Physics C Electricity & Magnetism
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Practice Problems: Electric Fields
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1. (easy) What is the magnitude of a point charge whose E-field at a distance of 25 cm is 3.4 N/C?

2. (easy) A small charge (q = 6.0 mC) is found in a uniform E-field (E = 2.9 N/C). Determine the force on the charge.

3. (easy) A dipole is set up with a charge magnitude of 2x10-7 C for each charge (one is positive and the other is negative.) The distance between the charges is 0.15 m.  What are the magnitude and direction of the E-field at the midpoint of the dipole? (Assume the positive charge is on the left.) Also determine the force magnitude and direction for an electron at that position in the field.

4. (moderate) Two charges (q1 and q2) are located on the x axis on a coordinate system. They are both positive, but the second charge has twice the magnitude of the first. q1 is at -0.5 m while q2 is at +0.5 m. Determine the overall direction of the E-field at the various positions listed below:
A. At the origin
B. At x = 0 and y is negative
C. At x = -0.5 and y is positive
D. At x > 0 but x < +0.5 m and y = 0
E. At x > +0.5 m and y = 0
F. At x = +0.5 m and y > 0
Choose your answers from the following:
Choice 1: Along the +x-axis
Choice 2: Along the +y axis
Choice 3: Along the –x axis
Choice 4: Along the –y axis
Choice 5: Between 1 and 89 degrees from the +x axis
Choice 6: Between 91 and 179 degrees from the +x axis
Choice 7: Between 181 and 269 degrees from the +x axis
Choice 8: Between 271 and 359 degrees from the +x axis

5. (moderate) Repeat the previous question for points A,B,D and E, except assume that the first charge is negative and the second charge is positive.

6. (moderate) Four equal charges are located on the corners of a square as shown below.  What is the magnitude of the E-field at the center position of the square?

7. (hard) Find the E-field (both magnitude and direction) at the center of the square charge distribution shown below.  Note that the charges are NOT equal.  Assume that the sides of the square have a length L.

Please supplement these problems with those found in your companion text.

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