Electrostatics

Electric Charges: the basis of electricity is charge. The charge on an atom is determined by the subatomic particles that make it up. Proton- has a positive charge and is located in the nucleus. Neutron- has no charge (is neutral) and is also located in the nucleus as it fills in the spaces between the protons. Electron- has a negative charge and is located outside of the nucleus in an electron cloud around the atom.

Particle charges: Electrons and protons have the same magnitude of charge (elementary charge, e). Electron (-e): x C Proton (+e): x C This is why electrons are forced to orbit around the nucleus. Electrostatic Forces hold atoms together. The Law of Charges- Like charges repel, and unlike charges attract. Because atoms have the same amount of protons and electrons they are electrically neutral. (Nitrogen has an atomic number of 7- 7 protons orbited by 7 electrons).

Subatomic Particle Sizes A proton and neutron have about the same mass x kg An electron has a much smaller mass x kg To put this into perspective it’s like comparing the sizes of a penny and large bowling ball. The proton is obviously the bowling ball and the electron is represented by the penny.

How do atoms become “charged?” Atoms become charged when they become more positive or more negative. How can this happen? Remove or add a proton or an electron. Protons and neutrons are bound together by the Strong Nuclear Force and it is very hard to separate them. Electrons, however, can be more easily removed.

Ions An atom with a deficiency of electrons is positively charged. An atom with an excess of electrons is negatively charged. ATOMS DO NOT GAIN OR LOSE PROTONS!!!!

Charge is a fundamental quality like mass. Charge is denoted as q. Charge has a fundamental unit of a Coulomb (C). Charges are usually really really small numbers (10 - ). So what is 1 C? –An object would have to have 6.25 x extra electrons to amount to –1 C of charge. –A lightning bolt is estimated to carry a charge of 10 C. Revisit the charges on an electron and proton.

Charges can ONLY be in multiples of e Remember: –-e = an electron = x C –+e = a proton = x C An object that has a net charge of 8.0 x C has a net charge of what multiple of e? Hint: How many electrons would need to be removed to create this charge? The net charge would be +5e, 5 electrons were removed

Multiples of Charges Chart 1e1e1.6 x e2e3.2 x e3e4.8 x e4e6.4 x e5e8.0 x

Electrostatic Force This is a non-contact force (like the gravitational force except instead of two masses exerting force on each other the two objects charges exert a force of repulsion or attraction). ANY charged object can exert the electrostatic force upon other objects- both charged and uncharged objects.

Coulomb’s Law- formula for electrostatic force Again this is similar to the gravitational force… F g = GmM r 2 charge (q) is now responsible for the force F e = kq 1 q 2 r 2 Just like G was a constant so is k. k is the electrostatic constant and = 8.99 x 10 9 Nm 2 /C 2

2x3x4x 1/4 = /9 = /16 =.61 1x Remember this…the relationship between the gravitational force and the distance from the object…this is the inverse square law 9.81 m/s 2 FgFg r2r2 F g = GmM r 2

F e = kq 1 q 2 r 2 FeFe r2r2

Try this… The distance between a proton and an electron in a hydrogen atom is 5.3 x m. Find both the gravitational force and the electrostatic force between the two particles.

Forces By the early 19th century, physicists had classified the apparent myriad of forces in nature to just 3 kinds: –Gravitational force –Electric force –Magnetic force

Forces By the end of the 19th century, they had narrowed the list to just 2 forces: –Gravitational force –Electromagnetic force

Forces The 20th century first added two new forces to this list that are observed only inside the atomic nucleus: –Gravitational force –Electromagnetic force –Weak force –Strong force

Forces And then found theoretical links that narrowed the list back to 2 kinds of forces: –Gravitational force –Strong/Electroweak force

The Electric Force We will turn our attention to the electric force, which is a force between objects with charge, just as the gravitational force is a force between objects with mass.

Electrostatics Electrostatics is the study of electric charge at rest. –(Or more or less at rest, in contrast with current electricity.)

Electrical Charges Electric charge is a fundamental property of matter. –Two types of electric charges Positive charge - every proton has a single positive charge. Negative charge - every electron has a single negative charge.

Electrical Charge An object with an excess of electrons is negatively charged. An object with too few electrons (too many protons) is positively charged. An object with the same number of electrons and protons is neutral.

Electrical Forces Like charges repel. Opposite charges attract.

Elementary Charges Protons carry the smallest positive charge. Protons and uncharged neutrons generally reside in an atom’s nucleus. Protons are held in the nucleus by the strong force.

Elementary Charges The smallest negative charge is the charge on the electron. In normal atoms, electrons orbit the nucleus. The electric force between electrons and protons supplies the centripetal force to keep electrons in the atom.

Elementary Charges The charges carried by the proton and electron are equal in size. The mass of the proton is about 2000 times the mass of the electron.

Units of Charge The SI unit of charge is the Coulomb. 1 Coulomb = the charge of 6.24 x electrons

Charge is Conserved Electric charge is conserved - –Electric charge moves from one place to another - no case of the net creation or destruction of electric charge has ever been observed. In solids, only electrons can move. In liquids, gasses, and plasmas, both positive and negative ions are free to move.

Conductors & Insulators Materials in which charges are free to move about are called conductors. Materials in which charges are not free to move about are called insulators.

Semiconductors Semiconductors are materials which are good insulators in pure form, but their conducting properties can be adjusted over a wide range by introducing very small amounts of impurities. –Silicon, germanium, etc. –Transistors, computer chips, etc.

Superconductors Superconductors are materials that lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273 o C). Recently, “high temperature” (above 100 K) superconductors have been discovered.

