physics

PHYS 152 Online: Gredig Activity 17 - Page 6 of 6 August 20, 2015

Table 3: Measurements No. x (m) y(m) V (V) 1 2 3 4 5

4 EVALUATION

8. (1 point) After running the simulations, what have you learned about electric potentials?

9. (1 point) Looking at your predictions, do you still agree with them? If so, describe the relevant feature, if not, elaborate on the di↵erences. What piece of information is relevant to understand the predictions?

10. (1 point) List one question about the topic, which you are not sure about yet.

PHYS 152 Online: Gredig Name: Fall 2015 Activity 17 August 20, 2015

This activity contains 6 pages (including this cover page) and 10 questions. Total of points is 15.

1 OBJECTIVE: #17: Electric Potential

Use this simulation to analyze electric potential.

• Understand the connection between electric fields and electric potential.

• Analyze the electric potential of line charges.

Computing the electric field is cumbersome, since it involves a vector ~E. This vector is analogous to a hilly landscape and at each location on the hill there is a sign standing and the sign points in the direction downward, so if you were to pour water, it would follow all the signatures. The signs would describe the hill quite well, but more useful would be altitude contours. Contours that describe equal altitudes, which is a scalar. In our analogy, this altitude would be the electric potential V . Just like we cannot measure the height h without a reference point (sea level, for example), we also cannot measure the electric potential without a reference. Therefore, we always talk about �h, and �V . It is important to distinguish �V from V , even though many internet sources and even textbooks confuse the two. Take V to be price, then �V would be the change that you get back, and V would be the price of the item you purchase, two very di↵erent things. How can you compute the electric potential, knowing the electric field?

�V = Vf � Vi = � Z f

i

~

E · d~r

It is important to have an initial point and a final point, when computing the electric potential di↵erence �V .

2 PREDICTION

1. (2 points) Use 10 positive charges (each is 1.5 C) and form a line. Then draw the electric field vector at several locations. Next think about the electric potential and draw field lines of equipotential (where the electric potential is constant). Where is the electric potential large and where is it small?

PHYS 152 Online: Gredig Activity 17 - Page 2 of 6 August 20, 2015

2. (2 points) Use 5 positive charges (1.5 C each) and 5 negative charges to form two parallel lines (similar to a capacitor). Then draw the electric field vector at several locations. Next think about the electric potential and draw field lines of equipotential (where the electric potential is constant). Where is the electric potential large and where is it small? ? Choose a coordinate system, and compute the electric potential at 4 di↵erent locations, given that the charges are distributed as follows, see Table 1.

3. (1 point) Fill out Table 2 by computing the electric potential at several locations.

PHYS 152 Online: Gredig Activity 17 - Page 3 of 6 August 20, 2015

Table 1: Charge Distribution charge (C) x (m) y (m)

1.5 -0.25 -0.25 -1.5 -0.25 0.25 1.5 -0.14 -0.25 -1.5 -0.14 0.25 1.5 -0.03 -0.25 -1.5 -0.03 0.25 1.5 0.08 -0.25 -1.5 0.08 0.25 1.5 0.19 -0.25 -1.5 0.19 0.25

Table 2: Prediction No. x (m) y(m) V (V) 1 2 3 4 5

4. (2 points) Use 10 positive charges and form a ring. Prediction: Then draw the electric field vector at several locations. Next think about the electric potential and draw field lines of equipotential (where the electric potential is constant). Where is the electric potential large and where is it small?