Chapter 32: Electrostatics

1. Chapter 32: Electrostatics Conceptual Physics Bloom High School

2. 32.1 Electrical Forces and Charges • Electrostatics- electricity “at rest” • Electrical forces- like gravity, can attract • Unlike gravity, can also repel • Charge- electrons (-), protons (+) • 1. Every atom has a (+) nucleus surrounded by (-) • 2. All (-) are identical in mass and charge • 3. All (+) are identical in mass and charge. The nucleus also contains neutrons, which are neutral. A proton is equal in mass to a neutron, but 2000x more massive than (-) • 4. Neutral Atoms have an equal number of (-) and (+)

3. Like attracts like (only in chem) • Like charges repel, opposite charges attract

4. 32.2 Conservation of Charge • Conservation of Charge- electrons may not be created nor destroyed! • Protons cannot be gained or lost from an atom • Electrons are only gained or lost in whole number quantities • There’s no such thing as losing 1.5 electrons! • Anion- an atom that has gained one or more (-) • Negative ion • Cation- an atom that has lost one or more (-) • Positive ion

5. 32.3 Coulomb’s Law • Universal Gravitation (Ch 12) • Fg=G(m1m2)/r2 • G=6.67x10-11 Nm2/kg2 • m=mass of an object (kg) • r=distance between objects (m) • F=gravitational force between particles (N) • Coulomb’s Law (Ch 32) • Fe=k(q1q2)/r2 • k=9x109 Nm2/C2 • q=charge on a particle (C) • r=distance between particles (m) • F=electrostatic force between particles (N)

6. Coulomb’s Law • Force is inversely proportional to distance • Larger r=weaker F • q can be positive or negative • Allows for an attractive force is q’s are opposite • (+q)(+q)  +F and like charges repel! • (-q)(-q)  +F and like charges repel!

7. Comparison of Fg and Fe • For a Hydrogen atom: • Fe=k(q1q2)/r2 • qe=-1.6x10-19 C • qp=1.6x10-19C • r=5.3x10-11 m • Fe=8.2x10-8 N • Fg=G(m1m2)/r2 • me=9.1x10-27 kg • mp=1.7x10-31 kg • Fg=3.7x10-47 N • Fe/Fg=2.2x1039 stronger!!!

8. 32.4 Conductors & Insulators • Conductors- outer electrons are not well fixed to the nuclei and are free to travel • Heat and electricity can be transmitted • Most metals, “salt”-based solutions • Insulators- outer electrons are fixed • No energy can be transmitted • Ceramics, rubber, dry wood, pure water

9. Semi- & Super- conductors • Semiconductors- can behave as a conductor or insulator • Germanium, silicon • Used in circuit boards • Superconductors- transmits energy without resistance (heat) • Only occurs at temperatures near 0K • Limited applications due to low temperature required

10. 32.5 Triboelectricity • Triboelectricity- charging by friction • Walking across a carpet and touching a door knob • Rubbing a balloon on your head and sticking it on a wall

11. 32.6 Charging by Induction • A charge is drawn away from a pair of touching objects, then the objects are separated • Induced- when something is forced to occur • Grounding- allowing a charge to flow into the Earth

12. 32.7 Charge Polarization • Electrically Polarized- when one side of a molecule has a larger electron cloud than the other • By design or by induction

13. Summary of Causes of Charges • 1. By friction, through rubbing objects together • 2. By contact, through touching of a charged object to another conductive object • 3. By induction, bringing a charged object near another object, but not necessarily with touching