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Questions 1, 2, and 3 are short free-response questions that require about 13 minutes to answer and are worth 8 points. Questions 4 and 5 are long free-response questions that require about 25 minutes each to answer and are worth 13 points each. Show your work for each part in the space provided after that part.(DIAGRAM IS NOT AVAILABLE TO COPY)Two charges, $q_{1}=+3 \mathrm{n} \mathrm{C}$ and $q_{2}=-12 \mathrm{nC}$ , are fixed in place and separated by a distance of 3 $\mathrm{cm} .$(a) What is the magnitude of the electric force between the two charges? Is the force attractive or repulsive?(b) At what point between the two charges is the electric field equal to 0?(c) What is the electric field at the midpoint between the two charges?(d) What is the magnitude of the electric force on a charge $q_{3}=+2 \mathrm{nC}$ placed in the middle of the two charges?
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Chapter 14
Practice Test 4
Section 2
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as an example of working with columns law, electric force, an electric field, we'll take a look at two charges that are sitting in space next to each other. one is positive and the other is negative. And let's just give those some values uh minus 12 anna columns on the right And a positive three and columns on the left separated by 30 centimeters. Okay, So columns law gives us two formulas for both the force of attraction between two charges, and that should be written with an absolute value around those two charges. Why? Because this equation gives you the magnitude of the interaction. The direction comes from basically knowing what signs of charges there are and whether situated in space. So the force always acts on a line joining the bear and it will Act one way if they are attracting each other, it will act opposite that on both charges if they repel. Um in the electric field has a similar type of relationship. Again, there's A. K. And the absolute magnitude of a. Q. Over by divided by the distance squared. Um but notice that the electric field assumes just one charge sitting there in space creating a field, whereas the force measures in interaction. So let's take a look at this example. Um the two charges will obviously feel an attraction and that will be a Newton's third law pair. So those are equal and opposite each other. Um And hence you don't want to be putting signs on your columns law calculation. But anyway, uh we can calculate the magnitude of that interactive force. The electrical constant is nine times 10 to the 9th in S. I. Units. And remember we want the absolute magnitudes of those charges. So we're going to put them in both with positive signs. And really it makes no sense to have a negative magnitude. Only astronomers make that happen. Um Yeah, but a vector magnitude is not a negative number. So let's see what we get in terms of Newton's, we've used everything in S. I. units. So this is 3.6 Times 10 to the -6. Um Newton's okay. The next situation we're going to think about is the electric field to add a point right in between those two charges. So we have our negative and positive charges and we are going to look at a point, we'll call that point P. So one 15 cm from both 15 centimeters and 15 centimeters. Now, the electric field is calculated at all points in space by taking however many point charges you have here there too, and adding up their individual electric fields. Um So electric field is a vector and it usually tells you um the force a positive charge would experience if it were placed at a point here we're talking. Look looking at point P. And in order to get the vector right, what you really want to do is imagine the vectors at that point that are created by your two charges. So in our situation, electric field always emanates outwards from a positive charge. And we could draw a bunch of lines coming out of that charge. But what we're interested in is that vector that's on the side with P similarly, electric field is going into um a negative charge. Sorry, that should be minus. And again, we could draw all sorts of factors going into that charge, but what we see is is at point P. Both fields point to the right. So our electric field is going to be the sum of their magnitudes will give us the magnitude of that electric field and the direction will be pointing to the right And again, to find those magnitudes, Remember, we do not want to put in a sign of a charge, we put in a magnitude of charge because if you don't do that, you will wind up subtracting something when you meant to add it and that would be bad. Yeah, and r E minus. We do the same thing. Of course. This is even more critical that we put in a absolute value of the charge. And if all come out, if all quantities are put in S. I. Units, the unit should be newtons per Coolum. Okay, and that works out to be 60 newtons per column. He plus plus c minus turns out to be 60 Newtons per Coolum. And the direction is to the right. And I'll tell you if you put in the wrong sign of charge in this this relationship, you'll think that the positive charge cannot create a electric field to the left and vice versa. The negative charge can't create an electric field to the right. And we see that that's definitely not true that the electric field points of different directions in space around each charge. Um So you have to be careful with that, and finally, if we put another charge into the situation, um So let's put a charge of 2-plus 2. Yeah, the columns there, right smack dab in the middle. We could work out the force on it based on the forest from the two charges nearby, but it is much easier to say F equals Q three times the electric field. Um Where e is the total electric field experienced by that charge at that point, which is 60 Newton columns practical. Um To the right standard minus nine. And that will be to the right as well. So, a thing to remember is that the electric field gives the sense of the force that would be directed on a positive charge. So this would be 1.2 Types 10 of -7 Newtons, and to the right.
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