Finding a good descriptive model of the electric circuit (for 8th graders)

[abcd112358]

Since this quesstion popped up in this thread I thought it might be better to create a new thread:

https://www.physicsforums.com/threads/a-question-about-an-electrons-movement-in-a-dc-circuit.997736/

The question is the following: What kind of descriptive model could/should one use while first introducing youg students to make the electric circuit less abstract?
The goal of course is to both make the whole system more understandable while not creating wrong preconceptions that might make life harder for students in later grades or even at university.

Some more informations to show where I am coming from:
I am a relatively unexperienced teacher and where I am teaching I am forced (by our curriculum) to teach my students (8th grade) some kind of descriptive model and the students must work with it too.
Here is a rough translation of the passages from the curriculum I am bound to fulfill:

1. The students use a descriptive model of an electric circuit to visualize the physical quantities current, voltage and resistor. They also use this model to describe the relations between these physical quantities. The students describe limitations of the model using specific situations.
2. While using the model, the students explain observations regarding electrical current and voltage in circuits with a maximum of three resistors. Meaning for example: Why do we see each lightbulb in their respective brightnes for any arangement of three lightbulbs in any combination of shunt connections and series connections.

Some problems regarding the search for a good model:
The concept of energy is taught in 9th grade and electro magnetism (including electric fields and the magnetic fields created by moving charges) are only taught in 10th grade. The students (vaguely) know that there is some kind of magnetic force around a current and that poitive and negavie charges attract each other.

Although I have a somewhat concrete problem here finding a more general answere to this model problem might be interesting to solve for many people.

Ps.: Please excuse all the mistakes I made while writing this. There are (sadly) several reasons why I became a physics teacher and not an english teacher.

 

[hutchphd]

I personally have no problem with the "marbles in a tube" description of electrons in a wire. The resistors are places where the tubes provide impediments ("friction" internally). Light bulbs are of course problematic because of their nonlinear R.
The distinction between conductors and insulators can be easily specified in this model and needs to be taught.
I wish I could recommend a resource but ignorance prevails.

 

[abcd112358]

I personally have no problem with the "marbles in a tube" description of electrons in a wire. The resistors are places where the tubes provide impediments ("friction" internally). Light bulbs are of course problematic because of their nonlinear R.
The distinction between conductors and insulators can be easily specified in this model and needs to be taught.
I wish I could recommend a resource but ignorance prevails.

What would be the advantages of the marble model compared to a watermodel with closed waterpipes?

I don't think the nonlinearity of the resistance is a problem for the exercises noted above.
I can see how you could explain observations in a series connection (all marbles still move at the same pace but there is more resistance/friction) and in a shunt connection (the marbles from the main tube need to split up to push the marbles in both tubes.

 

[hutchphd]

Because water in pipes is in fact a system we deal with daily, and the analogy is not perfect, I prefer the slightly more abstract model of balls in a tube. I think it is less likely to be thought of as "exact". And it is important to emphasize that the model cannot be generalized without serious modification. Soon you run into Quantum Mechanics, whereas the water analogy logically is a dead end.
Magnetic fields are problematic either way...I don't know a really good way to go there... calling for hellp!

 

[abcd112358]

Because water in pipes is in fact a system we deal with daily, and the analogy is not perfect, I prefer the slightly more abstract model of balls in a tube. I think it is less likely to be thought of as "exact".

This is a great point. I like it a lot.
In the marble model voltage would be represented by the force that pushes the marbles through the tube und current would be pretty much be simply represented by the number of marbles passing through at a time?

The model does not need to give any insight on QM effects or the creation of magnetic fields. The model is only meant to give the students an easy start for the subject.

 

[anorlunda]

Marbles in a tube analogize only current, not voltage, and are downright misleading with respect to power. It leads to the image of a bucket full of marbles at the far end as representing energy.

The question is the following: What kind of descriptive model could/should one use while first introducing youg students to make the electric circuit less abstract?

We need more information.

• How young are the students?
• What is the background of these students? Algebra? Calculus?
• What is the purpose of the instruction? To train electricians? To train in physics? To train in problem formation and analysis? To demystify electricity?
• What will these students take to continue following your course?

Edit: Whoops, I missed the title that says 8th graders. That answers how young.

 

[abcd112358]

Marbles in a tube analogize only current, not voltage, and are downright misleading with respect to power. It leads to the image of a bucket full of marbles at the far end as representing energy.We need more information.

• How young are the students?
• What is the background of these students? Algebra? Calculus?
• What is the purpose of the instruction? To train electricians? To train in physics? To train in problem formation and analysis? To demystify electricity?
• What will these students take to continue following your course?

Edit: Whoops, I missed the title that says 8th graders. That answers how young.

1. About 14 years old (+/- 1 year)
2. Later on there will be calculations involved (R=UI and calculation of the combined resistance of series connections and of shunt connections). They are able to solve simple linear equations and simple equations involving variables in fractions as long as they can solve those equations with some addition of fractions and then building the inverse on both sides. They have no idea about calculations involving vectors or differentiation. But I think if there is too much calculus involved then the descriptive model kind of missed its mark.
3. It is part of the curriculum of the physics classes in regular/general schools. The purpose is to help the students get a first mental image of electricity so that the subject might be a little bit less abstract. Later on for example when talking about induction this model would have been dropped and we would only talk about charges moving or simply the existence of electric currents. A qunatum mechanical analysis of electric currents won't happen in school and is only going to be tought at university or the likes.
4. I am not sure I understand your question correctly: Do you mean which informations from this model the students are going to need to use in later classes/grades?

 

[256bits]

The model does not need to give any insight on QM effects or the creation of magnetic fields. The model is only meant to give the students an easy start for the subject.

