Back To CourseIntroduction to Engineering
14 chapters | 123 lessons
Kip holds a PhD in Engineering from The University of Texas at Austin and was an occasional substitute lecturer in engineering classes at that institution.
When you turn on your kitchen faucet, water flows into your sink. Somewhere in your utility district there is a pump, or a tower holding elevated water, that creates pressure at the far end of the pipe - that pressure tries to push the water through the pipe. When your faucet's valve is closed, it holds off that pressure, but when you open the valve you remove that blockage and flow begins. The water moves because of the unresisted pressure. But the flowing water and the pressure that makes it flow are not the same thing.
Similarly, if you park your car on a hill and forget to set your parking brake, your car might roll down the hill. Gravity causes this, of course. But the moving car and the gravity making it move are entirely different things.
All motion in the universe happens because of forces. Forces create a tendency for things to move, and if nothing resists that motion then the things do move. But the motion and the force causing it are always separate physical concepts. This holds true with regard to the electrical phenomenon as well, where the corresponding concepts are current and voltage.
In electric circuits, current is the flow of electric charge. It's really electrons (charged particles) that move around in circuits. For unfortunate historical reasons, the electron's charge is defined as negative. The idea of charge, and the definition of what was a positive charge and what was a negative charge, existed before the electron was discovered; physicists never bothered to switch the convention. So when current flows in some direction in a circuit, the actual physical effect is electrons flowing in the opposite direction.
If current is moving charge, what is the force that causes this motion? You've probably heard that 'like charges repel - opposite charges attract.' This is a non-quantitative statement of Coulomb's Law. Just as Newton proposed that all objects with mass exert force (gravitational force) on one another, Coulomb proposed that all charged particles exert a force (electric force) on each other. Quantitatively, physicists write:
F = K * q1 * q2 / r^2
In this equation q1 and q2 quantify the two charges, and can be positive or negative, and r is the distance separating the two charges. K is a constant of nature that scientists originally determined by making experimental measurements. F is the force that each charge will feel as a result of being distance r from the other charge.
If q1 and q2 are both positive or both negative, F will be positive. We know that like charges repel, so as written above the equation defines the force trying to push the charges away from one another. If the q1 and q2 have opposite signs then F will be negative; a negative 'pushing apart' force is a 'pulling together' force.
As an aside, be aware that the Coulomb force has nothing to do with magnetism. There's also a magnetic force, and it's often very important in the operation of circuits, but it's an entirely different thing and unrelated to this lesson.
So this explains the origin of the force that makes current flow. Voltage is closely related to this force. You may remember from high school that work is defined as a force acting through a distance. The Coulomb force acts on electric charge, and when that charge moves through a distance the force has done work. Voltage quantifies the work done on charge as it moves in response to the Coulomb forces in the circuit.
In order for charge to 'move through a distance' it has to start from some location (A) in the circuit and travel to another location (B). So we can talk about the voltage between points A and B. Engineers often talk about the voltage 'at' one point in a circuit, but when they do this they have agreed to regard some other point as a zero voltage point (often called ground). Voltage only makes sense in relation to a pair of points in the circuit. Engineers are able to use this shorthand because the voltage between two points in the circuit will always turn out to have the same value no matter what path between the two points is used to calculate it. In scientific parlance, we say that path-independent quantities like this are conservative.
A clear difference between the natures of current and voltage emerges from these considerations. Current is defined at a point, as the flow of charged particles past that point. Voltage, on the other hand, is defined between two points. Physicists call quantities of the first type through variables and quantities of the second type across variables. Through variables always relate to some sort of material flow, while across variables always relate to the work (positive or negative) associated with the redistribution of material that flow achieves. Quantities of these two types appear and share similar relationships in the study of almost every physical domain (electrical, mechanical, hydraulic, etc.)
Current is the flow of charged particles past any point in an electric circuit. Coulomb forces act on the charged particles to create this flow, and voltage is the work performed by these forces as they move charge between two points in the circuit. The general paradigm of flow caused by forces, and work arising from these forces acting across a distance, are common to a broad array of physical domains. Flows (such as current) are defined with respect to individual points in a physical system. Scientists sometimes refer to such quantities as through variables, since flow passes 'through' individual points. Work-related quantities (such as voltage) are defined with respect to a pair of points. Scientists sometimes refer to these as across variables, since they're defined between or 'across' two points. Often the second point associated with a work-related value like voltage is implied; in electric circuits this is accomplished by identifying a specific 'ground,' or zero voltage point. Generally speaking, across variables are causes, and through variables are effects. In all of these domains, including electric circuits, across variables are independent of the path between the two points (i.e., they are conservative quantities).
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Back To CourseIntroduction to Engineering
14 chapters | 123 lessons
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