1. Electrostatics Chapter 20

2. History The word electricity comes from the Greek elektron which means “amber”. Amber becomes negatively charged when rubbed with wool, because amber attracts negative charges (electrons) and will take them from wool. As a result, the amber becomes "negatively charged," The “amber effect” is what we call static electricity. Electrostatics is the study of non-moving electric charges, sometimes called static electricity.

3. More History Experiments done over 200 years ago by Benjamin Franklin and others led to the arbitrary assignment of the name "negative" to the property of those particles that are transferred to hard rubber when it is rubbed with wool. Franklin did not, of course, know about elementary particles. We now know that the particles being transferred in rubbing are electrons. We also know that electrons are not the only particles that have the property of charge. Ben Franklin decided: charge on glass rod = positive charge on rubber rod = negative He could have decided to call the glass rod red and the rubber rob blue, or some other names but he didn’t.

4. Charge Charge is a property of certain sub-atomic particles and is not a substance than can be transferred from one particle to another. Particles either have charge, or they don't. There are many particles with non-zero charge including protons and electrons. Charging an object involves transferring sub-atomic particles that have charge from one object to another object. Amber Wool “negatively charged” object “positively charged” object

5. Charging by friction Charging by friction or rubbing is a process that results in a transfer of electrons between the two objects which are rubbed together. This method can be used to charge insulators. When wool is rubbed on amber, electrons are transferred to the amber and the amber becomes “negatively” charged. When a glass rod is rubbed with silk, electrons are transferred to the silk, and the glass becomes “positively” charged. Amber Wool

6. Atomic Structure – brief review To understand electrostatics it is first important to understand the basic structure of an atom

7. Neutral vs. Charged Objects Summary of Subatomic Particle ProtonNeutronElectron In nucleus Outside of nucleus Tightly Bound Weakly Bound Positive ChargeNo ChargeNegative Charge Relatively Massive Not very massive Positively ChargedNegatively ChargedUncharged Possesses more protons than electrons Possesses more electrons than protons Equal numbers of protons and electrons Charged objects have an equal number of electrons and protons. There are three ways an object can become charged, by friction, contact or induction.

8. Charge Interaction Experiments show that charging by friction results in 2 different types of charged objects All glass rods rubbed with silk are charged similarly and experiments show that similarly charged objects repel each other. All rubber rods rubbed with wool are charged similarly and experiments show that similarly charged objects repel each other. Experiments show that glass rods rubbed with silk are charged differently from rubber rods rubbed with wool and that differently charged objects attract each other. Like charges repel Unlike charges attract rubber

9. Law of Conservation of Charge If a system starts out with an equal number of positive and negative charges, there’s nothing we can do to create an excess of one kind of charge in that system unless we bring in charge from outside the system (or remove some charge from the system). Law of Conservation of Charge: – The net amount of electric charge produced in any process is zero.

10. Charge as a Quantity Symbol: q or Q Unit: C, Coulomb One Coulomb is a HUGE charge It takes 6.25 x 10 18 electrons to amount to this much charge

11. Coulomb Force (Electric Force) Using a torsion balance, Coulomb found that: the electric force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between the charges. Torsion Balance

12. Coulomb’s Law (Electric Force) q is charge, units: C q 1 and q 2 can be the charge on single charged particles or the charge on charged objects that repel or attract each other r is the distance between charges, units: m F E is Electric Force, units: N k c is the coulomb constant, 8.99x10 9 Nm 2 /C 2 Coulomb force is the force of attraction, or repulsion between two charged particles or objects.

13. Coulomb Constant

14. Coulomb Force As the distance increases by a factor of 2, the force decreases by a factor of 4, not 2, because it depends on distance squared.

