Chapter 20 & 21: Electrostatics

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1 There are four forces that exist in nature: Chapter 20 & 21: Electrostatics, that is, they only act over very small distances. and can act over very large distances. Rules of Electrostatics: 1. There are two kinds of charge that exist in nature ( charge and charge) and they have the property that one another, and one another. 2. The between charges varies as the inverse square of the, and directly with the. ( ) (quantized means small discrete packets that can not be further subdivided. For example you can have one electron or 2 electrons, but never half an electron) Charge: The basic unit of charge is the. (Although protons are ultimately made up of quarks) The basic unit of charge is the. It is almost always electrons that are moving when charge The SI unit of charge is the (C). Charge of 1e - = 1 proton = 1.6x10-19 Coulombs Picture Time: Object with: no overall charge: negative charge: positive charge: Example: Lisa rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens? (a) Protons are removed from the rod. (b) electrons are added to the rod. (c) the fur is also charged negatively. (d) the fur is left neutral.

2 Conductors and Insulators: are materials in which are free to. Example: are materials in which. Examples: Example: Which of the following best characterizes electrical conductors? (1) low mass density, (2) high tensile strength, (3) poor heat conductors, (4) charges move freely, (5) all the above. How to Charge Objects Consider the pith ball (pith is a material kind of like cork). Why is the neutral pith ball attracted to the negatively charged rod? Why does it bounce away? Consider the electroscope (an electroscope measures charge): Charge by - a physical transfer of charge from a charged object. How does this work? Charge by - A transfer of charge, but only two neutral objects touch. How does this work? Example: An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be: (1) negative, (2) positive, (3) neutral, (4) attracted, (5) depends on the materials. Why?

3 Coulomb s Law Where k = 9.0x10 9 N m 2 /C 2 (Coulomb s Constant) d = Distance between charges (m) Q = Charges (C) Coulomb s Law describes the forces that: 1. Binds to the nucleus. 2. Binds to form molecules. 3. Binds to form solids and liquids. Example: Two charges, and are 5.0 m apart. Calculate the force between them. Example: Find the net force on the 5.0 nc charge. (Draw Picture) Example: Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does the resulting mutual force between them change? Example: If the size of the charge value is tripled for both of two point charges maintained at a constant separation, the mutual force between them will be changed by what factor? The Electric Field: The electric force is another force that is able to act at a distance. (Just like ) Electric charges don t need to be to exert a force on each other. This means the electric force must be a. All around themselves. Any other in that electric field will experience an & will either be attracted or repelled by the original charge.

4 It is this electric field that extends out that allows that charge to act at a distance on another charge. Masses only attract so all gravitational fields point towards the mass creating them. Charges can both and. So the field might point a charge or from the charge creating the field, depending on its charge. The Test Charge To determine the direction of the electric field, a test charge is used. A test charge is a pretend point charge that is infinitely small and has a very very small positive charge so it will not disrupt the field it is in at all. The test charge is always, never negative. The way I remember it: Pretend Positive Point charges come from your Pants Pocket Drawing Electric Field Lines Draw the electric field for A positive Charge A negative Charge: Rules for Drawing Electric Fields: The lines must on charges and on charges, or at infinity. The drawn leaving a positive charge or approaching a negative charge is to the amount of charge. Field lines may not cross or touch each other. Field lines must meet conductors or charges perpendicular to the surface of the conductor or charge. Example 2 Charges: Example Charge & Plate: Quick Review: Back to Earth s Gravitational Field: Near the surface of the Earth, Earth s gravitational field is 9.8 N/kg downwards, toward Earth s surface. Do larger masses experience a larger gravitational field from Earth? What two variables does gravitational field depend on?

5 The electric field produced by a point or spherical charge is given by k = Coulomb s Constant (9.0x10 9 N m 2 /C 2 ) Q = The charge producing the field. Given in Coulombs d = The distance to the point in question The of the is based on the of for a positive charge. Another way to calculate the electric FORCE: F = Electric Force (N) E = Electric Field (N/C) Q = Charge placed in the field (C) Example: What is the electric field m away from a ( ) point charge? Example: Two charges are along the x-axis. Q 1 is 3.0 m from the origin and has a charge of -12.0mC. Q 2 is 4.5 m from the origin and has a charge of +4.0mC. (all charges are along the positive x-axis) a) Calculate the electric field 8.0 m from the origin. (DRAW THE PICTURE) b) b) What force will a mc charge experience if it is placed 8.0 m from the origin? Example: Two charges, +Q and Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? Why? (draw the picture) Example: Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines?

