# Chapter 21 Electric Charge and electric Field HW3

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Chapter 21, Electric Charge, and electric Field HW#3: Chapter 21: Pb. 1, Pb. 12, Pb. 13, Pb. 25, Pb. 31, Pb. 36, due Friday, Feb. 12 Charles Allison © 2000

21 -6 The Electric Field For a point charge: ε 0 is the permittivity of the free space=8. 85 x 10 -12 C 2/N. m 2 Charles Allison © 2000

21 -6 The Electric Field Force on a point charge in an electric field, the direction of the force depends on the charge sign Charles Allison © 2000

Electric Field: system of charges If we have a system of charges we can ask ourselves what the electric field is at any point in space, P? In this case we find the Electric field due to each charge in the system at that point, P, and we do a vector sum of these fields. Q 2 r 1 Q 1 P r 3 Q 3 Charles Allison © 2000

Problem 26 • Charles Allison © 2000

Problem 27 • Charles Allison © 2000

21 -6 The Electric Field Example 21 -7: E at a point between two charges. Two point charges are separated by a distance of 10. 0 cm. One has a charge of -25 μC and the other +50 μC. (a) Determine the direction and magnitude of the electric field at a point P between the two charges that is 2. 0 cm from the negative charge. (b) If an electron (mass = 9. 11 x 1031 kg) is placed at rest at P and then released, what will be its initial acceleration (direction and magnitude)? Charles Allison © 2000

21. 6 Electric Field 2 -D Problem: Two charges are positioned as seen below. Q 1= +2. 0 µC and Q 2= +8. 0µC. What is the magnitude and the direction of the Electric Field at point P? y q 1 r 1=0. 090 m P q 2 x r 2=0. 12 m k = 8. 99 x 10+9 Nm 2/C 2 = Coulomb’s constant Charles Allison © 2000

Problem 34 34. (II) Calculate the electric field at the center of a square 52. 5 cm on a side if one corner is occupied by a -38. 6μC charge and the other three are occupied by-27μC charges. Charles Allison © 2000

21 -6 The Electric Field Example 21 -8: above two point charges. Calculate the total electric field : (a) at point A and (b) at point B in the figure due to both charges, Q 1 and Q 2. Charles Allison © 2000