Resistor and current in a circuit

[jbriggs444]

But why should what goes into per second should be what goes out per second in any arbitrary section of circuit ?

One can chase this particular "why" question down one layer further.

If charges go into a section of a circuit more rapidly than they come out then like charges will build up in that section of the circuit. But like charges repel. They repel strongly. You can build up an excess of charge in a section. But that excess is so tiny that it is normally ignored -- unless one is considering capacitors.

Capacitors are specifically constructed to deal with the self-repulsion of like charges by bringing a similar distribution of opposite charges into close proximity.

 

[conscience]

If charges go into a section of a circuit more rapidly than they come out then like charges will build up in that section of the circuit. But like charges repel. They repel strongly. You can build up an excess of charge in a section. But that excess is so tiny that it is normally ignored

Thank goodness !

Something similar ( although incorrect ) I had intended in post 3 .

Could you please review post 3 ?

 

[sophiecentaur]

I have to teach a group of 9th grade students
role of resistor in a circuit and how it affects the current in the circuit .

You need to see what is required in the Curriculum. Any model you use that they can follow will be inaccurate and, if you teach them one inaccurate model and they are tested on another, they may find that they can't get any marks.
It is shocking that anyone should be called upon to teach such stuff at that level but it happens all over the place. Look at past question papers and see what the mark schemes require from the answers. There will be some Key Words that earn marks. Cynical, I know, but it's the least worst approach. The last thing they need is more confusion than necessary, I think. Tell them they will not get it fully and that they will be able to understand more when their Maths is better. I know that deferred gratification is not in fashion . . . . .

edit: p.s. "The electrons are moving with approximately uniform average speed"
This polysyllabic exposition may be a stretch for the vocabularies of some 9th graders.

Perhaps "The electrons don't actually speed up at all, from start to finish. " would do better.

 

[Nugatory]

But why should what goes into per second should be what goes out per second in any arbitrary section of circuit ?
Please consider any portion of the circuit and not specifically from one terminal of battery to other .

The water-in-pipes analogy that @rumborak mentions in #2 of this thread works pretty well for this question.

Suppose there were a part of the circuit in which the flow of charge in was greater than the flow out. Over time, the charge in that area would increase without bound. But because like charges repel, as the charge builds up the force pushing charge out of that part of the circuit also increases, which in turn increases the flow out. So as long as the flow out is less than the flow in, the flow out will increase; this can only end when the two are equal.

 

[conscience]

One can chase this particular "why" question down one layer further.

If charges go into a section of a circuit more rapidly than they come out then like charges will build up in that section of the circuit. But like charges repel. They repel strongly. You can build up an excess of charge in a section. But that excess is so tiny that it is normally ignored -- unless one is considering capacitors.

Capacitors are specifically constructed to deal with the self-repulsion of like charges by bringing a similar distribution of opposite charges into close proximity.

The water-in-pipes analogy that @rumborak mentions in #2 of this thread works pretty well for this question.

Suppose there were a part of the circuit in which the flow of charge in was greater than the flow out. Over time, the charge in that area would increase without bound. But because like charges repel, as the charge builds up the force pushing charge out of that part of the circuit also increases, which in turn increases the flow out. So as long as the flow out is less than the flow in, the flow out will increase; this can only end when the two are equal.

Both the above inputs are quite nice .Thanks a lot

Thank you @rumborak , @Tom.G

 

[conscience]

It is so simple that I don't think it needs an analogy. Even 9th graders can do it. Any analogy is going to be more complicated and less accurate than the real thing.

Your concern is valid and much appreciated .

I feel using analogy is a nice way while introducing something new . Every Introductory Physics texts vis a vis Halliday , University Physics use some or the other kind of analogy while starting with circuits .So I guess I will not be doing something totally absurd .

Most of the discussion in this thread has been related to my second question i.e how do you respond to a smart kid who questions that why should current be constant in the circuit . I neither intended nor mentioned anywhere that I was relating this question to the inclined plane analogy mentioned in the OP .Did I ? I was just looking for a simple explanation .

