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Static Electricity
9 questions By Tony R. Kuphaldt
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Question 4 of 9
How many electrons are there in 2 ×10−5 coulombs of charge? Determine the answer without using a calculator, and express the answer in scientific notation!
Reveal answerThere are approximately 1.25 ×1014 electrons in this quantity of charge.
Notes:One of the benefits of using scientific notation is that it allows us to easily perform multiplication and division using very large and very small numbers.
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Question 5 of 9
If a positive charge is forcibly moved closer to another positive charge, mechanical work must be performed. This is analogous to compressing a spring: work must be done to cause motion against the natural repulsion of the system:
This mechanical work becomes “stored” in the electric field between the charges, and is a form of potential energy. This, again, is similar to a mechanical spring, where the work done in compressing a spring is “stored” as potential energy in its compressed state.
Define voltage in terms of the change in potential energy resulting from this forceful motion of the charge, the way a physicist would.
Reveal answerVab = Ub − Ua qWhere,
Vab = Voltage between points A and B
Ub − Ua = Difference in potential energy between points A and B
q = Quantity of charge (moved), in Coulombs
Follow-up question: explain why, if a charge of twice the magnitude (2q) were moved from the same point A to the same point B, there would be twice as much work done and twice as much potential energy stored, but exactly the same voltage (potential) as gained by the smaller charge (q).
Notes:If time permits, it is quite enlightening for students to discover that voltage may be defined in simple mechanical terms of repulsion and work. Of course, a basic familiarity with work in the scientific sense of the term is a prerequisite for understanding this scenario.
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Question 6 of 9
A co-worker of mine was once installing sheet-styrofoam insulation in the walls of his new garage. The sheets of insulation came wrapped together in bundles for easy transport and storage.
One side of each sheet was covered in aluminum foil. This acted as a reflective surface for infra-red (heat) radiation, and improved the effectiveness of the insulating panel. It also happened to function as a conductor of electricity, much to the dismay of my friend.
My friend found that the sheets were physically stuck together by the force of static electric charges when they were delivered to his house. Upon trying to pry two of them apart from each other, he received a surprisingly large electric shock from touching the aluminum foil surfaces of the respective panels! Explain how the physical work he did in separating the panels from each other became manifest as voltage, based on your knowledge of voltage, work, and electric charges.
Reveal answerVoltage is defined as work per unit charge, which tells us that this fellow’s physical work of separating the attractive charges became translated into electrical voltage, which shocked him when the magnitude became great enough.
Notes:By the way, this is an absolutely true story! This scenario could be easily replicated on a dry (low-humidity) day, with two large sheets of styrofoam insulation and a brave person pulling them apart!
Someone clearly did an ill-advised search and replace turning “element” to “entry”, so this page mentions “entryary charge” instead of “elementary charge” and “the entry carbon” instead of “the element carbon”. Please fix this.
What is the math behind Q4? Showing the work might be a good idea.