Electric Charge and Electric Field

Chapter 21 Electric Charge and Electric Field

PowerPoint® Lectures for University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman Lectures by James Pazun Modified by P. Lam 7_7_2008 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Topics for Chapter 21

• Atomic charge model • Electrostatic force - Coulomb’s Law - qualitative • Conductors and insulators • Electrostatic force - Coulomb’s Law - quantitative • (intermission) • Electric field generated by point charges • Electric field generated by continuous charge distribution • Electric field lines • Electric dipoles

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Introduction • Four fundamental forces (interactions): – 1. Gravitational force - comes from interaction between masses or energy – 2. Electromagnetic force - electric forces come from interaction between stationary or moving charges ; magnetic forces come from interaction between moving charges. There are two types of charges, + and -. (Since whether a charge is moving or not depends on the frame of reference, hence electric force and magnetic forces are different viewpoint of the same “force”) – 3. Strong nuclear force – 4. Weak nuclear force

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley I. Atomic Charge Model - electron, proton, and neutron • Visualize a football stadium as an atom. Electrons would be garden peas with charge of e. • Protons would be basketballs or melons with charge of +e, and neutrons would reside next to the protons with zero charge. • All of the protons and neutrons could be in a small basket in the middle of the field. • In S.I. unit, e=1.6x10-19 Coulomb.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Neutral vs. Charged Materials • Most materials are electrically neutral, that is, it contains exactly the same number of protons (+’s) as electrons (-’s). • If we remove some electrons from a neutral material, it becomes positively charged. • If we add some electrons to a neutral material, it becomes negatively charged. • When two materials are rubbed against each other, some electrons from one material are transferred to the other materials, creating one positively charged material and one negatively charged material (net amount of charge is conserved). (What fundamental interaction is “rubbing”?) Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley II. Electrostatic force - Coulomb’s Law

• Coulomb’s Law quantifies the electrostatic force between stationary point charges (as along as the relative velocity between charges is much less than the speed of light; one can consider the charges as nearly stationary and Coulomb’s Law applies).

(1) Like charges repel; unlike charges attract (2) Direction of forces are along the line joining,the two charges 2 q1q2 r q1q2 9 Nm (3) The magnitude of the force is proportional to 2 ;| F |= k 2 ;k 9x10 2 r r C

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Charging by “rubbing” (electromagnetic interaction) • Rubbing two materials is to force the atoms from the two materials to be very close to each other; one material likes to accept extra electrons while the other material is willing to give up some electrons (explained by quantum mechanics).

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley A charged object can attract a neutral material • A charged object such as a plastic comb (e.g. charged by rubbing against you hair) can attract a piece of paper which is neutral by inducing electric dipoles in the paper; the 1/r2 dependences of Coulomb’s Law => net attraction.

Note: Both a positively charged comb and a negatively charged comb can attract a neutral object.

• A collection of atoms forms solid, the chemical bonding determine whether the solid is a conductor or insulator. • Materials with weak chemical bonds which allows easy movement of electrons are conductors. • Materials with strong chemical bond which does not allow the electrons to move freely are insulators. • Q. Which type of materials (conductors or insulators) can be charged more easily? • Q. How are the excess charges distribute over the conductor and insulator?

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley An application of electrostatic force - the photocopier • 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.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Electrostatic interaction in daily phenomenon • The H+ and OH- ions in water makes it a very good solvant and our body needs it. • Dissolving salt in water is a consequence of interaction of electrostatic charges; H+ and OH- from water interacts with salt to break it apart into Na+ and Cl-.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Examples of electrostatic force - I • A fascinating comparison of gravitational force to electrostatic force is shown in Example 21.1 and Figure 21.11. • Alpha particle=He2+ (Helium nucleus without the two electrons) • Find the ratio of the electrostatic repulsion to the gravitational attraction.

Find force vector on q3

Find force vector on Q.

• Coulomb’s states that charges can act on each other over long distances. Faraday came up with a field concept where he imagined that one charge (source charge; Q) produces a web of electric force field (E(r))over all space; the force it exerts on the second charge (q) at location r is simply q*E(r).

• If one measured the force on a postive test charge (qo) at all points relative to source charge (or charges), one can map out the entire electric field generated by the source charge(s).

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley 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.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Electric fields add as vectors • Regard Figure 21.22. • Review Problem-Solving Strategy 21.2. • Follow Example 21.9 and Figure 21.23.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Electric dipoles and water • As mentioned in the introduction, the dipole force of water is vital to chemistry and biology.

• Regard Figure 21.32. • Follow Example 21.14 and Figure 21.33.

r Potential energy U = -pr •E