1. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 11 Charge and Coulomb’s Law

2. © 2001-2005 Shannon W. Helzer. All Rights Reserved. The Atom  Atoms are composed of three particles: neutrons, protons, and electrons.  The nucleus of the atom is made up of the protons and the neutrons.  The electrons orbit about the nucleus.  The negatively charged electrons are attracted to the positively charged protons.  The neutrons act as spaces between the like-charged protons.  These charge interactions are responsible for the atom staying together. 11-1

3. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Ions  An atom with the same number of positive charges and negative charges is said to be neutrally charged.  If the atom looses an electron, then it will have one more positive charge than it has negative charges.  Therefore, it has a net positive charge and is known as a cation.  If the atom gains an electron, then it will have one more negative charge than it has positive charges.  Therefore, it has a net negative charge and is known as a anion.  Either way, both species are known as ions. 11-2

4. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Charge Repulsion  Like charges repel one another.  Watch the electroscope below as a negatively charged rod is brought close by.  The electrons in the rod repel those on the metal plate.  They in turn travel down to the needle and axel apparatus.  Here they repel one another causing the needle to rotate. 11-3

5. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Charge Attraction  Like charges repel; however, opposite charges attract.  Charging by induction demonstrates both at the same time.  The net result of charging by induction leaves a positive net charge on one sphere and a negative net charge on the other. 11-4

6. © 2001-2005 Shannon W. Helzer. All Rights Reserved. More Charge Repulsion and Attraction  Water molecules are known as “polar” molecules.  This statement means that they have a positive and a negative end.  Watch what happens on the molecular level as a positively charged rod is brought near to an ordinary glass of water.  The negative end of the water molecule is attracted to the positively charged rod causing the H 2 O molecules to rotate and align. 11-5

7. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Grounding  When a negatively charged rod is brought into close position to a metallic sphere, a charge separation occurs.  If a switch connected to the ground is opened, then the electrons will flow from the sphere into the ground.  The elimination of charge in this manner is known as grounding.  Your house is electrically grounded in order to prevent injury or fire. 11-6

8. © 2001-2005 Shannon W. Helzer. All Rights Reserved. Coulomb’s Law  A law used to determine how much force of attraction or repulsion exist between two charged particles. F C = _________________ with units of ______________________ k = _________________ with units of ______________________ q 1 & q 2 = _________________ with units of ______________________ d or r= _________________ with units of ______________________ The remaining slides will demonstrate the procedure used in conjunction with Coulomb’s Law. 11-7

9. © 2001-2005 Shannon W. Helzer. All Rights Reserved. WS 51 #1 Three charges are aligned as shown (note the scale on the graph). The distance between q 1 and q 2 is 0.04 m and the distance between q 3 and q 2 0.02 m. a. Make a guess. Do you think that charge q 2 will be attracted towards q 1 or q 3. Why? b. Calculate the force on q 2 due to q 1. Do not forget the direction. c. Calculate the force on q 2 due to q 3. Do not forget the direction. d. What is the net force (magnitude and direction) on q 2 ? e. Does your answer verify your guess in part a? 11-8

10. © 2001-2005 Shannon W. Helzer. All Rights Reserved. WS 51 #2  Due to the fact that the charges in this problem all lie on a straight line, this problem is done exactly like WS 51 #1.  However, note the fact that q 2 and q 3 have different charges in this problem.  Also, make sure you pay attention to the scale and convert it to the proper units. 11-9 Electric Charge Positions Scale: 1 Square = 0.05m q1q1 q3q3 q2q2

11. © 2001-2005 Shannon W. Helzer. All Rights Reserved. WS 51 #3  The charges in this problem are not on a straight line.  As a result, you must use analytical vector addition (Unit 6).  You must use trig in order to determine the angle between q1 and q2.  You must use the “Pthag” in order to find the distance between q 1 and q 2. 11-10 Scale: 1 Square = 0.5 cm q1q1 q3q3 q2q2 VectorMagnitudeAnglex componenty componentQuad --------------

12. © 2001-2005 Shannon W. Helzer. All Rights Reserved. WS 52 #1  For problems like this one you will need to use trig in order to find the angles between both sets of charges.  Always remember that the angles should be based upon 0  being to the right along the +x axis. 11-11 Scale: 1 Square = 1.0 cm q1q1 q3q3 q2q2 VectorMagnitudeAnglex componenty componentQuad --------------

13. © 2001-2005 Shannon W. Helzer. All Rights Reserved. WS 51 #4  A pie pan is placed on top of a Van de Graaff machine as shown. Once the machine is turned on, the pie pan hovers at a distance of 1.5 cm above the machine. If the charge on the top surface of the Van de Graaff machine (q1) is C and the charge on the pie pan is C, then calculate the mass of the pie pan. 11-12

14. © 2001-2005 Shannon W. Helzer. All Rights Reserved. This presentation was brought to you by Where we are committed to Excellence In Mathematics And Science Educational Services.

15. A AA 11-1 VectorMagnitudeAnglex componenty componentQuad --------------

16. © 2001-2005 Shannon W. Helzer. All Rights Reserved. A AA 11-1