Why do we need to do complicated analogies with balls bouncing down pegs at all? Why not just teach V=IR?

I have always intended to stick to V=IR .I assure you that I won't stretch the inclined plane analogy too far .Just a rough comparision , then V=IR all the way till the end .

 

[256bits]

The height represents potential difference .Balls are electrons , pegs are ions .
A child putting the balls back on top acts like a battery .
It can be seen that similar to the ball example , the current flowing in a circuit

how do you respond to a smart kid who questions that why should current

be flowing uphill against the potential. Is the raised part of the ramp the negative side of the battery, or maybe its the positive? Can you show side by side diagrams of the peg-ball experiment and v=IR and what matches with what. Shouldn't the balls, acting as electrons, be flowing uphill if the raised part of the ramp is at a higher potential, meaning positive, since the girl is acting as a battery, and a battery acts as a raise in potential, and current flows from positive to negative through the circuit resistance. Or do the balls show current, and the balls hitting the pegs shows current hitting ions. So what is current made up of then, if the opposite direction it is electrons flowing, then the whole thing does not make sense to me anymore and I am totally confused. Can we start over please, as I do not want to be thinking for the rest of my life as current being little balls hitting pegs, or was it electrons hitting pegs going the wrong way, even as a third year university electrical engineering student the vision will come back to haunt me. End Of Student Question.

Sorry to say but like the above student I just don't get it either.

 

[Dale]

I feel using analogy is a nice way while introducing something new . Every Introductory Physics texts vis a vis Halliday , University Physics use some or the other kind of analogy while starting with circuits .So I guess I will not be doing something totally absurd

It isn't absurd at all, but I personally think it is a really bad habit that the community of educators has gotten into. I wish that we would just stop. I think that teachers and textbook writers have perpetuated this habit more to show off their own cleverness than to actually help the students.

I mean, what is the underlying concept that you are trying to teach with the analogy. Presumably it is Ohms law. But trying to analyze randomly bouncing balls is horrendously more complicated than V=IR. Same with fluid mechanics (the other usual analogy), it is an analogy that is far more complicated than the real thing! Why spend valuable class time that way? How does the student benefit? Would they not benefit as much from spending the same time learning the real thing?

My ideal approach would be more hands on and real. I would get a voltmeter and a battery. I would show them how to measure the voltage. I would get a light bulb and show them how to measure current. I would plot voltage and current and show Ohms law physically.

At the end, not only would they understand Ohms law without analogies, but they would have learned some practical stuff that will be useful later in life too.

 

[conscience]

I mean, what is the underlying concept that you are trying to teach with the analogy. Presumably it is Ohms law. But trying to analyze randomly bouncing balls is horrendously more complicated than V=IR. Same with fluid mechanics (the other usual analogy), it is an analogy that is far more complicated than the real thing! Why spend valuable class time that way? How does the student benefit? Would they not benefit as much from spending the same time learning the real thing?

My ideal approach would be more hands on and real. I would get a voltmeter and a battery. I would show them how to measure the voltage. I would get a light bulb and show them how to measure current. I would plot voltage and current and show Ohms law physically.

At the end, not only would they understand Ohms law without analogies, but they would have learned some practical stuff that will be useful later in life too.

Fair enough . You have almost convinced me to drop this analogy thing altogether

How would you introduce the concept of potential difference to 9th graders being exposed to circuits for the first time?

Please note that facility to perform practicals is not available .

 

[Dale]

Please note that facility to perform practicals is not available

I would probably do it as a demonstration. It might depend on classroom size, number of students, and so forth.

 

[NTL2009]

Fair enough . You have almost convinced me to drop this analogy thing altogether

How would you introduce the concept of potential difference to 9th graders being exposed to circuits for the first time?

Please note that facility to perform practicals is not available .

But different people learn differently. If presented with the formula E = I * R, I'd just be "ho-hum, lots of things have that relationship. Eggs = Eggs per carton * cartons. So what makes electricity 'special'? ".

Suggestion for your analogy: String the balls together like a beaded necklace. Instead of pegs, use a channel lined with carpeting, and a means to compress and release the carpet-channel a bit to increase and decrease friction, and pull the string of balls through the channel. This way, all the balls move at the same rate in the 'circuit'.

