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VII. Electricity Topics Of the four fundamental forces, the most important (and strongest) in our everyday lives is the electrostatic force. This is the mutual repulsion and attraction of subatomic particles. We feel this force whenever we attempt to put one solid object through another. Most macroscopic objects (like a table) are electrically neutral, but only because huge numbers of positive and negative electric charges are in balance. Electric forces bind electrons to protons to form atoms, bind atoms together to form molecules, and bind molecules together to form solids and liquids. In this unit, we also introduce the abstract concepts of vector and scalar fields, which are important for further study in this domain. Questions How does matter hold itself together? Why can’t I pass my hand through a table? What is electric charge? Is it something an object “obtains” or is it more fundamental? Why do mathematical models work so well in describing the physics of electricity? How can I produce a large electric spark? To what uses can I put this spark? How is electricity stored? Knowledge and skills By the end of this unit, students will be able to o Calculate the number of fundamental charges in a macroscopic charge. o Determine the charge state of an electroscope based on the charging process and the position of the electroscope leaves. o Reason about how static charges are generated when two materials are brought in contact. Understand and comment on a triboelectric sequence. o Determine the net charge of a collection of charged objects. o Describe the processes of charge by induction, polarization, and conduction. o Calculate two-dimensional vector forces using Coulomb’s Law for a set of three arbitrarily placed charged objects. o Sketch the electric field of a small collection of positively and negatively charged objects. o Calculate values of the two-dimensional electric field in the presence of a set of three arbitrarily placed charged objects. o Reason about the strength of the electric field based on the density of field lines. o Relate electric potential to electric potential energy, and make calculations based on this relationship. Also, solve problems using potential and kinetic energy based on charged particles moving through simple electric potentials. o Relate electric potential to electric field. Be able to reason about equipotential diagrams … in particular, be able to locate regions in an equipotential diagram where the electric field is strongest or weakest, and be able to determine the direction of the electric field from this diagram. o Likewise, be able to sketch an equipotential diagram if given an electric field pattern. o Calculate simple (one-dimensional) electric field values from electric potential functions, and vice versa, using algebra. o Determine the electric charge of two leaves in an electroscope from the angle at which they are hanging and the masses of the leaves. o Calculate the capacitance of a parallel-plate capacitor given its geometry and dielectric. Make educated guesses about the storage capacity of a capacitor given its geometry. By the end of this unit, students will understand that o Like mass, electric charge is a property of fundamental particles, and cannot be created or destroyed. o The fundamental unit of electric charge can be determined through Millikan’s oil drop experiment. o Neutrally charged objects can experience an attractive electric force when polarized by a nearby charged object. o The storage capacity of a capacitor depends only on its geometry and the additional capacity provided by a polarizable (dielectric) substance. o A useful but limited analogy can be made between electric potential and height. o Seen from an energy perspective, differences in electric potential are the cause of the motion of charged particles. o Seen from a force perspective, electric fields cause the acceleration of charged particles. o The electric field, acting as a “mediator” of force, gets rid of the messy “action at a distance” problem Newton associated with the inverse-square law of gravity. o Electric fields obey the “principle of superposition” which means the electric fields surrounding individual particles can be added together like vectors to create the electric field of the aggregate. o Inside conductors (such as metals), the electric field must equal zero. Thus conductors are equipotentials, and can act as “cages” that block externally applied electric fields. o Electric potential, like electric potential energy, is only important in differential or relative terms. That is, you could add a constant to the total amount of electric potential possessed by any or all objects in the universe and the subsequent behavior would be identical to what we see now. o Electric current is a measure of the net amount of charge which passes by some location in a certain amount of tim By the end of this unit, students will be familiar with the following vocabulary words o charge, positive, neutral, negative, fundamental charge, electric force, inversesquare law, vector field, scalar field, superposition, charge by induction, charge by conduction, charge by polarization, capacitor, dielectric, permittivity of free space, dielectric constant, parallel-plate capacitor, Millikan oil drop experiment, current By the end of this unit, students will be familiar with the relevant equations from the course textbook concerning electric forces, fields and potential Performance tasks Students will make conclusions about the triboelectric sequence by measuring the electric charge transfer between two objects using an electroscope. Students will conduct experiments that directly demonstrate the ability of a neutrally charged object to experience an attractive electric force via polarization. Such experiments might involve small paper fragments, a comb, rabbit fur, water, etc. Students will charge and discharge a capacitor through an ammeter and light bulb, demonstrating the relationship between current, charge, and time.