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Voltage, Current, and Resistance
11 questions By Tony R. Kuphaldt
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Question 4 of 11
Describe what “electricity” is, in your own words.
Reveal answerIf you’re having difficulty formulating a definition for “electricity,” a simple definition of “electric current” will suffice. What I’m looking for here is a description of how an electric current may exist within a solid material such as a metal wire.
Notes:This question is not as easy to answer as it may first appear. Certainly, electric current is defined as the “flow” of electrons, but how do electrons “flow” through a solid material such as copper? How does anything flow through a solid material, for that matter?
Many scientific disciplines challenge our “common sense” ideas of reality, including the seemingly solid nature of certain substances. One of the liberating aspects of scientific investigation is that it frees us from the limitations of direct sense perception. Through structured experimentation and rigorous thinking, we are able to “see” things that might otherwise be impossible to see. We certainly cannot see electrons with our eyes, but we can detect their presence with special equipment, measure their motion by inference from other effects, and prove empirically that they do in fact exist.
In this regard, scientific method is a tool for the expansion of human ability. Your students will begin to experience the thrill of “working with the invisible” as they explore electricity and electric circuits. It is your task as an instructor to foster and encourage this sense of wonder in your students’ work.
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Question 5 of 11
What units of measurement are used to express quantities of voltage, current, and resistance?
Reveal answer- • Voltage is measured in the unit of the volt (V).
- • Current is measured in the unit of the ampere, or amp (A).
- • Resistance is measured in the unit of the ohm (Ω).
Notes:Like all other physical quantities, there are units of measurement appropriate for specifying electrical quantities. In this sense, the “volt,” “amp,” or “ohm” are no different from “gram,” “yard,” or “bushel.”
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Question 6 of 11
How many physical points must be referenced when speaking of the following electrical quantities?
- • Voltage
- • Current
- • Resistance
In other words, does it make sense to speak of voltage at a single point, or between two points, or between three points, etc.? Does it make sense to speak of current at a single point, between two points, between three points, etc.?
Reveal answer- • Voltage is always measured between two points.
- • Current may be measured at a single point (at a cross-section of a conductive path).
- • Resistance is always measured between two points.
Follow-up question: explain, if you can, the relevance of these facts to electrical safety. For example, why is it important to know that voltage is always a quantity existing between two points (rather than existing at a single point) when considering your personal safety?
Notes:One of the more confusing aspects of electrical theory is that both voltage and resistance are quantities relative between two points. It is meaningless to speak of the amount of voltage “at this point” in a circuit, at least without implying a reference point such as ground. The same goes for resistance: it makes no sense to speak of how much electrical resistance there is “at this point” in a circuit either.
Conversely, it makes perfect sense to speak of either voltage or resistance lying “between this point and that point,” because both these quantities are relative between exactly two points.
Current, on the other hand, is the coordinated motion of electric charge through a conductor. As such, it may be measured at any single point in a circuit, like measuring the flow of water in a river at any one particular cross-sectional sample.
I’m having trouble with combo circuits
I can understand that voltage is potential energy to move an electron between two points and that something like a battery stores this potential independently of current flow. However, if Ohm’s Law states that V=IR and anything multiplied by zero is zero then V=0x has to be V=0. No current, no voltage. How to reconcile this seeming paradox?