“Creating” an Electric Charge When you “create” an electric charge (by rubbing your feet on a carpet) you are actually separating existing charges - not creating charges. One object ends up with an excess of electrons (- charge), and the other a deficit of electrons (+ charge).

Charging by Friction If one neutral material has more affinity for electrons than another (neutral) material, it will attract electrons from the other. One material becomes negatively charged, the other positively charged.

Charging by Contact If a charged object is brought in contact with a neutral object, charges will be repelled from (or attracted to) the charged object. The neutral object will gain a charge of the same sign as the charged object.

Grounding Providing a path from a charged object to the Earth is called grounding it. Charges will be attracted from (or repelled to) the Earth by the charged object. Since the Earth is so large, both the charged object and the Earth are neutralized.

Electrical Forces The electrical force between 2 charges depends on: –The size of each charge More charge means more force. –The distance between the charges More distance means less force.

Electrical Forces The electrical force between 2 charges is: –Directly proportional to each charge. –Inversely proportional to the square of the distance between the charges.

where k = 9.0 x 10 9 N m 2 /C 2 Coulomb’s Law F electric = k Q q r 2

Polarization Bringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. –Like charges in the neutral object are repelled by the charged object. –Unlike charges in the neutral object are attracted by the neutral object. The neutral object returns to normal when the charged object is removed.

Electric Dipoles An object that is electrically neutral overall, but permanently polarized, is called an electric dipole. –Example: H 2 0 molecule

Charging by Induction 1.Bring a charged object near (but not touching) a neutral object. 2.Ground the neutral object. 3.Remove the ground. 4.Remove the charged object 5.The neutral object now has a charge opposite to the charged object.

ANY charged object, whether positively charged or negatively charged, will have an ATTRACTIVE interaction with a neutral object. A balloon when rubbed on your head becomes charged by picking up extra electrons from your hair. That same balloon, because it is charged, will attract a neutral object like pieces of paper

So we are able to predict the charge on objects based on their interaction with other objects. They can either both be positive or both be negative. They can have opposite charges or one object is charged and the other is neutral.

Try this… On two occasions, the following charge interactions between balloons A, B and C are observed. In each case, it is known that balloon B is charged negatively. Based on these observations, what can you conclude about the charge on balloon A and C for each situation. positive or neutral negative positive (if it was neutral it wouldn’t repel C) positive

Why does the balloon stick to the wall? When a balloon is rubbed with a piece of cloth electrons are transferred between the two objects. Usually the balloon attracts extra electrons and then receives an overall negative charge. balloons static

When the balloon is placed against the wall the excess electrons will repel the electrons in the wall and be attracted to the positive charges.

What happens to your hair when you rub a balloon on your head? The balloon, after being rubbed and then pulled away, removes some of the electrons in your hair which give each strand a positive charge. Like charges want to repel and each strand is repelling from the others and “sticking up.”

Getting Shocked As you walk across a carpet, electrons are transferred from the rug to you. Now you have extra electrons. Touch a door knob (conductor) and ZAP! The electrons move from you to the knob.

Lightning Lightning is a REALLY big shock. Positive charges tend to go up, negative charges tend to go down. When the attraction reaches a critical level you get a lightning bolt.

Objects that tend to give up electrons and become positive: Glass Nylon Fur Hair Wool

Objects that tend to attract electrons and become negative: Rubber Polyester Styrofoam Saran Wrap PVC

Insulators and Conductors Different materials hold electrons differently. Insulators don’t let electrons move around within the material freely. –Ex. Cloth, Plastic, Glass, Dry Air, Wood, Rubber Conductors do let electrons move around within the material freely. –Ex. Metals- Silver, Copper, Aluminum

Try this… A charged plastic rod is brought close a neutral metal sphere. How would the distribution of charges be in the metal sphere?

Try this… Which of the diagrams below best represents the charge distribution on a metal sphere when a positively charged plastic tube is placed nearby?

Law of Conservation of Charge Charges within a closed system may be transferred from one object to another, but charge is neither created nor destroyed.

The diagram below shows the initial charges and positions of three metal spheres, R, S, and T, on insulating stands. 0e0e -8e+6e R ST Sphere R is brought into contact with sphere S and then removed. Then sphere S is brought into contact with sphere T and removed. What is the charge on sphere T after this procedure is completed? Note that the net charge of the system is -2e.

0e0e-8e RS When the spheres come in contact the charge will be distributed evenly between both spheres. -4e +6e T

0e0e-8e ST -4e+6e -4e R -4e + 6e = 2 +2e = 2 +e+e +e+e +e+e

Note that the charge of the system is conserved- the initial charge is the same as the final charge. -4e +e+e+e+e R ST -4e + e + e = -2e

Electroscopes- instruments used to detect charge The yellow arms or leaves on both instruments will move to show the charge.

Charged Electroscope Excess of the same charge in both leaves causes them to diverge/repel, Uncharged Electroscope Leaves are neutral so they are not diverging/repelling or converging/attracting. +-

Steps in charging an electroscope by INDUCTION: 1. Uncharged electroscope: +- Leaves are just hanging straight down. Net charge is zero.

2. A negatively charged rod is brought near the electroscope: Net charge is zero

Electrons move to the leaves: Net charge is still zero

Leaves diverge: Net charge of zero

The electroscope is grounded: Grounding is the process of removing the excess charge on an object by means of the transfer of electrons between it and another object of substantial size.

Electrons go to the ground: Net charge is now positive

Leaves converge and ground is removed:

Negatively charged rod is removed:

Electrons redistribute throughout the electroscope and move up toward the top: --

Leaves now have the same charge and diverge: