What Determines the Strength of Electrostatic Interactions?

Transcription

1 What Determines the Strength of Electrostatic Interactions? 1. Fin a Pattern Charles Coulomb use a torsion balance to measure the force that one charge sphere exerts on another charge sphere to fin how the force between two electrically charge objects epens on the magnitues of the two charge objects an on their separation. Coulomb coul not measure the absolute magnitue of the electric charge on the metal spheres. However, he coul ivie the charge in half by touching a charge metal sphere with an ientical uncharge metal sphere. a. Explain the mechanism behin his metho of iviing the charge. Why i he have to use metal spheres? Woul the foam spheres work? b. The table below provies ata that resembles what Coulomb might have collecte. Fin patterns in the ata an evise a mathematical relationship base on these observations. Use graph paper to help (hint: you nee to make 3 graphs). Remember to ecie which variables are inepenent an which variable is the epenent. Then examine the epenent variable as you change only one inepenent variable at a time. Experiment Charge q 1 Charge q 2 Distance r Force F q1 on q2 1 1 (unit) 1(unit) 1(unit) 1 (unit) 2 ½ 1 1 ½ 3 ¼ 1 1 ¼ 4 1 ½ 1 ½ 5 1 ¼ 1 ¼ 6 ½ ½ 1 ¼ 7 ¼ ¼ 1 1/ ¼ / /16 q 1 q 2 r c. Write an expression that shows F q1 on q2 as a function of q 1, q 2 an the istance between their centers.

2 2. Reason (The following activities were borrowe from E&M TIPERs) The iagrams show two charge objects an their separation. Rank the force that the left object exerts on the right object from the STRONGEST to the WEKEST force. Explain how you mae the ranking q +2q +3q q -q -q C D ½ E q 2 +3q +3q 1/3 For each of the cases compare the force that the left object exerts on the right object to the force that the right object exerts on the left object. Draw an label force iagrams. Explain how you know.

3 3. Reason In each iagram below are small charge objects fixe on a gri. ll charge objects q are ientical. ll other charge objects are a multiple of Q. Draw the electric force iagram for each charge q, then rank the magnitues of the net electrical forces exerte on q. When you are one, ientify thir law pair forces for each iagram. +9Q -3Q +Q

4 4. Reason In each iagram below are small charge objects fixe on a gri. ll charge objects q are ientical. ll other charge objects are a multiple of Q. Draw the electric force iagram for each charge q, then rank the magnitues of the net electrical forces exerte on q. -3Q C D +9Q -3Q

5 5. Reason In each iagram below are small charge objects momentarily fixe on a gri. charge object of is place on the left a istance from the +3q charge object. The two objects are release simultaneously in a frictionless environment. Next to each particle is its mass m (or some multiple of m). a) Compare the magnitue of the force that particle exerts on particle with the force that particle exerts on particle. Write the result of your comparison as a ratio. b) Rank the magnitue of acceleration of each particle. Show all your work. Scenario 1 Scenario 2, m +3q, m, 2m +3q, m Scenario 3 Scenario 4,2 m +3q, 2m, m +3q, 2m Di You Know? Coulomb s law: The magnitue of the electric force that object 1, with electric charge q 1, exerts on object 2, with electric charge q 2, when they are separate by a center-to-center istance r is given by the expression below. Note that this is also the equal magnitue electric force that object 2, with electric charge q 2, exerts on object 1, with electric charge q 1 : F q1 on q 2 =F q2 on q 1 k q 1 q 2 r 2 where k = 9.0 x 10 9 N m 2 / C 2. We assume that the objects are much smaller than their separation r (i.e. point-like objects).

6 Combining Mechanics an Electrostatics Use Coulomb s Law to answer the following questions ON SEPRTE SHEET OF PPER: 1. Samantha rubs two latex balloons against her hair, causing the balloons to become charge negatively with 2.0 x 10-6 C. She hols them a istance of 0.70 m apart. What is the electric force between the two balloons? Will the balloons attract or repel? Explain. 2. Two pieces of puffe rice become equally charge as they are poure out of the box an into Kirk s cereal bowl. If the force between the puffe rice pieces is 4 x N when the pieces are 0.03 m apart, what is the charge on each of the pieces? 3. When sugar is poure from the box into the sugar bowl, the rubbing of sugar grains creates a static electric charge that repels the grains, an causes the sugar to go flying out in all irections. If each of the two sugar grains acquires a charge of 3.0 x C at a separation of 8.0 x 10-5 m, with what force will they repel each other? 4. oppo the clown carries two mylar balloons that rub against a circus elephant, causing the balloons to separate. Each balloon acquires 2.0 x 10-7 C of charge. How large is the electric force between them when they are separate by a istance of 0.50 m. 5. Inez uses hairspray on her hair each morning before going to school. The spray spreas out before reaching her hair partly because of the electrostatic charge on the hairspray roplets. If two roplets of hairspray repel each other with a force of 9.0 x 10-9 N at a istance cm, what is the charge on each of the equally-charge rops of hairspray? 6. onnie is usting the house an raises a clou of ust particles as she wipes across a table. If two 4.0 x C pieces of ust exert an electrostatic force of 2.0 x N on each other, how far apart are the ust particles at that time? 7. Each of two hot air balloons acquires a charge of 3.0 x 10-5 C on its surface as it travels through the air. How far apart are the balloons if the electrostatic force between them is 8.1 x 10-2 N?