6 Measuring the Charge on an Electron Millikan s Oil Drop Experiment In 1909 Robert Millikan measured the charge of an electron using an oil drop experiment In 1923 he received the Nobel Prize for his work. Millikan shot X-rays at a spray of oil drops, giving the oil drops a negative charge. The charged oil drops then went into an electric field controlled by Millikan. The oil drops were then attracted to the positively charged plate which created the field. Millikan varied the electric field in between the plates, until the electric force on the target oil drop balanced the force of gravity and the oil drop stayed suspended between the plates. Example: A 3.2x10-8 kg oil drop is suspended in an electric field of strength N/C. (a) What is the charge on the oil drop? (b) how many extra electrons does the oil drop have? Cool things electrostatics explains: 1. Shocking fingers & lightning rods On a shaped object, charges are spread. On an irregularly shaped object, charge tends to accumulate at areas of curvature/smallest radii. In other words, charge accumulates at. 2. Faraday Cage The Electric Field inside a conducting surface is o Conducting cup on a stand: (draw the pictures, there are 3) Explanation: The negative charges from the polarized inside get neutralized as the positive ball comes in contact with them. The charge from the positive ball is now left on the outside of the cup. o Negative rod with a conducting sphere: (draw the pictures, there are 3)

7 A faraday Cage is a enclosure in which charge will always flow to the outside, thus leaving the inside. Not only do Faraday cages block, they more importantly block. Applications of a faraday cage: Electric Potential Energy When going for a hike there are two things to consider: 1. How high up are you going? (What is your change in elevation?) 2. How much energy will it take to get there? (How much potential energy will you have once you get there?) Even though they are climbing the same mountain the hiker with the ( ) load will be doing more work (or will have more potential energy at the top) If the hiker trips, his potential energy will convert into energy and he will fall down the hill. Gravity is similar to two charges cause they attract each other just like masses do. When unlike charges are separated, it takes. That work is stored in the charges as Electric The larger the charge, the larger the PE Just as a hiker hiking up a hill does work and that work is stored as Gravitational Potential Energy. However, no one really talks about how much work someone did climbing a mountain or how much potential energy they had once they got there. Instead everyone talks about the of the peak. The same is true for charged objects When hiking people talk about height. With charges people talk about Electric Potential Difference The difference in electric Potential is the done moving a between two points in an electric field. However, it is rarely a positive test charge moving. Usually it is an actual charge (Q) so the electric potential difference is measured as ΔV = Potential Difference (J/C = Volts or V) W onq = Work on charge being moved. (J) Q = Charge being moved. (C)

8 The difference in electric potential is the radio of the work needed to move a charge to the strength of that charge. work is done moving a charge farther away from where it wants to be. This increases ΔV. work is done when a charge moves towards where it wants to be. This is a decrease in ΔV. Charges move to decrease their Potential Energy Masses will always try to move to lower their by decreasing their height. (This is why meatballs roll off the table and onto the floor ) Charges will always move to lower their potential energy also by their potential difference. But it gets a bit trickier because the charge can either be positive or negative. Think of potential as having the potential to move. o A negative charge will move away from another negative and towards a positive all on it s own. As it does this, its ΔV decreases. o A positive charge will move away from another positive and towards a negative all on it s own. As it does this, its ΔV decreases. Example: The electric potential of an electron decreases by 600. V. a) How much work is done by the electron? b) In this scenario, would the electron be moving towards another negative charge or towards a positive one? Equipotential Lines & Electric Field Lines When going hiking, it is helpful to bring a map. Hikers use topographical maps. Each contor (line) on the map is at the same height. They could be called equi-elevational lines. Equipotential Lines & Electric Field Lines (Draw the two pictures)

9 Example: The lamp will not glow when it is held with both ends equidistant from the charged Van de Graaff generator. But when one end is closer to the dome than the other, a current is established and it glows. Why? Why does anyone care about electric potential? What happens when two chemicals are mixed together? Why? All elements strive to have o Some elements do this by giving away valence electrons o Some elements do this by gaining valance electrons When a atom (K) is placed next to a atom (S), they react and from the potassium atom to the sulfur atom. This means, there must be an electric potential difference between potassium & sulfur. Batteries harness this flow of electrons o In a battery, elements are placed close to each other, without touching, so they do not react. o A connects the two elements. o The wire allows the from one element to the other. o This movement of electrons generates! As the zinc (Zn) electrons, it goes from zinc into an dissolved in water. As the dissolved hydrogen ions (H + ) electrons, they become hydrogen gas and leave the container. Eventually the chemicals run out, and the battery is considered. Rechargeable batteries can be reset o A dead rechargeable battery can be plugged into the wall. o The (voltage) from the returns the electrons and chemicals to their original starting position to be used again. o This process is not perfect so rechargeable batteries eventually die as well.