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Electrostatics

Electrostatics. Chapter 17. Types and Sources of Charge. Positive (proton) and negative (electron) Charges stem from ionized atoms. Those with too few electrons are positively charged, those with too many negatively charged Electric charge is conserved Electric charge is quantized

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Electrostatics

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  1. Electrostatics Chapter 17

  2. Types and Sources of Charge • Positive (proton) and negative (electron) • Charges stem from ionized atoms. Those with too few electrons are positively charged, those with too many negatively charged • Electric charge is conserved • Electric charge is quantized • Millikan oil drop: found the charge for a single electron to be -1.6 x 10-19 C.

  3. Conductors • Loosely bound electrons in the outermost shell • Electrons can easily move from one atom to the next, allowing charges to be lost or gained easily and current to flow. • Metal • Salt Water

  4. Insulators • All electrons are tightly bound • Requires large amounts of energy for charge to change or to move. • Rubber • Pure Water • Air

  5. Others • Semiconductors • Naturally insulators but can be conductors under the right conditions • Superconductors • Perfect conductor when at or below a certain temperature

  6. Charging by Friction • Rubbing two objects together to transfer electrons from one object to another. • Led to the discovery of charge • Elektron means amber in Greek

  7. Charging by Conduction • Grounding-a method of discharging where a charged object is touched against a much larger neutral object.

  8. Charging by Induction

  9. Balloons

  10. Coulomb's Law • k=8.99x109 Nm2/C2 • Inverse square • Almost exactly the same as Newton’s Law of Universal Gravitation • q is measured in Coulombs (huge) • F will be positive with like charges and negative with unlike charges; therefore positive means repulsive and negative means attractive.

  11. Practice Problems • Two charges are 1.2m apart. If the first has a charge of 6μC and the force between them is .45N, what is the charge of the second?

  12. 2D Coulomb’s Law • Charges A, B, and C form a right triangle. Charge A is 3C, charge B is 2.5C, and charge C is -7C. The distance between A and B is 4.7m and between B and C is 3m. What is the force on each charge?

  13. Electric Fields • The equivalent to g when discussing gravity. • In order to determine electric field, place a positive test charge near other charges and measure the force. • The electric field is independent of the size of the test charge, like g is independent of the mass of an object.

  14. Electric Field for a Point Charge • When the electric field is caused by a point charge, we can substitute out force by using Coulomb’s Law

  15. Practice Problems • If charge A is 6μC and charge B is -4μC, what is the electric field at point C?

  16. Electric Field Lines • Show what the electric field looks like in a specific direction • Direction of lines show direction • Spacing of lines show strength

  17. Two Charge Systems

  18. Current • Measures how much charge passes a point in a certain amount of time. • Measured in Amperes, or Amps (A)

  19. Electric Potential Energy • Like a mass has a potential energy in a gravitational field, a charge will have potential energy in an electric field. • As the charge moves, electric potential energy becomes kinetic energy.

  20. Potential Difference • Since we are usually dealing with large quantities of moving charge rather than a single stationary charge, it is easier to discuss a change. • Potential difference=voltage • Measured in volts • Found by finding the difference in potential energy between two points for a test charge and then dividing by the test charge.

  21. Resistance • A measure of how much an object resists the flow of electricity. • Electrons move from atom to atom. In the process, energy is lost in heat. • The higher the waste, the more the resistance. • Depends on material, temperature, and shape (thickness and length) • Symbol is R • Unit is the Ohm (Ω)

  22. Ohm’s Law • Relates resistance, current, and voltage. • Good for some objects, but not all objects (diode). • Power equation-relates the rate at which energy is converted from electrical energy to mechanical energy to factors in the circuit.

  23. Electric Power

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