Slide 1 / 86 Slide 2 / 86 New Jersey Center for Teaching and Learning AP Physics B Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be Electric Force used for any commercial purpose without the written permission of the owners. NJCTL maintains its and Field website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning 2012-11-26 community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org www.njctl.org Slide 3 / 86 Slide 4 / 86 Electric Force and Field Click on the topic to go to that section Atomic Structure · Charging by Rubbing · Charging by Conduction · Charging by Induction · Atomic Structure Grounding · Electroscope Basics · Electric Force · Electric Field · Electric Force in 2D · Return to Table Electric Field in 2D of Contents · Slide 5 / 86 Slide 6 / 86 Atomic Structure Atomic Structure · Protons and electrons have equal and opposite electric charge. · All matter is made up of atoms. · By convention, electrons are said to have a negative charge. · Each atom contains a central "nucleus". · Similarly, protons are said to have a positive charge. · The nucleus contains protons and neutrons, · Neutrons have no charge. and the nucleus is considered to be at rest. · Atoms are electrically neutral...not because they contain no · Electrons move around the nucleus in the charge...but because they have equal numbers of protons and empty space of the atom. electrons...their total charge adds up to zero.
Slide 7 / 86 Slide 8 / 86 1 An atom in it's normal state has no charge. This is Charge due to the fact that atoms have only neutrons A · Charge is neither created nor destroyed, it is have no protons conserved. B have no electrons C · Opposite charges attract & like charges repel have an equal numbers of protons and electrons D · As a result negatively charged electrons are attracted to the positive nucleus are too small to measure charge E · The magnitude of charge on an electron is denoted answer by e . An electron is said to have a charge of -e and a proton a charge of +e . Slide 9 / 86 Slide 10 / 86 2 What symbol is used for the magnitude of charge 3 Which of the following is the correct force on an electron or proton? between two negative charges? A C A e B B m C C p D L E D answer answer E Slide 11 / 86 Slide 12 / 86 4 Which of the following is the correct force 5 Which of the following is the correct force between two positive charges? between one positive charge and one negative charge? A A B B C C D D answer answer E E
Slide 13 / 86 Slide 14 / 86 Atomic Structure Atomic Structure This is NOT what an atom looks like!!! The nuclei of atoms are much more massive than electrons. Each proton or neutron is 2000 times more massive than an electron with each nucleus If an atom was magnified so containing at least one proton. that the nucleus was the size of a baseball, the atom would That's one reason that when electric charge moves be the size of our gym. it's usually the result of electrons moving, not protons. And the electrons would be too small to see. Atoms are The other reason is that in solids, the nuclei are almost all empty space. locked together so they can't move...regardless of their mass. Since everything, including us, is made of atoms, that means everything, including us, is mostly empty space. Slide 15 / 86 Slide 16 / 86 Solids Solids Insulators · A form of matter whose nuclei form a fixed · Have strongly bound electrons structure. which cannot move within the solid. · Nuclei, and their protons, are "locked" into position. Conductors · Some electrons move freely inside · Some electrons are bound more tightly to the solid. their atoms than other electrons. · Like charges repel, therefore the · In conductors, some electrons are free to electrons spread apart as far as move through the solid. possible. · In insulators, no electrons are free to move Slide 17 / 86 Slide 18 / 86 Charging by Rubbing Rubbing two objects together · Electrons, carrying negative charge, move from one object to another. · As a result, rubbed objects each gain a net charge that is equal and opposite to the other. Charging by Rubbing Return to Table of Contents without rubbing ...rub with some fur after rubbing
Slide 19 / 86 Slide 20 / 86 A neutral plastic rod is rubbed by a 6 piece of animal fur. Describe the charge on each item. A Both items will be neutral. B The fur and the rod will both have a negative net charge. The rod will have a negative net charge and the fur will Charging by Conduction C have a positive net charge. The rod will have a positive net charge and the fur will D have a negative net charge. answer Return to Table of Contents Slide 21 / 86 Slide 22 / 86 Charging by Conduction Charging by Conduction Negatively Charged Neutral Charge Charge = -4e (charge = -4e) (charge = 0) - - - - - - - - If the spheres touch, their + - + + + + - + + + - electrons push as far apart - - - - - - as they can (like charges repel) + + + + + + + + - - - - - - - (identical spheres very far apart) Identical spheres Slide 23 / 86 Slide 24 / 86 Charging by Conduction 7 An object with a charge of +4Q touches a neutral object. What are the new charges on each object? Negatively Charged Negatively Charged (charge = -2e) (charge = -2e) A no charge and +4Q - Once they are - - - moved apart again, B +2Q and +2Q + + + + the charges cannot - - - - get back to where C +4Q and +4Q answer + + + + they came from. D It canot be determined. - This results in an - - - equal distribution (very far apart) of charge.
Slide 25 / 86 Slide 26 / 86 Charging by Induction Negatively Neutral Charged - - - - - + + + + - - + + + + - Charging by Induction - - - - Return to Table of Contents Slide 27 / 86 Slide 28 / 86 Charging by Induction Charging by Induction Negatively Neutral Charged (but polarized) Negatively Positively Charged Charged - - - - - - - + - + - - - - - - - - - + + + + + + - + - Negatively - + + + + + + - - + + + + - Charged - + - - - + + + Rod + - - - - Neutral - (very far away) Slide 29 / 86 Slide 30 / 86 Charging by Induction Charging by Induction Positively Negatively Negatively Positive Charged Charged Charged Charge Negatively - - Charged - - - - Rod + + + + + + + + - + - - - - - - - - - - + + - - - - - + + + + + + + - + + + + - + - - - - - - + - - Negatively Charge
Slide 31 / 86 Slide 32 / 86 8 A negatively charged sphere A is brought near a neutral sphere B without touching it. Sphere B is then touched by a student. What is the new charge on sphere B? A Positive B Negative answer C Neutral Grounding D Cannon be determined. Return to Table of Contents Slide 33 / 86 Slide 34 / 86 Grounding Grounding Electrical circuits and devices are The earth is an enormous conductor. usually grounded to protect from accumulating a net charge that When a wire is attached between the earth and another could shock you. conductor, excess electrons will flow to the earth leaving the conductor neutral. This is "grounding". To ground an electrical device a conductor must run from the Electrons flow to and from us to the earth all the time. device into the ground. Grounding Pin When you touch an object with a net charge, you may get a Plugs for many electrical devices shock. This is because the conductor wants to get rid of its have a third grounding pin that excess electrons. To do this, electrons flow through you to connects to a wire in the outlet (symbol for "ground") the ground. which goes to the ground. Slide 35 / 86 Slide 36 / 86 9 Sphere A carries a net positive charge, and sphere B is neutral. They are placed near each other on an insulated table. Sphere B is briefly touched with a wire that is grounded. Which statement is correct? A Sphere B remains neutral. Electroscope Basics B Sphere B is now positively charged. C Sphere B is now negatively charged. The charge on sphere B cannot be determined without D additional information. answer Return to Table of Contents
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