire, with zero resistance, that is the end of the voltage source, and if the wire has sufficient resistance not to damage the voltage source, then statement number 4 is wrong.

Couldn't have said it better myself, except to add that in the first condition, the wire is ideal, it still doesn't really make sense to say "the voltage source will be considered as a wire" but rather to say "this is an impossible situation"

 

[mohamed el teir]

I am not sure what statement number 4 means. If the wire is an ideal wire, with zero resistance, that is the end of the voltage source, and if the wire has sufficient resistance not to damage the voltage source, then statement number 4 is wrong.

yes i am talking about ideal wires

Couldn't have said it better myself, except to add that in the first condition, the wire is ideal, it still doesn't really make sense to say "the voltage source will be considered as a wire" but rather to say "this is an impossible situation"

the voltage of the source is just v and we conclude since it is shorted that v is 0 therefore there is no battery primarily and there is just wire, it is like we are testing for the possibility of existence of a battery under this situation, but if we know in advance the existence of a battery with a specific voltage (10 for example), this voltage rise will not meet the required voltage drop to make the net voltage 0 so it may explode producing an open circuit not a wire

 

[phinds]

yes i am talking about ideal wires

the voltage of the source is just v and we conclude since it is shorted that v is 0 therefore there is no battery primarily and there is just wire, it is like we are testing for the possibility of existence of a battery under this situation, but if we know in advance the existence of a battery with a specific voltage (10 for example), this voltage rise will not meet the required voltage drop to make the net voltage 0 so it may explode producing an open circuit not a wire

I disagree and stand by my original statement.

Your png does not load

EDIT: I think the problem is that you do not understand the normal conventions of drawing circuits. Your #4 is just not valid. You asked a question but seem to want to argue about the answer in this case.

 

[berkeman]

the voltage of the source is just v and we conclude since it is shorted that v is 0 therefore there is no battery primarily and there is just wire, it is like we are testing for the possibility of existence of a battery under this situation, but if we know in advance the existence of a battery with a specific voltage (10 for example), this voltage rise will not meet the required voltage drop to make the net voltage 0 so it may explode producing an open circuit not a wire

An ideal voltage source has zero output resistance, so shorting it results in infinite current. A real voltage source has a finite output resistance, so shorting it results in a large (and potentially destructive) current. Please do not ever do this in real life.

 

[drvrm]

Mohamed el teir,
i was looking at your circuits and i do not understand the logic of the four drawings-1. you have shortcircuited perhaps the cell/which we do not do or forbid people in the lab- it may lead to burns/accidents.
if one wants to draw a circuit he puts up a load- a bulb and puts in an ammeter and a voltmeter as by looking at the source and wires you can not say which way the current is moving and what is the state of voltage source. so if you have a multimeter you can say definately about the current etc - so my request will be to pick up a school science textbook a nd draw electrical circuits by their 'rule' otherwise people will be just guessing what your aim is!

 

[mohamed el teir]

I disagree and stand by my original statement.

Your png does not load

sorry the attachment was by mistake, what do you think will happen if a battery is shorted other than damage ? or you think that the battery primarily can't be shorted ?

An ideal voltage source has zero output resistance, so shorting it results in infinite current. A real voltage source has a finite output resistance, so shorting it results in a large (and potentially destructive) current. Please do not ever do this in real life.

yes i agree, i want to say that in case 4 when we apply KVL to the loop : v-0i=0, so we have two possibilities, that v is 0 (the volt of battery is 0 so it is just like a conducting wire not actual battery) and also i is 0 therefore it is just wire loop, and the 2nd possibility if we know there is a battery 10 v for example therefore 10-0i=0 leads to i = 10/0 = infinite (in ideal case) or 10/(very small resistance) (in real case) therefore damage occurs

