Static Electricity Notes CP Physics

Electrical Forces Created by the attraction and repulsion of charged particles (ions) Part of electromagnetic force

Electromagnetic Forces vs. Gravitational Force Attraction or repulsion between two charged objects Can act over a distance (two objects do not have to touch) Stronger than gravitational force Attractive force only between two objects Can act over a distance (two objects do not have to touch) Weaker than electromagnetic force (weakest of four universal forces)

Fundamental Rule of Electrical Phenomena Like charges repel Opposite charges attract

Charged Objects Objects become charged when electrons are removed Ions = charged atoms Atoms lose electrons = positive ion Atom s gain electrons = negative ion Materials that become negatively charged Rubber Silk Materials that become positively charged Glass Hair Fur Plastic

Conservation of Charge Cannot create or destroy charges, can only transfer charges from one material (object) to another Actually separating charges to form (+) and (-) charged objects Note: charges cannot be divided, they always move in whole numbers

Coulomb’s Law States that electrical force varies inversely with the square of the distance and directly with the product of the charges Unit of charge = Coulomb (C) Electrical forces between very large objects tend to cancel out, but electrical forces between very small objects (atomic level) can be very large Equation: Fel = kQ1Q2 k = 9.0 x 109 N•m2/C2 d2 Q = charge in coulombs, d = distance in meters, k = constant

Conductors and Insulators Conductors: outer electrons move easily, have loosely held electrons, ex. Metals Insulators: outer electrons held tightly so they do not move easily, ex. Glass, plastic, rubber, wood Semiconductors: materials that are usually insulators but when impurities added become conductors, ex. Silicon, germanium Superconductors: materials with no resistance to electrical flow – do not lose any energy to friction or heat, most work at absolute zero (-273 oC)

Charging Objects: Friction Rubbing two surfaces together transfers electrons Results in a (+) object and a (-) object Two objects are attracted to each other Ex. Clothes in dryer, combing hair

Charging Objects: Conduction Touching a neutral object with a charged object results in the neutral object taking on the same charge. The two objects then repel each other. Until the objects are grounded, this form of charging is permanent.

Charging Objects: Induction Bring a charged object near the neutral object. Charges within the neutral object separate so that the side closest to the charged object takes on the opposite charge and is attracted to the charged object. This process is only temporary unless the object can be divided as in steps c and d shown below.

Grounding Process of touching an object so that a path to the ground exists and charges can move from the object into the ground.

Lightning Electrical discharge between oppositely charged clouds or the ground Results from charging by induction – negatively charged clouds induce positive charge in ground below them

Charge Polarization Occurs in insulators Charged object near insulator causes charge rearrangement within atoms or molecules Atoms – electrically polarized – one side of atom is slightly positive while opposite side is slightly negative Ex. Balloon rubbed against hair will stick to the wall

Charge Polarization: Water Many molecules are electrically polarized in their natural state. One example is water – the oxygen atom has a slight negative charge while the side of the molecule with the hydrogen atoms is slightly positive.