PHYS-222 Worksheet 2
Problem 2-1
a toy model of proton:
A proton consists three quarks, with charges + 32 e, + 32 e, − 31 e respectively, where e is electron charge (elementary charge). Let’s construct a toy model for proton. Three quarks are located on vertices of an equilateral
triangle (See Fig. 1). The length of the edges is equal the diameter of proton 10−15 m. What is the magnitude
of the force on the negative charged quark? (the answer is between 1N - 100N)
b
q1 = + 32 e
a = 10−15 m
b
b
q2 = + 32 e
q3 = − 31 e
Figure 1: problem 2-1
problem 2-2
P-21.98
y
−Q
+Q
a
x
O
Figure 2: problem 2-2
A semicircle of radius a is in the first and second quadrants, with the center of curvature at the origin.
Positive charge +Q is distributed uniformly around the left half of the semicircle, and negative charge −Q is
distributed uniformly around the right half of the semicircle (Fig. 2). What are the magnitude and direction
of the net electric field at the origin produced by this distribution of charge? (the direction is ? the magnitude
E ∝ aQ2 )
1
problem 2-3
three point charges are arranged along the x-axis. Charge q1 = −4.5nC is located at x = 0.2m, charge
q2 = +2.5nC is at x = −0.3m. (See Fig. 3) Find a point along the axis, where the electric field E is zero.
(-1.76 m, 0.086 m )
q2 = +2.5nC
b
b
x2 = −0.3m
q1 = −4.5nC
x1 = +0.2m
Figure 3: problem 2-3
keys: 1. 88.8 N 2.
2Q
π 2 ε0 a2 ,
direction, +x 3. -1.76 m
2