Mohamed el teir,
i was looking at your circuits and i do not understand the logic of the four drawings-1. you have shortcircuited perhaps the cell/which we do not do or forbid people in the lab- it may lead to burns/accidents.
if one wants to draw a circuit he puts up a load- a bulb and puts in an ammeter and a voltmeter as by looking at the source and wires you can not say which way the current is moving and what is the state of voltage source. so if you have a multimeter you can say definately about the current etc - so my request will be to pick up a school science textbook a nd draw electrical circuits by their 'rule' otherwise people will be just guessing what your aim is!

ofcourse my purpose is the implementation of these parts in larger circuits not dealing with the drawn circuits alone, in norton theorem for example to get norton current you make a short circuit instead of the resistance you are working on, if this SC is connected in parallel with current source or battery i just wanted to know the case

 

[phinds]

sorry the attachment was by mistake, what do you think will happen if a battery is shorted other than damage ? or you think that the battery primarily can't be shorted ?

You are missing the point. The assumption for such drawings unless otherwise stated, and you did not state otherwise, is that the elements are ideal. You can't have an ideal voltage source with an ideal short circuit across it. Of course you can short circuit a battery, but it would either explode or overheat enough to possibly start a fire, unless it was a low-power capable battery.

As I said in a previous post, the problem here is that you are refusing to understand that there are conventions for such drawings.

 

[mohamed el teir]

You are missing the point. The assumption for such drawings unless otherwise stated, and you did not state otherwise, is that the elements are ideal. You can't have an ideal voltage source with an ideal short circuit across it. Of course you can short circuit a battery, but it would either explode or overheat enough to possibly start a fire, unless it was a low-power capable battery.

As I said in a previous post, the problem here is that you are refusing to understand that there are conventions for such drawings.

yes elements are ideal, why it is refused to short circuit ideal battery ? mathematically it would give infinite current (which translates to very large current in reality due to the very small resistance causing damage) so is it refused only by convention to short circuit an ideal battery?
take a look at this:

if you want the equivalent norton circuit at terminals a-b (in the 2 ohm resistor area),to get R norton you will remove the 2 ohm resistor, put SC instead of battery and OC instead of current source so the equivalent R will be 1 ohm connected in parallel with 0 giving 0 resistance, to get i norton you will put SC instead of 2 ohm resistor, here we see that the battery is shorted, will we say i norton is infinite ? and since v thevenin (= 10) divided by R thevenin (= R norton = 0) equals i norton we will get infinite also, is norton inapplicable here ?

 

[jbriggs444]

yes elements are ideal, why it is refused to short circuit ideal battery ? mathematically it would give infinite current

Mathematically, there is no real-valued current that can satisfy the requirement that current times resistance = potential difference. Infinity is not a real number. If you insist on using an ideal power source in conjunction with an ideal zero resistance then mathematics does not define the resulting current at all. You are not allowed to divide by zero.

 

[mohamed el teir]

Mathematically, there is no real-valued current that can satisfy the requirement that current times resistance = potential difference. Infinity is not a real number. If you insist on using an ideal power source in conjunction with an ideal zero resistance then mathematics does not define the resulting current at all. You are not allowed to divide by zero.

so why it is commonly said that in open circuit resistance is infinite (v/i if v = 10 for ex so it will be 10/0) ? aren't the cases so close ? in open circuit infinite resistance means no current is flowing and in short circuit infinite current means no resistance is present ?

 

[phinds]

@mohamed el teir, an ideal short on an ideal voltage source is impossible because it is an inherent contraction in terms. An ideal short has zero volts across it and an ideal voltage source has a non-zero voltage output. Do you really think you can have both at the same time?

 

[Dale]

* number 1 : the current will flow normally
* number 2 : the voltage source will not give current but Vb-Va = V of the voltage source (10 for example)
* number 3 : no current will flow
* number 4 : the voltage source will be considered as a wire Va-Vb=0

Both 1 and 2 are fine. Both 3 and 4 are impossible for ideal components.

 

[phinds]

Both 1 and 2 are fine. Both 3 and 4 are impossible.

