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1Physics 314 Written Homework #3 September 10, 2020

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PROBLEMS (6 points each). Do these problems on a separate piece of paper and attach them to the cover page. Make sure that the copy your upload to Blackboard is readable. Note: Show your work in a nice logical progression. The easier it is for me to follow your work the greater the chance you’ll earn partial credit. When appropriate draw pictures and explain your procedure in words. Warning: Magical formulae that appear out of thin air without derivation will be given a zero score!

1. Two 10-cm-diameter charged disks face each other, 20 cm apart. The left disk is charged to −60 nC and the right disk is charged to +60 nC. a. What is the electric field E, both magnitude and direction, at the midpoint between the two disks? [4 pts.] b. What is the force F, both magnitude and direction, on a −2.0 nC charge placed at the midpoint? [2 pts.] 2. Air “breaks down” when the electric field strength reaches 3.0 × 106 N/C , causing a spark. A parallel-plate capacitor is made from two 5.0 cm × 5.0 cm electrodes. How many electrons must be transferred from one electrode to the other to create a spark between the electrodes? [6 pts.] 3. An ammonia molecule (NH3) has a permanent electric dipole moment 5.0 × 10−30 cm. A proton is 4.0 nm from the molecule in the plane that bisects the dipole. What is the electric force of the molecule on the proton? [6 pts.] 4. Find the electric flux through a rectangular area 3.0 cm by 5.0 cm between two parallel plates where there is a constant electric field of 30. N/C for each of the following cases: a. When the area is parallel to the plates. [2 pts.] b. When the area is perpendicular to the plates. [2 pts.] c. When the normal to the area makes a 30o angle with the electric field. [2 pts.] 5. A net flux of 1.0 x 104 Nm2/C pass inward through the surface of a sphere, which has a radius of 6.0 cm. a. How much charge is inside the sphere? [4 pts.] b. How precisely can we determine the location of the charge? [2 pts.]...