15. Multiple Charge Problem Four charges are arranged in a square with sides of length 2.5 cm. The two charges in the top right and bottom left corners are +3.0 x 10^-6 C. The charges in the other two corners are -3.0 x 10^-6 C. What is the net force exerted on the charge in the top right corner by the other three charges? To solve any problem like this, the simplest thing to do is to draw a good diagram showing the forces acting on the charge. You should also let your diagram handle your signs for you. You have to be very careful to add these forces as vectors to get the net force. In this problem we can take advantage of the symmetry, and combine the forces from charges 2 and 4 into a force along the diagonal (opposite to the force from charge 3)

16. Charge and Mass of Elementary Particles ParticleCharge, (C)Mass, (kg) electron-1.6x10 -19 9.109x10 -31 proton+1.6x10 -19 1.673x10 -27 neutron01.675x10 -27 If an object has a… + charge  it has less electrons than normal - charge  it has more electrons than normal

17. Coulomb force and Gravitational force between an electron and proton in hydrogen The Coulomb force is MUCH larger, a factor of 10 39 larger

18. Conductors Conductors are materials in which charges flow easily. In electrical conductors, the outer electrons (also known as valence electrons) are loosely bound. They are relatively free from individual atoms. We say that these electrons are delocalized. When electrons are gained by the conductors, the other electrons will flow automatically so that electron re- distribution in the conductors occur. When electrons are lost by the conductors, the other electrons will also flow automatically so that electron re- distribution in the conductors occur All metals are conductors of electricity

19. Insulators Materials that do not allow electrons to move freely inside them are called electrical insulators. An electrical insulator has electrons that are all in fixed positions. The addition or removal of electrons at any one part of the insulator does not result in the electrons in other parts of the same insulator to move. Thus, we say that the charge is localised (or confined) to the region. Examples of insulators are wood, plastics, ebonite, glass, fur, silk.

20. Charging by contact (or conduction) Charging by conduction (or contact) is useful for charging metals and other conductors. If a charged object touches a conductor, some charge will be transferred between the object and the conductor, charging the conductor with the same sign as the charge on the object.

21. Charging by induction Charging by induction - also useful for charging metals and other conductors. A charged object is used, but this time it is only brought close to the conductor, and does not touch it. If the conductor is connected to ground (ground is basically anything neutral that can give up electrons to, or take electrons from, an object), electrons will either flow on to it or away from it. When the ground connection is removed, the conductor will have a charge opposite in sign to that of the charged object.

22. Charging by Induction, the steps Step 1 Place a negatively-charged rod near the single conductor (e.g. metallic sphere) that is sitting on an insulated stand. Make sure that the single conductor is initially uncharged. Step 2 Note that the moment the negatively-charged rod is placed near the conductor, the charge distribution in the conductor is disturbed immediately. This happens because the negative charges repel the conductor’s electrons to its further end. - - - - - - insulating stand + + + + + + - - - - - - - - - - - - - -

23. Charging by Induction, the steps Step 3 Next, touch the conductor for a short while. As the human body is regarded as a good conductor of electricity, the excess charges in the conductor(electrons in this case) will flow through the body down to earth. Touching the charged conductor in this case is called grounding. - - - - - - insulating stand + + + + + + - - - - - - So long as the finger is still touching the conductor, excess charges (electrons in this case) continue to flow down to the earth through the body.

24. Charging by Induction, the steps Step 4 (Once the excess electrons are discharged) Keeping the negatively-charged rod stationary, remove the finger from the conductor. - - - - - - insulating stand + + + + + + - - - - - - + + + + + +

25. Charging by Induction, the steps Step 5 (part i) Finally, the negatively-charged rod is removed. insulating stand + + + + + + Step 5 (part ii) The positive charges remaining in the single conductor will re-distribute themselves uniformly. It is actually the negative charges (electrons) that move so that the positive charges will be uniformly distributed. insulating stand + + + + + +

26. Charge Polarization If a rubber balloon is charged by rubbing it with animal fur, the balloon can subsequently be stuck to the surface of a wooden cabinet or a whiteboard. This interaction between a neutral object and any charged object can be explained by charge polarization and our usual rules of opposites attract and likes repel. Electrostatic painting employs charge polarization of the tiny paint particles in order to quickly paint a charged metal object.

27. Lightning

28. Van de Graaff Van de Graaff electrostatic generator: simulates lightning from cloud to ground

29. Sources Physics by Zitzewitz www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/ estatics/ http://webphysics.davidson.edu/course_mate rial/py230l_wc/demo/illustration22_1.html http://members.aol.com/physicsfirst/ http://members.aol.com/physicsfirst/ http://physics.bu.edu/~duffy/PY106/Charge.h tml