Then a descriptive model would be using ideal components of an electrical circuit, so that E = I R.
I don't see anywhere that it says you have to use an analog model.

 

[anorlunda]

It is part of the curriculum of the physics classes in regular/general schools. The purpose is to help the students get a first mental image of electricity so that the subject might be a little bit less abstract.

OK, if that's the goal, I would focus on three demonstrations.

1. A battery and a light. The lesson is that a battery converts chemical energy to electric energy and the light converts electric energy to light energy. If the light gets warm to the touch, then they can feel that some gets converted to heat energy. I would make the lesson about energy, energy transmission, and energy conversion. Electricity is extremely useful for those things. There would be no added value to mention Voltage & current & series & parallel those details don't add to the basic understanding and questions about them can't be answered without deeper study. I wound not even mention them.
   
   Tell the students that they can learn how a battery works when they study chemistry, and how an LED or a light bulb, makes light when they study more advanced physics, but the non-abstract lesson is that energy exists in many forms, energy is conserved, and that there are very many devices that can convert one form of energy to other forms.
2. Move a wire loop past a magnet and making a LED light up for an instant, or a needle on a meter move. (see Youtube demo here) If you rotate the loop of wire, you could make the needle move both directions, thus illustrating AC power. That non-abstract demo is vitally important because that is the mechanism of how electric power is generated, transmitted to the student's home, and then consumed in the home. In the future, some of the students may find careers in that process.
3. Rubbing silk on fur to make static electricity. You can choose different materials to make positive and negative charges. You can show how a spark is produced when we touch the charged item to ground. You can show how like charges repel and opposite charges attract. (Charge a balloon, then see how the balloon sticks to your hair.) Challenge the students to think what is different about the object when it is charged and what are we seeing when we see the spark. That echoes some of the experiments that scientists puzzled over centuries ago, and leads to interesting discussions about how electricity ideas were developed historically.

Lessons 2&3 are about fields; important things that we can't see or feel directly but that exist in 3D space. That could lead to an interesting physics discussion about different kinds of fields. Even without calculus, students can see that a temperature field has no direction, but a wind field has both magnitude and direction.

 

[abcd112358]

OK, if that's the goal, I would focus on three demonstrations.

1. A battery and a light. The lesson is that a battery converts chemical energy to electric energy and the light converts electric energy to light energy. If the light gets warm to the touch, then they can feel that some gets converted to heat energy. I would make the lesson about energy, energy transmission, and energy conversion. Electricity is extremely useful for those things. There would be no added value to mention Voltage & current & series & parallel those details don't add to the basic understanding and questions about them can't be answered without deeper study. I wound not even mention them.
   
   Tell the students that they can learn how a battery works when they study chemistry, and how an LED or a light bulb, makes light when they study more advanced physics, but the non-abstract lesson is that energy exists in many forms, energy is conserved, and that there are very many devices that can convert one form of energy to other forms.
2. Move a wire loop past a magnet and making a LED light up for an instant, or a needle on a meter move. (see Youtube demo here) If you rotate the loop of wire, you could make the needle move both directions, thus illustrating AC power. That non-abstract demo is vitally important because that is the mechanism of how electric power is generated, transmitted to the student's home, and then consumed in the home. In the future, some of the students may find careers in that process.
3. Rubbing silk on fur to make static electricity. You can choose different materials to make positive and negative charges. You can show how a spark is produced when we touch the charged item to ground. You can show how like charges repel and opposite charges attract. (Charge a balloon, then see how the balloon sticks to your hair.) Challenge the students to think what is different about the object when it is charged and what are we seeing when we see the spark. That echoes some of the experiments that scientists puzzled over centuries ago, and leads to interesting discussions about how electricity ideas were developed historically.

Lessons 2&3 are about fields; important things that we can't see or feel directly but that exist in 3D space. That could lead to an interesting physics discussion about different kinds of fields. Even without calculus, students can see that a temperature field has no direction, but a wind field has both magnitude and direction.

Maybe I communicated the situation not clearly. Sorry about that.
These lessons are nice and I already did them in different grades fitting to the curriculum, but they won't give the students a mental image of what is going on inside the wire that is analoguos to reality.

Two other problems:
1. Energy is taught in later years. I could meantion energy nevertheless but I am not sure I should open this additional can of worms while my students are already occupied with learning general stuff about circuits.
2. I have to teach them about current, voltage and resistance in 8th grade and I also have to give them some kind of analoguos model with which the students need to be able to explain simple observations regarding circuits (like I meantioned in my first post).

A came to this question because of the discussion about the waterfall model in the other thread and the fact that the waterfall model is that lackluster. Maybe I should have phrased my question broader without adding the situation brought uppon me by my country's curriculum.
The problem that I have to use such a model and can't "simply" talk about a simplified version of reality might be a personal problem that is only present in my country, but I think the serach for a relatively good analoguos model for basic electric circuits might be interesting for the rest of the world nevertheless.

 

[abcd112358]

Then a descriptive model would be using ideal components of an electrical circuit, so that E = I R.
I don't see anywhere that it says you have to use an analog model.

Maybe it is because of my lackluster translation. Alternative translations (instead of "decriptive") might be "graphic", "demonstrative" or maybe "vivid", although I think "graphic" would be the next best translation.

And yes. All the parts of these circuits are usually treated as ideal. Later on the students are going to learn about non omic resistors but that has imo nothing to do with the search for a basic analoguos model.

 

[atyy]

Can you just picture the electrons as balls of charge?
Then you can start with discharge of a capacitor through a light bulb?
You can use the concept that like charges repel.
And voltage is related to the difference in charge across the capacitor plates.
And think of the light bulb as some sort of friction, so that heat is generated, and heat of the right form is visible light.