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Chapter 21

Chapter 21. Electric Charge and Electric Field. Goals for Chapter 21. To study electric charge and see how charge behaves in conductors and insulators To calculate force with Coulomb’s Law To consider the electric field as a map of force on a test charge

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Chapter 21

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  1. Chapter 21 Electric Charge and Electric Field

  2. Goals for Chapter 21 • To study electric charge and see how charge behaves in conductors and insulators • To calculate force with Coulomb’s Law • To consider the electric field as a map of force on a test charge • To see how electric fields superimpose • To visualize and consider the path of electric field lines around a charge or charges • To see the unique applications of electric dipoles

  3. Introduction • Water pervades the science of chemistry and biology. It’s not only what we drink when we’re thirsty, but it’s been called “the universal solvent.” • Even if we were to only look at water, and water as a solvent, we would see a simple problem like salt dissolving in water is the interaction of electrostatic charges, of ions and dipoles.

  4. Electric charge • Glass rods, plastic tubes, silk, and fur can be used to demonstrate the movement of electrons and how their presence or absence make for powerful forces of attraction and repulsion.

  5. The photocopier, a very clever tool • The world may have come to take copiers for granted, but they are amazing devices. They use charge to hold fine dust in patterns until the pattern may be transferred to paper and made permanent with heat.

  6. How is the atom arranged? Why is it easiest to move electrons? • Visualize a football stadium as an atom. Electrons would be garden peas in the highest seats with charge of −1. Protons would be basketballs or melons with charge of +1, and neutrons would reside about the protons with no charge. All of the protons and neutrons could be in a small basket on the 50-yard line.

  7. Consider lithium as a cation, an anion, and a neutral • Let’s study the subatomic arrangement of lithium with all charges balanced and the way only electrons move to make the atom an ion (+ or −).

  8. Movement of charges—charging by conduction • Materials that allow easy passage of charge are called conductors. Materials that resist electronic flow are called insulators. The motion of electrons through conducts and about insulators allows us to observe “opposite charges attract” and “like charges repel.”

  9. Electrons move freely and charges may be induced • Take a child’s toy, a rubber balloon. If you rub the balloon vigorously on a fuzzy sweater then bring the balloon slowly toward a painted concrete or plaster wall, the balloon will stick to the wall and remain for some time. The electrostatic force between static electrons and the induced positive charge in the wall attract more strongly than the weight of the balloon.

  10. Static electricity about an insulator can shift • The motion of static charges about a plastic comb and light bitsof paper can cause attractive forces strong enough to overcomethe weight of the paper.

  11. Charges will “seek” motion to ground • An uncharged conductor can attract the charge imparted to paint droplets.

  12. Charles Coulomb determined the electrostatic force law • Coulomb’s Law allows the calculation of electrostatic attraction or repulsion.

  13. Examples of electrical force calculated—I • A fascinating comparison of gravitational force to electrostatic force is shown in Example 21.1 and Figure 21.11. • Regard Problem-Solving Strategy 21.1. • See also Example 21.2 and Figure 21.12.

  14. Examples of electrical force calculated—II • Consider Example 21.3 and Figure 21.13. • See also Example 21.4 and Figure 21.14.

  15. Electric fields may be mapped by force on a test charge • If one measured the force on a test charge at all points relative to another charge or charges, an electric field may be mapped. • This experiment is often done in one’s mind (called a “gedanken experiment”).

  16. Electric fields I—the point charge • Fields of force may be sketched for different arrangements of charge. • Consider Example 21.6 and Figure 21.19.

  17. Electric fields II—charges in motion within a field • Consider Example 21.7. • Consider Example 21.8 and Figure 21.21.

  18. Electric fields add as vectors • Regard Figure 21.22. • Review Problem-Solving Strategy 21.2. • Follow Example 21.9 and Figure 21.23.

  19. A field around a ring or line of charge • Review Example 21.10 and Figure 21.24. • Review Example 21.11 and Figure 21.25.

  20. A field around a disk or sheet of charge • Review Example 21.12 and Figure 21.26. • Review Example 21.13 and Figure 21.27.

  21. Electric field lines map out regions of equivalent force I

  22. Electric dipoles and water • As mentioned in the introduction, the dipole force of water is vital to chemistry and biology.

  23. Consider force and torque on a dipole • Regard Figure 21.32. • Follow Example 21.14 and Figure 21.33.

  24. The electric field of a dipole revisited • Consider Example 21.15. • Figure 21.34 illustrates the example.

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