You are here:

Physics/Potetial and Field

Question
I am having difficulty with these two problems. Any help would be greatly appreciated.

1. Draw the electric field lines surrounding these point charges knowing that q_1 = -4 micro coulombs and q_2 = 6 micro coulombs.
(goes with first picture)

2. a) Using the equipotential surfaces above, draw the electric field lines
(goes with secod picture)
b) Determine the magnitude of the electric field at point A.

Hello Nick,

1. This website will help you draw the electric field lines around those 2 charges.
http://www.physicsclassroom.com/class/estatics/u8l4c.cfm
Scroll down about 60% of the way to the group of 4 sketches with the title "Electric Field Line Patterns for Objects with Unequal Amounts of Charge". The bottom right sketch is the one that applies to your problem. Except that the sketch has the one with positive charge on the left. If you print that, cut out the bottom right sketch, and turn it upside down, you will have the positive charge on the right. So then the only question is how dense to draw the lines.

Drawing electric field lines is really only a visual, learning aid. But to help you learn, I suggest that you use the ratio of the charges as a guide for the density of the lines. You could draw 6 lines from q2 and 4 lines to q1. It would probably be better to draw 9 and 6, or 12 and 8. As long as the ratio is correct.

Notice that the 4 rules just below the sketches the website gives to summarize the discussion above the sketches.

2. This website will help you draw the electric field lines on this figure.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/equipot.html
Notice that it says in the first paragraph that "Equipotential lines are always perpendicular to the electric field." Or rephrased in a way to apply to your given figure: Electric field lines are always perpendicular to the equipotential lines. Scroll down to the bottom frame. That is close to what you should do to your figure. However, the 2 charges in your figure apparently have the same polarity and magnitude. Notice that the closer you get to either, the more negative the equipotential is. So if you look back at the first website I gave you and scroll around looking at the other sketches, you should find how the field lines should be drawn when the charges are both negative and equal. So draw it that way keeping the intersections of the field lines and the equipotential lines perpendicular.

I hope this helps,
Steve

Physics

Volunteer

Steve Johnson

Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University