Models of Electricity

Introduction

What is voltage? Which way does electrical current flow? Does it go from the positive side of a battery to the negative side or the other way around? We'll spend the rest of these web pages exploring these questions but before we begin experimenting, let me set you up for the answers.

Models of Light

When I first encountered scientific models describing the behavior of light I wanted to know ," is light a particle or is light a wave?" After many discussions I realized that light "is" light but that it behaves "like" a particle when it interacts and that it behaves "like" a wave when it travels. The models help me to think about this new thing called light in familiar terms. After all, I've spent years throwing pebbles into ponds and watching the resulting waves. But light is not exactly like those pebbles or those water waves it is a unique new thing.

Electricity

It is the same with electricity. In many activities, you are going to encounter models for voltage and current. Voltage and current are probably new things in your experience, they were new to Franklin, Volta, and Maxwell. Yet, in a very human way these people created models to describe voltage and current in more familiar terms. We will perform experiments then try to predict their outcomes and understand their results by employing models for electric current and voltage. Keep in mind that when we describe current as a flow of positive charges we are using a model meant to make thinking easier. Choosing the "right" model, the simplest one that gives a good enough answer, is part of the art of scientific explanation and teaching. Choosing the right model will become easier as you gain experience with electricity. Soon you might be describing the flow of fluid through pipes using a model in which fluid flow is compared to electrical current!

Current

Along these lines, current has been modeled as the flow of an electrical fluid, voltage was the pressure which drove that fluid. The "high-tension line" is a leftover from this model. The high tension is actually high-voltage and has nothing to do with the tightness of the stretch of the line. But even when you hear electric current being modeled as the flow of some fluid like water watch for places where the model breaks down, does electric current have inertia like water, is it incompressible? Watch for the places where models break down. Electric current is not exactly like the flow of water and its "inertia" or "compressibility" are quite different concepts from the ones we are used to applying to water.

In one model current is the flow of negative charges, in another it is the flow of positive charges. The actual electric current is more complicated than a simple flow of positive and negative particles and yet the simple model of current as a flow of one type of particles is usually good enough to provide the correct description of the results of an experiment. We all know that the principles of general relativity provide the most accurate model to date for understanding motion and yet, for almost all experiments Newton's laws of motion are good enough. So we'll usually model electric current as the flow of positive particles, occasionally we'll model it as the flow of negatives, and rarely we'll switch back and forth between models. Whichever model we choose we'll be looking for ways to increase our understanding.

Electrical engineers use the model of current as the flow of positive charges, high school science teachers often use the model of current as the flow of negative electrons.

Voltage

One model for voltage is that it is like height. So picture an electric circuit on a table, then visualize voltage at various points around the circuit as the height. The higher the voltage the higher the height. Just like height you need a reference point from which height is measured. This reference point on maps of the earth is called sea level. The reference point in your electric circuit is called ground or common. Voltage is the energy per electric charge, it is measured in joules per coulomb. Height is a measure of the energy per unit mass it is measured in meters^2/second^2 or alternately in joules per kilogram. So height gives energy per unit mass and voltage gives energy per unit charge. They are similar.

The Waterfall Model

So, if you model one part of an electric circuit as a waterfall then a high waterfall models a high voltage difference, if a lot of water flows over this high fall, then it models a high voltage circuit with a lot of current. If a small amount of water goes over a high fall it models a high voltage circuit with a little current. A low waterfall models a low voltage circuit with either a tiny flow of water or a huge flow depending on the current

Conclusion

So. We'll use models to help us understand the new concepts of electricity. Remember that the model is not exact. Current is like the flow of a fluid in many, but not all, ways.

 Scientific Explorations with Paul Doherty © 2000 2 August 2000