It even works if you compress one section more than the other. That would be analogous to a 100 Ohm R in series with a 10,000 Ohm R. The balls can't move through the 100 Ohm R any faster than they can through the constricted 10,000 Ohm R, as they are all connected and pulled by the same force.

 

[Nugatory]

How would you introduce the concept of potential difference to 9th graders being exposed to circuits for the first time?

Start with gravitational potential differences instead of electrical. Everyone has a fairly solid intuitive understanding of what it means to be moving "uphill" and "downhill", and topographical maps with curves of equal gravitational potential are readily available and easy to read.

 

[conscience]

Start with gravitational potential differences instead of electrical. Everyone has a fairly solid intuitive understanding of what it means to be moving "uphill" and "downhill", and topographical maps with curves of equal gravitational potential are readily available and easy to read.

Excellent suggestion ! Thanks

Probably one last question

How do you introduce concept of "infinity" while explaining standard definition of electric potential ? Just as the teacher mentions the word "infinity" students are bemused . Questions like what is infinity ,why only infinity , how far is infinity and so on , crop up .

So how do we explain the notion of infinity in context of electric potential ?

 

[cnh1995]

So how do we explain the notion of infinity in context of electric potential ?

Perhaps you'll find @jim hardy's little thought experiment here helpful.

https://www.physicsforums.com/posts/5331150/
https://www.physicsforums.com/posts/5347090/

 

[Ram C]

Great analogy!...
Here are another analogies of series and parallel circuits:
One can clearly compare series and parallel circuits with water flowing analogy. Suppose we have a water tank a some height above the building. Let water is drawn from tank to some terminal at ground through one single pipe. The pipe is then distributed into 3 taps. Line from first tap is going to 2nd tap and then to 3rd tap. This is like series circuit. Because quantity of water (electric current) is same but potential difference is reducing at each tap. You will get lower pressure at last tap while same quantity of water which passed through tap 1. But if you take separate water line for each tap directly from tank, then you will get equal pressure at each tap but with different quantity of water (electric current) according to the diameter of pipe/tap. This is analogous to parallel circuit.

 

[Physics_Kid]

no, this isn't a good way to look at it

electron drift is very slow a few mm per second
the current in a circuit is the number of electrons ( charges) passing a particular point in a circuit at a given time, not how fast they are travelling

passing a point ??
its passing through an area, many times called flux.

i would model it using a block crossing sandpaper on a flat table. the tricky part is to show how the block itself loses energy (eV) as it crosses the sandpaper while keeping constant velocity.

 

[Fernando Perfumo]

Why not you show them the real thing? I bet they will understand it very well and never forget.
Do simple experiments using lamps, batteries, one amperimeter, a couple of voltimeters and a set of long and short or thick and thin wires. Of course use harmless voltages and fuses
I still remember when I was a child a tv series showing laboratory experiments on physics... simple rockets, static electricity... I really liked that.

 

[Quandry]

OK, so here is the crunch question. Nugatory mentioned Curriculum. I will now mention qualified tutors.
It is OK to learn and understand these things, but is it fair on the 9th graders.
There are many teaching aids on the net which I suggest you review. Most of the elementary stuff is for 4th graders.
For example https://www.pbslearningmedia.org/re...e.lp_electric/electric-circuits/#.WXfgysaB3u4

 

[sophiecentaur]

It is OK to learn and understand these things, but is it fair on the 9th graders.

Agreed. Non Scientists, who never really got a grasp of the subject (but who were bright enough in other directions to get influential jobs) like to think that all kids have the potential to understand EM, QM and Thermodynamics so why not hit kids with it as early as possible? They (those politicians) see a way to make their mark by having the education system force feeding the subject at an inappropriate age. Never mind the casualties who, ever afterwards, either find Science incomprehensible or think it's a trivial subject - as presented in popular media. And that is actually the majority of the adult population.
Education, like the Judiciary, should be insulated against political influence. Fat chance of that, though.