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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Unit 11Charge and Coulomb’s Law

© 2001-2005 Shannon W. Helzer. All Rights Reserved.

The AtomAtoms 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.

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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

IonsAn 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.

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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Charge RepulsionLike 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.

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© 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.

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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

More Charge Repulsion and AttractionWater 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 H2O molecules to rotate and align.

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© 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.

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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Coulomb’s LawA law used to determine how much force of attraction or repulsion exist between two charged particles.

FC = _________________ with units of ______________________

k = _________________ with units of ______________________

q1 & q2 = _________________ with units of ______________________

d or r = _________________ with units of ______________________

The remaining slides will demonstrate the procedure used in conjunction with Coulomb’s Law.

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1 22C

k q qF

r

© 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 q1 and q2 is 0.04 m and the distance between q3 and q2 0.02 m.

a. Make a guess. Do you think that charge q2 will be attracted towards q1 or q3. Why?b. Calculate the force on q2 due to q1. Do not forget the direction.c. Calculate the force on q2 due to q3. Do not forget the direction.d. What is the net force (magnitude and direction) on q2?e. Does your answer verify your guess in part a?

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1q 2q 3q21F 23F

41 6.0 10q C

42 3.0 10q C

43 3.0 10q C

1 22C

k q qF

r

© 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 q2 and q3 have different charges in this problem.Also, make sure you pay attention to the scale and convert it to the proper units.

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Electric Charge Positions

Scale: 1 Square = 0.05m

q1 q3q2

21F

23F

1 22C

k q qF

r

© 2001-2005 Shannon W. Helzer. All Rights Reserved.

WS 51 #3The 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 q1 and q2.

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Scale: 1 Square = 0.5 cm

q1 q3

q2

Vector Magnitude Angle x component y component Quad

-------------- -------------- -------------- -------------- -------------- --------------

23r21r

1 22C

k q qF

r

© 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.

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Scale: 1 Square = 1.0 cm

q1

q3

q2

Vector Magnitude Angle x component y component Quad

-------------- -------------- -------------- -------------- -------------- --------------

23r21r

1 22C

k q qF

r

© 2001-2005 Shannon W. Helzer. All Rights Reserved.

WS 51 #4A 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.

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6106.4 81013.2

CF

W

1 22C

k q qF

r

© 2001-2005 Shannon W. Helzer. All Rights Reserved.

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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

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A

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Vector Magnitude Angle x component y component Quad

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© 2001-2005 Shannon W. Helzer. All Rights Reserved.

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A

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