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Voltage, Current, and Resistance
11 questions By Tony R. Kuphaldt
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Question 7 of 11
Lightning is a natural, electrical phenomenon. It is caused by the accumulation of a large electrical charge over time resulting from air, dust, and water droplets transporting small electrical charges.
Explain how the terms voltage, current, and resistance relate to the process of lightning. In other words, use these three terms to explain the cycle of charge accumulation and lightning discharge.
Reveal answerAs electric charges accumulate between clouds and earth, the voltage between these points increase. Air under normal conditions is a good insulator of electricity: that is, it possesses very high electrical resistance. So, at first there is no current resulting from the rise in voltage between clouds and earth. However, when the voltage exceeds the air’s “ionization” potential, the air becomes a good conductor of electricity (its electrical resistance decreases dramatically), resulting in a transient current as the accumulated electric charge dissipates in the form of a lightning bolt.
Notes:I usually avoid spending a lot of time on technical definitions, because undue emphasis on the definitions of words tends to reinforce rote memorization rather than true comprehension. If students must master certain definitions, though, it is best to develop that mastery in the context of application: ask the students to use their new vocabulary, not just recite it.
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Question 8 of 11
Is it possible to have a condition where an electrical voltage exists, but no electric current exists? Conversely, is it possible to have a condition where an electric current exists without an accompanying voltage? Explain your answers, and give practical examples where the stated conditions are indeed possible.
Reveal answerIt is not only possible, but quite common in fact, to have a condition of voltage with no current. However, the existence of an electric current must normally be accompanied by a voltage. Only in very unique conditions (in “superconducting” circuits) may an electric current exist in the absence of a voltage.
Notes:This question challenges students’ comprehension of voltage and current by asking them to explain the relationship between the two quantities in practical contexts. Do not allow students to simply give a “yes” or a “no” answer to either of the stated conditions. Encourage them to think of examples illustrating a possible condition.
The term “superconducting” may spur some additional questions. As usual, do not simply tell students what superconductivity is, but let them research this on their own. Your more inquisitive students will probably have already researched this topic in response to the answer!
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Question 9 of 11
Voltage is also known by another name: electromotive force, or EMF. Explain what this other name for voltage means.
Reveal answerElectromotive force literally means, “a force that motivates electrons.”
Notes:This sheds some light on why the letter “E” is often used to symbolize voltage in mathematical equations. Some of your students may wonder later why the letter “I” is used to represent current. Let them research this on their own!
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?