Chapter 32: Electrostatics

Conceptual Physics

Bloom High School

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 (+)

Like attracts like (only in chem)

• Like charges repel, opposite charges attract

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

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)

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!

Comparison of Fg and Fe

• For a Hydrogen atom:– Fe=k(q1q2)/r2

– qe=-1.6x10-19 C

– qp=1.6x10-19 C

– 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!!!

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

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

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

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

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

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