Static Electricity Legacy Problem #7 Guided Solution

The successful solution of a physics problem involves reading the problem very carefully and identifying the known quantities in the problem statement, and then identifying the unknown quantity in the problem statement. This should be done before ever picking up a calculator and plugging and chugging into some physics equation. Here we notice there are three numerical quantities stated in the problem statement. First there's q1, 3.6 times 10 to the negative 12 coulombs. I know that's a q value, a quantity of charge, because I notice the unit c on the given quantity. The second quantity is q2, 6.8 times 10 to the negative 9th coulombs. That also is a quantity of charge as indicated by the unit c. And finally we know the d value, 0.019 centimeters, the distance of separation between the two charge objects. What we wish to calculate is we wish to calculate the force of electrical attraction. This force of electrical attraction can be calculated using the Coulomb's law equation which states F equal k, the proportionality constant, times q1 times q2 divided by d squared. In this equation the value of k is a constant value of 8.99 times 10 to the 9th newtons times meters squared per coulomb squared. The fact that the value of k has in it units of coulombs squared and units of meters squared means that we must do our substitutions of q1 and q2 and d into the equation in such a manner that the units c and meters cancels. That means I need to take my distance value of 0.019 centimeters and convert that to units of meters before ever substituting into the equation. Failing to do so will surely result in a miss. Now converting from the given centimeters to units of meters involves using the fact that there are 100 centi things in anything, 100 centi dollars in a dollar, centimeters in a meter, etc. And so I need to move the decimal place two places. I need to think about that. A centimeter is about the width of your little finger and so if you have 0.019 centimeter distance of separation, that's about 150th of the width of your finger. So now when you move the decimal place you have to think should I move it to the left or to the right two decimal places. If you were to move it to the right you would be saying that that little distance about 150th of the distance of the width of your fingernail is equal to 1.9 centimeters. Now think of it. Is that true or not true? Actually it's absurd. You moved the decimal place the wrong way, you need to move it the other direction such that you get for your distance 0.00019 meters. Now substitute all known quantities into the given equation and solve for the force in units of newtons. Now one thing you'll notice in discussing this problem is I ignored the fact that q1 and q2 were positive and negative respectively. This is not important information. It's not important because when you plug the plus and minus into the equation to calculate a force and you were to use that plus minus you'd get a minus value for the force which simply means it's an attractive force. Why is it not important to know that it's a negative force value? Because if all that negative force value means is attractive you knew that anyway by virtue of the fact that you knew that one object was positive and the other was negative.