1202_LabManual.pdf

# Predictions justified with physical theory experiment

• Lab Report
• 268

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predictions justified with physical theory experiment is physically sound and tests phenomenon in question results interpreted with theory to clear, appropriate conclusion Quantitativeness statements are vague or arbitrary analysis is inappropriately qualitative uncertainty analysis not used to evaluate prediction or find result numbers, equations, units, uncertainties missing or inappropriate consistently quantitative equations, numbers with units, uncertainties throughout prediction confirmed or denied, result found by some form of uncertainty analysis results, conclusions based on data Total 91

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P HYSICS L AB R EPORT R UBRIC 92
LAB 4: ELECTRIC FIELD AND POTENTIAL Many forces in nature cannot be modeled as contact forces, such as those you have used to describe collisions or friction interactions. Forces sometimes characterized as “action-at-a- distance” involve objects exerting forces on each other although not in physical contact. The gravitational force, in fact, fits this characterization. You are just now learning about another action-at-a-distance force: the electric force. Action-at-a-distance can be difficult to fit into our physics framework for two reasons. First, it is hard to conceive of objects interacting when they are not touching. Second, objects that interact by these action-at-a-distance forces form systems that can have potential energy. The concept of action-at-a-distance does not satisfactorily describe where this potential energy resides. The notion of a "field" solves these problems. In a field theory, an object affects the space around it, creating a field. Another object entering this space is affected by that field and experiences a force. In this picture the two objects do not directly interact with each other; one object creates a field and the other object interacts directly with that field. The magnitude of the force on an object is the magnitude of the field at the space the object occupies (caused by other objects) multiplied by the property of that object that causes it to interact with that field. In the case of the gravitational force, that property is the mass of the object. In the case of the electrical force, it is the electric charge. The direction of the electrical or gravitational force on an object is along the direction (towards or opposite) of the field (at the object's position). The potential energy of the system can be envisioned as residing in the field. Thinking of interactions in terms of fields solves the intellectual problem of action-at-a- distance. It is, however, a very abstract way of thinking about the world. We use it only because it leads us to a deeper understanding of natural phenomena and inspires the invention of new devices. The problems in this laboratory are primarily designed to give you practice visualizing fields and their associated potentials, and in using the field concept to solve problems.

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• Fall '17
• Aaron Wynveen

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