Physics 242 – Electric Charges

Types and Sources of Electric Charge

Two kinds of electric charge – positive negative electrons

protons

http://csep10.phys.utk.edu/astr162/lect/light/bohr.html

Bohr Model of Atomhttp://www.physics.ucla.edu/~dauger/orbitals/

Quantum Mechanical Model

Materials with equal numbers of neg. and pos. charge are said to be electrically neutral.

Physics 242 – Electric Charges

Types and Sources of Electric Charge

THANKS BEN!

Ben Franklin actually did a great deal of experimentation in the area of electricity.

However, Ben Franklin chose the wrong sign convention for charge – it has stuck ever since.

Physics 242 – Electric Charges

Properties of Electric Charge

• Charge is quantized. • Positive and Negative charges have same magnitude

e = p = 1.6 x 10-19 CSI unit of charge is the Coulomb

• Charge is conserved.

http://www-personal.umich.edu/~jcv/imb/imb.htmlhttp://www.wikipedia.org/wiki/Proton_decay

p → e+π0

Proton Decay

Physics 242 – Electric Charges

Atoms and Materials

Classification of Materials – Conductors, Insulators, Semiconductors

Conductors

Electrons are free to moveabout the material

Insulators

Electrons are bound to the atoms- can’t move (conduct).

Different elements will have differenttypes of bonds – different strengths.

Physics 242 – Electric ChargesHow do we charge an object?

A. Tribocharging - Charging by rubbing

Positive (+)

Air

Human Hands

Rabbits Fur

Glass

Human Hair

Mica

Wool

Cats Fur

Silk

Aluminum

Paper

Cotton

Rubber Balloon

Hard Rubber

Gold/Platinum

Polyester

Vinyl

Saran Wrap

Negative (-)

Triboelectric Series

  Electrostatic Voltages

Means of Static Generation 10 – 20 % Humidity

65 – 90 % Humidity

Walking across carpet 35,000 1,500

Walking across vinyl floor 12,000 250

Worker at bench 6,000 100

Vinyl envelopes for work instructions

7000 600

Common poly bag picked up from bench

20,000 1,200

Careful when you fill up your gas tank!http://esda.org/esdbasics1.htm

Physics 242 – Electric ChargesHow do we charge an object?

A. Tribocharging - Charging by rubbing

Conductors – charge will distribute around the surface to try until equilibrium is achieved. - charge only resides on surface, not inside.

Insulators – charge cannot move, so it remains localized

around the contact region.

On symmetric objects, charge distributes uniformly

On non-symmetric objects, charge collects at sharp points.

Physics 242 – Electric ChargesHow do we charge an object?

B. Induction – Inducing a charge without touching.

Conductors

Charging by induction (two conductors)

Charging by induction (1 conductor and ground)

Charge is physically moved in/on the materials.

Physics 242 – Electric Charges

Conductors – An Example

+3q +3q

-3q

+3q +3q

+3q

• Outside the conductor, the net charge is +3q• Sphere equivalent to a point charge• Positive charge doesn’t move, neg. charge does all the moving.

Physics 242 – Electric ChargesHow do we charge an object?

B. Induction – Inducing a charge without touching.

Insulators

Free StandingAtoms are randomly oriented

Charge does not actually moveIn/on the insulator – redistributes. Polarizes the material.