Electrostatics The Shocking Truth

What is Electrostatics? Definition: Electricity at rest (stationary) Static means to stand and is used in Mechanical Engineering to study forces on bridges and other structures. Statue, stasis, stationary, ecstatic, status

Examples of Electrostatics Thunder and Lightning Static shocks from carpet and doorknobs Rubbing balloons on hair and sticking to the wall Rubbing Comb and attracting paper

Electrostatics and You! Wow! A balloon! Wow! A comb (and bits of foil)! Wow! A Kleenex tissue!

Polarization of Charge

Da-da da-da da-da, Charge! An object that has electrical interactions with its surroundings is said to be CHARGED. There are “positive” and “negative” charges. Like charges repel (+&+; -&-) Opposites attract (+&-).

Conductors and Insulators Allows flow of electricity by having free roaming electrons. Examples: Metals, water, humid air, ionized gas, plasma, graphite, wet wood Insulator: Hinders flow of electricity by having tightly bound electrons. Examples: Glass, ceramics, dry wood, rubbers, plastics, cloth, air.

Electrical Forces Charge: Positive (+) and Negative (-) by convention. Protons (+) are attracted to Electrons (-). Neutrons have neutral (no) charge. Like charges repel; opposite charges attract.

Model of the Atom

A Beautiful Comparison Newton’s Law of Gravitation Fg = G m1m2 d2 Coulomb’s Law of Attraction F = k q1q2 d2 Both are inversely proportional to distance squared. Product of masses versus product of charges Newton’s: Attractive only. Coulomb’s: Attractive and repulsive.

Electroscopes Test sample touches external ball. Charges spread out onto foil leaves. Since all charges are the same the leaves separate by repulsion - either positive and positive, or negative and negative. Pith balls also work.

Induction versus Conduction Charging by Conduction: Transferring charges by touching a charged object to an uncharged one. Example: Electroscope

Induction versus Conduction Charging by Induction: Transferring charges without touching a charged object to an uncharged one. Example: Large amounts of negative charges in storm clouds induces separation of charge on ground and lightning results.

Charge Induction

Induction versus Conduction Charging by Induction or Conduction: A balloon is charged negatively and sticks to the wall. Is it charging the wall by induction, conduction or something else? Answer: Neither. Even though it touches it is more like induction than conduction. Few charges are transferred to the wall since the wall is an insulator.

Van de Graaff Generator The American physicist Robert Jemison Van de Graaff invented the Van de Graaff generator in 1931. Charge transferred onto moving belt and up to the metal dome on top. Silicon tape on lower roller and rubber belt cause charges to build up on belt that are taken up and collected on the dome. The results are shocking!

Van de Graaff Generator Demonstration 1. Los Angeles Sparks. Demonstraton 2. Space Invaders. Demostration 3. Any longhaired freaks?

Jacob’s Ladder (Climbing Arc) How Does a Jacob's Ladder Work? The explanation is that an arc starts at the bottom and due to the fact that hot air rises, the arc tends to move up the diverging rods until they are too far apart for the voltage provided by the power source. The circuit breaks and a new arc is formed at the bottom. Like a real lightning the charges jump across the separation. Notice that higher up the rods are pulled together because there they are more flexible. This basically is how lightning works. Even the smell and generation of Nitrides is similar. Homemade Jacob's Ladder/ Climbing Arc

How can we move and use charges? Michael Faraday (English physicist, 1831) found that: Change a magnetic field, create an electric current (see video). Wire in presence of strong magnet made current (see video). Double the wires, double the current. Many wrappings of wire around a nail with an applied current makes a temporary magnet.

Hand Generator (The Ancient) How Does a Hand Generator Light the Bulb? Coils of wire on the axle are rotated between the poles of strong magnets. This generates alternating current (try attaching a galvanometer and observe the readings). The current is strong enough to heat the filament in the light bulb. Give it a try. Notice that when the circuit is opened it is much easier to turn the crank. Why?

Hand Generator (The Modern) These are much more efficient than the ancient ones. They have their own storage cells too. Flashlight, AM/FM Radio, Warning Siren. Maybe someday we will have to ride a bicycle-like device while we use our computers.

Put it in Reverse! Just as motion can make electricity, the reverse works as well. Electricity can be used to generate motion = electric motors

In Summary… Objects become electrically charged in three way: By friction By conduction (contact) By induction (no contact). Electrons can flow in wires when moved relative to a magnet. This is the basis for electric motors.