Static Electricity Audio Guided Solution SE8Q2

Problem Set SE8, Question 2:

A metal sphere is charged by losing 3.98 x10¹³ electrons while a second sphere is charged by losing 6.35 x10¹³ electrons. The two spheres are 18.7 cm apart. Determine the amount of electrostatic force (in Newtons) between the two spheres.

Audio Guided Solution

Problem 14 can be thought of as a relatively routine calculation of the electrostatic force between two spheres. To calculate the electrostatic force, one needs to know the distance of separation, which is stated here as 18.7 cm, and one also needs to know the quantity of charge on the two objects, Q1 and Q2. Neither Q1 nor Q2 are given, however, it does mention that both of the spheres have lost electrons, giving it an overall positive charge, and the quantity of charge can be calculated by the number of electrons lost. The charge of one electron is 1.6 x 10-19, and then multiply that number by 3.98 x 10-13 to find Q1, and by 6.35 x 10-13 to find Q2. Once you have Q1 and Q2, you say F equals K, 9 x 10-9, multiplied by Q1, multiplied by Q2, divided by the distance squared in meters. So don't use the 18.7 cm in the denominator, rather use the 0.187 m in the denominator to get all units to cancel, with the exception of newtons, the unit on force. Good luck!

Solution

Habits of an Effective Problem Solver

An effective problem solver by habit approaches a physics problem in a manner that reflects a collection of disciplined habits. An effective problem-solver...

... reads the problem carefully and develops a mental picture of the physical situation. If necessary, sketches a simple diagram of the physical situation to help visualize it.
... identifies the known and unknown quantities; records them in an organized manner. A diagram is a great place to record such information. Equates given values to the symbols used to represent the corresponding quantity - e.g., Q₁ = 2.4 μC; Q₂ = 3.8 μC; d = 1.8 m; F_elect = ???.
... uses physics formulas and conceptual reasoning to plot a strategy to solve for the unknown quantity.
... identifies the appropriate formula(s) to use.
... performs substitutions and algebraic manipulations in order to solve for the unknown quantity.

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Coulomb's Law and Force Analysis