Gads. I can't believe I overlooked that 3 is also impossible. I blame it on the fact that I just barely believe in current sources

Good catch, Dale.

 

[jbriggs444]

so why it is commonly said that in open circuit resistance is infinite (v/i if v = 10 for ex so it will be 10/0) ? aren't the cases so close ? in open circuit infinite resistance means no current is flowing and in short circuit infinite current means no resistance is present ?

With no real disrepect intended, one reason is that physicists and engineers are more casual about infinities than mathematicians.

 

[Dale]

so why it is commonly said that in open circuit resistance is infinite (v/i if v = 10 for ex so it will be 10/0) ? aren't the cases so close ? in open circuit infinite resistance means no current is flowing and in short circuit infinite current means no resistance is present ?

The problem with 3 and 4 is not about whether you want to allow infinity as a valid value. The problem is that both circuit 3 and circuit 4 are not self-consistent.

For circuit 3 the current source says "the current is I" while the open circuit says "the current is 0". The current cannot be both I and 0, so the circuit is not self consistent.

For circuit 4 the voltage source says "the voltage is V" while the short says "the voltage is 0". The voltage cannot be both V and 0, so the circuit is not self consistent.

 

[phinds]

so why it is commonly said that in open circuit resistance is infinite (v/i if v = 10 for ex so it will be 10/0) ? aren't the cases so close ? in open circuit infinite resistance means no current is flowing and in short circuit infinite current means no resistance is present ?

You are carrying Ohm's Law too far when you try to take it to the realm of infinity.

 

[phinds]

take a look at this:
View attachment 90903

This is a perfectly acceptable circuit. The current through the 1 ohm resistor is 10 amps, the current through the 2 ohm resistor is 5 amps. That's 15 amps, 3 of which is provided by the current source and 12 of which is provided by the voltage source. The current source has 10 volts across it, which is fine, and the voltage source has 12 amps through it, which is fine.

This floundering around with specific cases really isn't the best way to learn this stuff. Clearly, you need to go back and study the basics of voltage sources and current sources. It's all very simple, it just takes a bit of getting used to.

 

[mohamed el teir]

The problem with 3 and 4 is not about whether you want to allow infinity as a valid value. The problem is that both circuit 3 and circuit 4 are not self-consistent.

For circuit 3 the current source says "the current is I" while the open circuit says "the current is 0". The current cannot be both I and 0, so the circuit is not self consistent.

For circuit 4 the voltage source says "the voltage is V" while the short says "the voltage is 0". The voltage cannot be both V and 0, so the circuit is not self consistent.

right, about 1 and 2 : are they consistent because:
1 : the source says current is I and SC doesn't affect it (the current in the rest of wire is also I)
2 : the source says voltage rise = V and open circuit doesn't affect it (the voltage drop on the open circuit is also V)
correct ?

 

[mohamed el teir]

This is a perfectly acceptable circuit. The current through the 1 ohm resistor is 10 amps, the current through the 2 ohm resistor is 5 amps. That's 15 amps, 3 of which is provided by the current source and 12 of which is provided by the voltage source. The current source has 10 volts across it, which is fine, and the voltage source has 12 amps through it, which is fine.

This floundering around with specific cases really isn't the best way to learn this stuff. Clearly, you need to go back and study the basics of voltage sources and current sources. It's all very simple, it just takes a bit of getting used to.

i didn't say it is wrong circuit, i said how to get norton current in the a-b area (across the 2 ohm resistor) ? it will lead to short circuit on the voltage source on the right
(as in norton you replace the R(L) (the 2 ohm here) with SC)

 

[Dale]

right, about 1 and 2 : are they consistent because:
1 : the source says current is I and SC doesn't affect it (the current in the rest of wire is also I)
2 : the source says voltage rise = V and open circuit doesn't affect it (the voltage drop on the open circuit is also V)
correct ?

Yes.

