# Physics 121 Fall 2004

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```					Physics 121 Practice Problem Solutions 03
Electric Field

Contents:
121P03 - 1Q, 4P, 6P, 8P, 13P, 21P, 23P, 39P

•   Recap & Definition of Electric Field
•   Electric Field Lines
•   Charges in External Electric Fields
•   Field due to a Point Charge
•   Field Lines for Superpositions of Charges
•   Field of an Electric Dipole
•   Electric Dipole in an External Field: Torque and Potential
Energy
•   Method for Finding Field due to Charge Distributions
–   Arc of Charge
–   Ring of Charge
–   Disc of Charge
–   Infinite line of Charge
•   Summary                                                      1
Fall 2008
PROBLEM 121P03 - 1Q: The figure shows three electric field lines. What is the direction of the
electrostatic force on a positive test charge placed at (a) point A and (b) point B? (c) At which
point, A or B, will the acceleration of the test charge be greater if the charge is released?

Fall 2008
PROBLEM 121P03 - 4P: What is the magnitude of a point charge that would create an electric
field of 1.00 N/C at points 1.00 m away?

Fall 2008
PROBLEM 121P03 - 6P: Two particles with equal charge magnitudes 2.0 x 10-7 C but opposite signs
are held 15 cm apart. What are the magnitude and direction of the Electric Field E at the point midway
between the charges?

Fall 2008
PROBLEM 121P03 - 8P*: In the figure, two fixed point charges q1 = +1.0 x 10-6 C and q2 = +3.0 x 10-6 C
are separated by a distance d = 10 cm. Plot their net electric field E(x) as a function of x for both positive
and negative values of x, taking E to be positive when the vector E points to the right and negative
when E points to the left.

Fall 2008
PROBLEM 121P03 - 13P: What are the magnitude and direction of the electric field at the center of the
square in the figure, if q = 1.0 x 10-8 C and a = 5.0 cm?

Fall 2008
PROBLEM 121P03 - 21P*: A thin glass rod is bent into a semicircle of radius r. A charge +q is uniformly
distributed along the upper half, and a charge -q is uniformly distributed along the lower half, as shown
in the figure. Find the magnitude and direction of the electric field E at P, the center of the semicircle.

P

Fall 2008
PROBLEM 121P03-23P:In Fig. 23-35 , a nonconducting rod of length L has charge -q uniformly distributed
along its length. (a) What is the linear charge density of the rod? (b) What is the electric field at point P, a
distance a from the end of the rod? (c) If P were very far from the rod compared to L, the rod would look like
a point charge. Show that your answer to (b) reduces to the electric field of a point charge for a >> L.
y       dq = ldx

x                         r
kdq ˆ
dEP     2
i along x-axis by symmetry                             r=a+L-x
r
l = linear charge density = -q/L                                    dq = ldx
L
k l dx
L
dx                     1
L                       Note
EP                     ˆ  kl ˆ
i      i                  kl ˆ
i                            d            
(a  L - x)2              (a  L - x)2          (a  L - x) 0                      1           (-1)(-1)
 a  L - x   (a  L - x)2
0                         0                                                 dx            

kq ˆ  1  1    1 qˆ  L 
EP       i              i
L a  L a    4 0 L  a(a  L) 


q 1  1 ˆ                  For L << a, the result
 EP             2  1  L/a 
i       approaches the point
4  0 a           
charge formula
Fall 2008
PROBLEM 121P03 - 39P: A uniform electric field exists in a region between two oppositely charged
plates. An electron is released from rest at the surface of the negatively charged plate and strikes the
surface of the opposite plate, 2.0 cm away, in a time 1.5 x 10-8 s. (a) What is the speed of the electron as
it strikes the second plate? (b) What is the magnitude of the electric field E ?

Fall 2008

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