PES 1120: Homework assignment 1 [24 points]

Handed out: Friday January 31Due in: Friday February 7, at the start of class

Show all working and reasoning to receive full points.

Part 1: fully graded questions [18 points]

Question 1 [12 points]Three point charges, q1, q2, and q3 are placed along a straight, vertical line as shown. At thepoint P , which is directly to the right of q1, the net electric field created by three charges iszero.

a)[2 points] If q1 and q3 are both positive charges, is q2 positive or negative? For full pointsyour explanation must include a diagram showing the three individual electric fields.b)[2 points] Assuming q2 is negative, draw in a new diagram a sketch of the electric fieldlines that are present around the three point charges.c)[8 points] Assuming q2 is negative and q1 = 10 µC and the distance between the chargesare as given in the diagram, find the values of q2 and q3.

Question 2 [6 points]In an inkjet printer, letters are built up by squirting drops of ink at the paper from a rapidlymoving nozzle. The ink drops, which have a mass of 1.0× 10−8 g each, leave the nozzle andtravel toward the paper at 20 m/s, passing through a charging unit that gives each drop apositive charge q by removing some electrons from it. The drops then pass between paralleldeflecting plates 2.5 cm long where there is a uniform vertical electric field with magnitude9.0× 104 N/C.a)[4.5 points] If a drop is to be deflected 0.50 mm by the time it reaches the end of thedeflection plates, what magnitude of charge must be given to the drop? Assume gravity isnegligible.b)[1.5 points] Proof that it was OK in part a) to ignore gravity. Your reasoning must besupported by calculations.[Note : Similar to book problem on page 593 (9th edition) or page 637 (10th edition).]

Part 2: completion grade only [6 points]

In this section you must fully complete 3 problems from the textbook to score 6 points intotal. If you do everything correct in a problem, you will get 2 points for it. If you getsomething wrong but make a reasonable attempt, you will get 1 point for a problem. Noattempt or an unconvincing attempt will score zero.

To get the full 2 points you must:

• Write down all known and unknown variables clearly

• Draw a diagram

• Include all mathematical and logical steps, including assumptions

• Get the correct answer with the correct units and significant figures

Please grade this section yourself using the answers at the back of the book. The grader willcheck that you have not cheated by awarding yourself too many points.

This week’s questions are:

• Chapter 21, Problem 51 [2 points]

• Chapter 22, Problem 19 [2 points]

• Chapter 22, Problem 49 [2 points]

PES 1120: Homework assignment 1 [24 points]

Handed out: Friday January 31Due in: Friday February 7, at the start of class

Show all working and reasoning to receive full points.

Part 1: fully graded questions [18 points]

Question 1 [12 points]Three point charges, q1, q2, and q3 are placed along a straight, vertical line as shown. At thepoint P , which is directly to the right of q1, the net electric field created by three charges iszero.

a)[2 points] If q1 and q3 are both positive charges, is q2 positive or negative? For full pointsyour explanation must include a diagram showing the three individual electric fields.b)[2 points] Assuming q2 is negative, draw in a new diagram a sketch of the electric fieldlines that are present around the three point charges.c)[8 points] Assuming q2 is negative and q1 = 10 µC and the distance between the chargesare as given in the diagram, find the values of q2 and q3.

Question 2 [6 points]In an inkjet printer, letters are built up by squirting drops of ink at the paper from a rapidlymoving nozzle. The ink drops, which have a mass of 1.0× 10−8 g each, leave the nozzle andtravel toward the paper at 20 m/s, passing through a charging unit that gives each drop apositive charge q by removing some electrons from it. The drops then pass between paralleldeflecting plates 2.5 cm long where there is a uniform vertical electric field with magnitude9.0× 104 N/C.a)[4.5 points] If a drop is to be deflected 0.50 mm by the time it reaches the end of thedeflection plates, what magnitude of charge must be given to the drop? Assume gravity isnegligible.b)[1.5 points] Proof that it was OK in part a) to ignore gravity. Your reasoning must besupported by calculations.[Note : Similar to book problem on page 593 (9th edition) or page 637 (10th edition).]

Part 2: completion grade only [6 points]

In this section you must fully complete 3 problems from the textbook to score 6 points intotal. If you do everything correct in a problem, you will get 2 points for it. If you getsomething wrong but make a reasonable attempt, you will get 1 point for a problem. Noattempt or an unconvincing attempt will score zero.

To get the full 2 points you must:

• Write down all known and unknown variables clearly

• Draw a diagram

• Include all mathematical and logical steps, including assumptions

• Get the correct answer with the correct units and significant figures

Please grade this section yourself using the answers at the back of the book. The grader willcheck that you have not cheated by awarding yourself too many points.

This week’s questions are:

• Chapter 21, Problem 51 [2 points]

• Chapter 22, Problem 19 [2 points]

• Chapter 22, Problem 49 [2 points]

Chapter 21, problem 51

Chapter 22, problem 19

Chapter 22, problem 49