 

[phinds]

i didn't say it is wrong circuit, i said how to get norton current in the a-b area (across the 2 ohm resistor) ? it will lead to short circuit on the voltage source on the right
(as in norton you replace the R(L) (the 2 ohm here) with SC)

You are missing the point of the Norton equivalent, which is to draw a circuit with parallel resistance and a serial current source based on removing the external load. There is no problem with that. At A/B, you remove the external load (the voltage source) and you have a simple Norton equivalent of 3 amps with a parallel resistance of 2/3rds of an ohm.

AGAIN, you really would do better to study the basics, I think, than to keep coming at it by examples. Once you have the basics under your belt, the examples become trivial.

 

[mohamed el teir]

You are missing the point of the Norton equivalent, which is to draw a circuit with parallel resistance and a serial current source based on removing the external load. There is no problem with that. At A/B, you remove the external load (the voltage source) and you have a simple Norton equivalent of 3 amps with a parallel resistance of 2/3rds of an ohm.

AGAIN, you really would do better to study the basics, I think, than to keep coming at it by examples. Once you have the basics under your belt, the examples become trivial.

we took norton equivalent in the lectures in a different way, i don't invent examples, i am telling you the style of questions they give us and the steps of solution they told us, first of all in the question it tells you what is the external load to work on (2 ohm here) then to solve these are the steps explained to us in the lectures :
1- remove this load and replace batteries by short circuits and current sources by open circuits and get R between a,b (here it will be 0)
2- replace the place of the load with SC and calculate the current in that SC (here the SC will be parallel to the battery and that is what i was asking about)
you took the load as the battery but as i am saying the examiner enforces you to choose a specific load (2 ohm here), they forbid us in the lectures from using voltage sources as external loads in norton just resistances

 

[phinds]

we took norton equivalent in the lectures in a different way, i don't invent examples, i am telling you the style of questions they give us and the steps of solution they told us, first of all in the question it tells you what is the external load to work on (2 ohm here) then to solve these are the steps explained to us in the lectures :
1- remove this load and replace batteries by short circuits and current sources by open circuits and get R between a,b (here it will be 0)
2- replace the place of the load with SC and calculate the current in that SC (here the SC will be parallel to the battery and that is what i was asking about)
you took the load as the battery but as i am saying the examiner enforces you to choose a specific load (2 ohm here), they forbid us in the lectures from using voltage sources as external loads in norton just resistances

In that case, the question makes no sense. A Norton equivalent has a current source in series with the load and a resistor in parallel with the load and an undefined load voltage. You can't do that in this circuit if the 2 ohm resistor is the load and the voltage source stays in the circuit.

 

[Perq]

I have problem understanding what OP's problem is.
Wire is not source of voltage, no.

Also, there is no "voltage" in point. Voltage of a point is 0, because voltage is difference of electric potential.

On your 2), voltage between "a" and "b" will be 0 (assuming perfect wire, with no resistance).

In the end, such question makes no sense - in most simple words, loss of voltage of the wire (its resistance multiplied by current flowing through it) must be equal to voltage between the ends of voltage source. You cannot say that "wire is source of voltage", as if you were to disconnect the circuit (as you did on "2)") you'd get 0 voltage.

 

[mohamed el teir]

I have problem understanding what OP's problem is.
Wire is not source of voltage, no.

Also, there is no "voltage" in point. Voltage of a point is 0, because voltage is difference of electric potential.

On your 2), voltage between "a" and "b" will be 0 (assuming perfect wire, with no resistance).

