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Static Charges 3: Electric Fields, Electric Potential Energy, and Electric Potential (Voltage) Conceptual Physics, Hewitt Introduction to Chapter 33, Electric Fields and Potential 1. Based on the introduction, describe what a field is in your own words. What examples of fields are given here? 33.1 Electric Fields 2. If you throw an object into the air, does it come back down again by interacting directly with the earth’s mass? Explain. 3. How much of the space around a charge is occupied by the charge’s electric field? What do stronger electric fields potentially do? 4. An electric field has both strength (magnitude) and direction. In other words, electric fields are vector quantities. How is the direction of the electric field determined? - + Draw arrows on the diagrams shown to represent the direction of the electric fields around the positive and negative charges. Section 33.4 Electrical Potential Energy Review Concept: Doing work is the process of transferring energy. The amount of work done determines the amount of energy transferred. 5. What determines how much gravitational potential energy is stored when an object is elevated near Earth’s surface? 6. How is an electric potential energy transferred to a pair of charges? What must be done to the charges? 7. In the situations below, draw an arrow to indicate the direction in which the shaded charge must be pushed in order to increase its electric potential energy stored in the charge pair: + 8. + + - Under what circumstances will electric potential energy transform into kinetic energy? (This, by the way, is the beginning of understanding how batteries work.) Section 33.5 Electric Potential My note: it can be confusing to have two important concepts with such similar names: electric potential energy and electric potential. Don’t blame me for this. I will always refer to the concept introduced in this section—electric potential—by its other name: voltage. Of course, it doesn’t help that voltage is also often referred to as electromotive force, or EMF. Who’s in charge of this operation, anyway? Review Concept: In physics, doing work is the process of transferring energy. The amount of work done determines the amount of energy transferred. Section 33.4 Electrical Potential Energy 1. What determines how much gravitational potential energy an object has when it is elevated? 2. How is electric potential energy transferred to a charge? Where does the charge have to be? What must be done to it to increase the electric potential energy the charges store? 3. In the situations below, draw an arrow to indicate the direction in which the shaded charge must be pushed in order to increase the electric potential energy stored in the charge pair: + 4. + + - Under what circumstances will a charge’s electric potential energy transform into kinetic energy? (This, by the way, is the beginning of understanding how batteries work.) Section 33.5 Electric Potential 5. The idea of voltage (electric potential) can be understood visually. If the charge pairs below on the left store a certain amount of electric potential energy, then adding a negative charge would double the energy stored. Three charges would result in three times as much total stored energy, and so on. + + - + - - Adding charge to an electric field is equivalent to adding mass to a gravitational field—both processes add to the total potential energy stored. However, voltage is the total amount of electrical potential divided by the total charge (the units end up being Joules per Coulomb, which we call a VOLT). In each of the three situations above, the voltage is the same, because the voltage is the amount of energy per charge, not the total amount of energy present. discuss batteries. More on this later, when we So, to summarize, electrical potential energy is the TOTAL energy stored by all of the charges in the system, while voltage is the energy stored PER CHARGE. 6. What does the text say is the reason why the electric potential (voltage) is the same in a given location, no matter how much charge is present in that location? OK. Ready for this? Let’s start with a positive charge. + Next, let’s place a smaller negative charge next to the positive charge. Keep in mind that the SIZE of the charge does not matter here, only the amount of charge present. The positive and negative particles each carry the SAME amount of charge here, even though they are different sizes. + - This is very similar to mass at rest on the surface of the earth. Now let’s move the negative charge away from the positive charge. This requires applying a force along a distance, otherwise known as doing work. When we do work on the negative charge, the charge pair gains electric potential energy. + - Force The location of the charges with respect to each other stores a certain amount of electric potential energy. The total amount of electric potential energy depends on the strength of the charges, and on the distance between them. However, we do not even need to have the negative charge present to talk about the electric potential (voltage) associated with the particular location where it was placed. Talking about the voltage is like saying, “every Coulomb of charge in this location will have X Joules of potential energy.” But only one charge has to be present for a voltage to exist. This is why high voltages are not necessarily dangerous. The Van de Graff generator we use in class charged to a very high voltage (probably thousands of volts), and the voltage increases as you approach the sphere. But the sphere doesn’t hold enough charge (only a very small fraction of a Coulomb) to result in large amounts of electric potential energy. This is a bit like saying there is the POSSIBILITY of a large amount of gravitational potential energy at the top of a hill. But if I don’t park my car there, no gravitational potential energy is stored yet. It might be in the future, but for now, there is only the POTENTIAL for gravitational potential energy at the top of the hill, and no ACTUAL energy is stored until a large mass is placed there. 7. Here are four concepts we have encountered so far. Do your best to explain each in your own words. Use drawings. Please use a separate sheet. a. Electric Field b. Electrostatic Force c. Electric Potential (voltage) d. Electric Potential Energy