PHYS 308 Homework
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   Homework #1: Due Friday January 30 by 5:10pm
  1. problem 1.35 in your textbook. This is a simple use of the old physics-2 expression for the electric force, F=kq1q2/r2, with k=1/(4πε0)=8.99x109N⋅m2/C2. Use the mass of the electron me=9.11x10-31kg.
  2. Find the unit vector that points from (x,y)=(-4.5, 7.6) towards (x,y)=(3.4, -6.9). Show your work. Your final answer needs to have the separate x- and y-components shown explicitly as I did in class.
  3. Find the unit vector that points from (x,y)=(5.2cm, 3.1cm) towards (x,y)=(3.4cm, -6.9cm). Show your work. Your final answer needs to have the separate x- and y-components shown explicitly as I did in class.
  4. The electric field E at the surface of Earth has an average magnitude of E=120 N/C, and points towards Earth's surface. The average radius of Earth is 6,370km. Watch your units here.
       a. Find the magnitude of the net charge on Earth.
       b. Explain (a sentence or two) whether this charge is positive or negative, and how you know this.
  5. Three concentric metal spheres have the following radii and charges: r1=10cm, q1=(-)12μC; r2=25cm, q2=(+)18μC; r3=45cm, q3=(-)15μC. Find the magnitude and list the direction of the electric field E at the following distances from the center: (a)6.0cm, (b) 30cm, (c) 62cm.
   That's all for homework #1.