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Basics of Electrostatics: Electric Charges, Forces, and Fields

This physics topic covers the fundamentals of electrostatics, including electric charges, electrostatic forces, and electrostatic fields. It explores the qualitative and quantitative aspects of electrostatic phenomena and their applications.

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Basics of Electrostatics: Electric Charges, Forces, and Fields

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  1. PHYSICS 112 Dr. Anatoli Frishman frishman@iastate.edu

  2. Physics Electromagnetism Optics Atoms & particles Mechanics Thermal properties • Geometrical optics • Wave optics • Electrostatic • Electric current • Magnetism Condensed Mater High energy Biophysics Classical physics Quantum physics Relativistic physics Quantum relativistic physics

  3. Electromagnetism (electric and magnetic phenomena) This is about: electric charges, electromagnetic forces, and electromagnetic fields I. Electrostatics This is about: non-moving electric charges, electrostatic forces, and electrostaticfields. (With very good approximation, electrostatics could be applied to slow moving charges. Slow means that the speed of the considered charge is much smaller then speed of light: v<<c) • Electric charge • 1a. Qualitative description • There are two and only two types of electric charges • Charges of the same type repel, and charges of different type attract • (this is already qualitative description of electrostatic forces) • These two types are referred to as positive and negative (+q and –q) • Charges can be treated algebraically, and the net amount of electric charges in an isolated system is conserved (is not changed in any process). This is the law of conservation of electric charges

  4. 1b. Elementary charge: • 1c. Electric properties of different materials • (very brief, qualitative description) • Microscopic picture (atom) • Insulators • Conductors • Semiconductors • Superconductors • 1d. “Games” with electric charges • Charging by contact • Charging by induction • Electroscope • Electrometer

  5. The electroscope disk Positive Negative Neutral Gold leaves (or vane)

  6. Charged rod Inducted charge Positive Negative Neutral Inducted charge repulsion

  7. Charged rod (closer) Positive Negative Neutral Stronger repulsion

  8. If we ground the electroscope while the rod is there, the charges in the electroscope that were “escaping” from the rod flow to the ground. Positive Negative Neutral No repulsion

  9. Then we cut the grounding… Positive Negative Neutral No repulsion

  10. And remove the rod… The electroscope is now charged.The charge spreads now all over the object. Electroscope charged by induction Positive Negative Neutral Repulsion

  11. 2. Electrostatic forces (Coulomb’s law) r Q1 Q2 Units: Example:

  12. Coulomb’s law in vector form: Q1 Q2 Q1 Q2 r r Q1 andQ2 have the same sign Q1Q2 >0 Q1 andQ2 have opposite signs Q1Q2 <0 Principle of superposition:

  13. Example: Q1 Q2 Q3 r r

  14. Example: Q Q a Q Q

  15. Example: Compare the gravitational attraction and the electric repulsion of two electrons -Q Q r ~ 1.5 m Big! Example: One of your friends can resist a force of 100 lb (450 N) with his arms apart. You give him two charged balls with charges Q and Q to hold on each hand. How large a charge Q can he hold outstretched? Less than a cell in your body!

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