In the end, such question makes no sense - in most simple words, loss of voltage of the wire (its resistance multiplied by current flowing through it) must be equal to voltage between the ends of voltage source. You cannot say that "wire is source of voltage", as if you were to disconnect the circuit (as you did on "2)") you'd get 0 voltage.

i didn't say wire is source of voltage, what i meant from the very beginning that in numbers 3 and 4 current and volt will be 0 which means there are no voltage or current sources just wires, didn't say source = wire, and i said also the purpose of the question and the relation with norton theorem so if you want to understand just read all posts i will not repeat what i said

 

[mohamed el teir]

The problem with 3 and 4 is not about whether you want to allow infinity as a valid value. The problem is that both circuit 3 and circuit 4 are not self-consistent.

i have just remembered something, i understand the consistency principle and i am totally convinced that number 3 is wrong because the wire must have the current 0 and I at the same time, but regarding number 4 i think the concept of infinity has a rule, now we have short circuit saying voltage is zero, but this saying is based on the resistance being zero so multiplying it by current gives zero, but this in case current has a real value, if current is infinite we can't say voltage is zero, like in open circuit, we have zero current in open circuit, but we can't say voltage drop is zero, because resistance is infinite, so what do you see ?

 

[jbriggs444]

i have just remembered something, i understand the consistency principle and i am totally convinced that number 3 is wrong because the wire must have the current 0 and I at the same time, but regarding number 4 i think the concept of infinity has a rule, now we have short circuit saying voltage is zero, but this saying is based on the resistance being zero so multiplying it by current gives zero, but this in case current has a real value, if current is infinite we can't say voltage is zero, like in open circuit, we have zero current in open circuit, but we can't say voltage drop is zero, because resistance is infinite, so what do you see ?

What do we see? One sentence, with no proper capitalization. A string of run on thoughts ending in a mistake. If resistance is zero then resistance is not infinite.

You could decide to extend the rules of circuit analysis this way: A finite voltage applied across a zero resistance results in an infinite current whose sign is the same as the voltage. But such an extension would not be predictive in the other direction. What voltage results from an infinite current applied across a zero resistance?

This problem is essentially the reason why the real numbers are not normally augmented by plus and minus infinity -- the resulting number system is not a field. It does not obey all of the normally expected rules of arithmetic.

 

[mohamed el teir]

If resistance is zero then resistance is not infinite.

do you mean : If resistance is zero then "current" is not infinite. ??

 

[jbriggs444]

do you mean : If resistance is zero then "current" is not infinite. ??

No. I mean that if resistance is zero then resistance is not infinite.

You wrote:

i have just remembered something, i understand the consistency principle and i am totally convinced that number 3 is wrong because the wire must have the current 0 and I at the same time, but regarding number 4 i think the concept of infinity has a rule, now we have short circuit saying voltage is zero, but this saying is based on the resistance being zero so multiplying it by current gives zero, but this in case current has a real value, if current is infinite we can't say voltage is zero, like in open circuit, we have zero current in open circuit, but we can't say voltage drop is zero, because resistance is infinite, so what do you see ?

 

[mohamed el teir]

we have short circuit saying voltage is zero, but this saying is based on the resistance being zero so multiplying it by current gives zero, but this in case current has a real value, if current is infinite we can't say voltage is zero

"we have short circuit saying..."

like in open circuit, we have zero current in open circuit, but we can't say voltage drop is zero, because resistance is infinite

"like in open circuit..."

 

[jbriggs444]

this in short circuit

this in open circuit

Thus demonstrating the need for proper punctuation.

 

[Dale]

i have just remembered something, i understand the consistency principle and i am totally convinced that number 3 is wrong because the wire must have the current 0 and I at the same time, but regarding number 4 i think the concept of infinity has a rule, now we have short circuit saying voltage is zero, but this saying is based on the resistance being zero so multiplying it by current gives zero, but this in case current has a real value, if current is infinite we can't say voltage is zero, like in open circuit, we have zero current in open circuit, but we can't say voltage drop is zero, because resistance is infinite, so what do you see ?

Take one step back. Before you even get to the point of figuring out how much current goes through the voltage source you have to set up a system of equations which describes the circuit. This is done before solving. The equation for the wire says that the difference in voltage is 0. The equation for the source says that the same difference in voltage is V. So the equations describing the components are inconsistent, even before you ever go to solve for current.