Physics 16: 7-12 Electric Fields
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 16: Sections 7 to 12
- Section 7: We can characterize the effects of a charge as a force field, just as we characterize the effects of a mass by a gravitational field. We test the gravitational field generated by mass M by measuring the force exerted on a point mass m placed in the field: F = ma = m (Gm/r2). Likewise, we measure the electrical field of a charge Q by measuring the force exerted by Q's field on a point charge q placed within the field: F = q (Qk/r2) so F/q = E = Qk/r2.
- Section 8: Field lines are a useful conceptual model. The lines describe the direction a positive charge in the field would move; the number or density of lines can indicate the relative strength of the field.
- Section 9: Conductors are materials that allow electrons to move more or less freely, redistributing themselves in response to nearby electrical charge. Non-conductors (insulators) don't have free electrons.
- Section 10: We've already seen how a hydrogen atom in a strong bond with a highly electronegative element such as oxygen or nitrogen winds up having its lone electron spend all its time near the electronegative atom. The local positive charge concentration on the hydrogen can form bonds with local negative areas on other atoms or molecules. It is these "hydrogen bonds" that hold the two strands of the DNA double helix together. Since these bonds are not as strong as the inter-atom bonds that hold the hydrogen to its nitrogen partners, they can be easily broken and reformed by enzymes fitted to that purpose, without destroying the rest of the strand. Hydrogen bonds make possible the rapid duplication of DNA during cell reproduction, and the rapid translation of DNA to RNA during most cellular metabolic processes.
- Section 11: Static electricity has many practical applications: chief among them in modern society is the use of differences in charge to attract ink during copy and print operations on modern printers.
- Section 12: Early concepts of electricity envisioned a material fluid (similar to the caloric or heat fluid) that could move from one object to another, carrying charge. We sometimes find this concept still useful as a way of envisioning the force of an electrical field as a flow of charge potential. We can map the flow of "electrical stuff" from its center in a charged object through any surface at some distance from the charge. If we completely enclose a surface, then (according to Gauss's law) the flow of electrical stuff, the electric flux ΦE, through the total area is equal to the charge source divided by a constant, ε0 — the permittivity constant we've already seen in Coulomb's law. Keep this constant in mind, because it becomes very important later on when we look at theories of light.
- The Electric Field and Electric Force:
- Electric Flux:
- Gauss's Law:
Read the following weblecture before chat: Electric Fields
Use the simulation below to explore how field lines vary as you introduce new charges and move them around. Turn on the grid so that you can precisely place charges. Use the sensors to display net field direction and force values at specific points. What happens when you
- Place a +1 nC charge at the center of the display and add a +1 nC one space unit (5 small units) away? How does the field change if you double the distance?
- Place a +1 nC charge at the center of the display and add a -1 nC one space unit (5 small units) away? How does the field change if you double the distance?
- Place three charges in a row
- all positive
- all negative
- with outside charges positive and central charge negative
- with left and center charges positive and right side charge negative
- Experiment with charges in a triangle formation.
- Experiment with charges in a square formation.
Physics simulation Java Applets are the product of the PHET Interactive Simulations project at the University of Colorado, Boulder.
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