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Voltage, Current, and Resistance
11 questions By Tony R. Kuphaldt
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Question 1 of 11
Voltage is commonly defined as “electrical pressure.” The unit of the volt, however, may be defined in terms of more fundamental physical units. What are these units, and how do they relate to the unit of the volt?
Reveal answer1 volt is equal to 1 joule of energy imparted to 1 coulomb of charge (6.25 ×1018 electrons):
V = W QWhere,
V = Voltage (volts)
W = Work, or potential energy (joules)
Q = Charge (coulombs)
Notes:Note that I use the letter “V” to denote voltage rather than “E” as I usually do. This is because in general physics work, “E” usually stands for either “Energy” or “Electric field”. Some electronics reference books use the letter “E” for voltage, while others use the letter “V”, or even use the two letters interchangeably.
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Question 2 of 11
Electric current is measured in the unit of the ampere, or amp. What is the physical definition for this unit? What fundamental quantities constitute 1 ampere of electric current?
Reveal answer1 ampere of electric current is the rate of electron motion equal to 1 coulomb per second:
I = Q tWhere,
I = Electric current (amperes)
Q = Charge in motion (coulombs)
t = Time (seconds)
Notes:It may be helpful at this point to review the number of electrons constituting one coulomb of charge: 6.25 ×1018 electrons.
Technically, current’s mathematical definition involves calculus:
I = dQ dtHowever, students at this stage may not be ready to explore derivatives yet, and so the equation give in the answer for (average) current will suffice.
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Question 3 of 11
Explain what the electrical terms voltage, current, and resistance mean, using your own words.
Reveal answerVoltage: electrical “pressure” between two different points or locations.
Current: the flow of electrons.
Resistance: opposition, or “friction,” to the flow of electrons.
These three are related through Ohm’s Law.
Notes:While it is easy enough for students to look up definitions for these words from any number of references, it is important that they be able to cast them into their own words. Remembering a definition is not the same as really understanding it, and if a student is unable to describe the meaning of a term using their own words then they definitely do not understand it! It is also helpful to encourage students to give real-life examples of these terms.
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?