30-Apr-15 Physics 1 (Garcia) SJSU Electric Charges and Forces There are two types of charge: positive charges and negative charges REPEL ATTRACT Like charges repel, opposite charges attract.
30-Apr-15 Physics 1 (Garcia) SJSU Electrons, Protons, & Atoms Helium atom Electrons Protons Electrons carry negative charge. Protons carry positive charge. Electrons are loosely bound to atoms Protons are tightly locked within the nucleus. Objects are usually neutral but can easily acquire or lose electrons to become charged.
30-Apr-15 Physics 1 (Garcia) SJSU Charging by Friction/Contact Electrons move easily so an object can become charged by rubbing electrons off the object’s surface. Brush your hair with a plastic comb. Walk across a carpet with plastic-soled shoes. Electrons taken off of fur onto rubber rod
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Electroscope The “legs” of an electroscope separate when the electroscope is charged due to the repulsion of like charge on the two legs. Charged ebonite rod deposits negative charge (electrons) onto electroscope. “Legs”
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Dial Electroscope Alternative design for electroscope Charged Neutral
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Van de Graff Generator Van de Graff deposits large quantities of excess charge on its globe. A person with long hair can become a human electroscope.
30-Apr-15 Physics 1 (Garcia) SJSU Conservation of Charge + + + + + Rod & Fur Neutral Electrons are rubbed off the hairs of a piece of fur, collecting and charging a plastic rod. Is the fur also charged? Positive or negative. Charge cannot be created nor destroyed. Objects become charged by transfer of charges.
30-Apr-15 Physics 1 (Garcia) SJSU Conductors & Insulators Materials, such as metals, in which electrons flow freely are called electrical conductors. Materials, such as plastic and wood, in which electrons do not move easily are called electrical insulators. Materials, such as silicon, that can act as conductors or insulators under different conditions are called semiconductors.
30-Apr-15 Physics 1 (Garcia) SJSU Plastic Insulators Plastic insulation on step of van de Graff and charge wand keep charge from leaving the metal balls. Plastic insulation on wire prevents charge to leave wire except at the ends, where the conducting metal is exposed.
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Tesla Coil Air is an insulator but at high voltages it can conduct electricity as lightning. Portable Tesla coil
30-Apr-15 Physics 1 (Garcia) SJSU Tesla Coils Very high voltages achieved by large Tesla coils. Homemade, backyard model Fry’s Superstore, Fremont CA
30-Apr-15 Physics 1 (Garcia) SJSU Coulomb’s Law Electric force has magnitude given by (Electric Force) = (k) x Object A Object B ( Charge of Object A ) x ( Charge of Object B) ( Distance ) x ( Distance ) DISTANCE Force Equal and opposite forces (Newton’s Third law)
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Induce the Can Charges separate in the can. Force of attraction is stronger since opposite charges are closer. Metal Can (Neutral) Charged Object + + + + + + Attraction Force Repulsion Force Can Rolls
30-Apr-15 Physics 1 (Garcia) SJSU Charging by Induction Can separate charges by inducing them to opposite sides of a conducting object. Start Neutral Induce +’s to one side Allow –’s to Escape Finish with net + charge
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Electrophorus The electrophorus consists of a plastic plate and a metal disk with an insulated handle. - - - - - - - - - - - - - - - - - - - - - - - - + + + + + - - - - - Charge plastic plate by rubbing it with fur Bring metal disk to plate; separate charge Remove electrons; plate positively charged
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Electroscope & Induction The legs of the electroscope separate when charged rod brought near the electroscope. Charge induction pushes electrons into the legs. Negatively charged rod repels electrons so they move as far away as possible. “Legs” + + + + + +
30-Apr-15 Physics 1 (Garcia) SJSU Lightning Storms Charge separates inside of thunderclouds. The lower part of the cloud is negatively charged. This induces a positive charge on the ground. When the voltage difference is high enough, a lightning bolt can occur.
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Lightning Rod Charge flows readily to and from sharp metal points. Purpose of a lightning rod is to discharge a building before lightning occurs. Discharge Lightning Rods
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Electric Pinwheel Pinwheel turns as charge leaves the Van de Graff from the points at the tips of the spokes. Van de Graff Charged Air molecules
30-Apr-15 Physics 1 (Garcia) SJSU Charge Polarization Charge polarization occurs when the positions of the electrons and protons in an atom are skewed. Neutral Polarized Charged Rod Polarized Atoms
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Static “Cling” Charged object attracts a neutral insulator by inducing charge polarization in the neutral object. Charged Balloon Neutral Wall Polarization Charged Comb Neutral Paper
30-Apr-15 Physics 1 (Garcia) SJSU Check Yourself What causes a stream of water to be deflected when you bring a charged object next to it?
30-Apr-15 Physics 1 (Garcia) SJSU Electric Field Lines Electric field lines indicate the direction of electric force if a positive charge was placed in the electric field.
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Streamers Paper streamers attached to the van de Graff show the direction of the electric field. Similar to hair- raising demo
30-Apr-15 Physics 1 (Garcia) SJSU Lab: Electric Field Lines In lab you map out electric field lines and electric potential
30-Apr-15 Physics 1 (Garcia) SJSU Electric Potential Energy Potential energy associated with electric force, similar to potential energy associated with gravitational force. Gravity Force Electric Force
30-Apr-15 Physics 1 (Garcia) SJSU Electric Potential Electric potential is potential energy per charge, like temperature is internal energy per molecule. 5000 °F ? Both situations are safe because the total amount of energy is small Check Yourself: Is this safe?
30-Apr-15 Physics 1 (Garcia) SJSU Capacitors Capacitors are used to store separated positive and negative electric charges. Basic design is pair of conducting plates insulated from each other. Industrial capacitor
30-Apr-15 Physics 1 (Garcia) SJSU Demo: Discharging a Capacitor Charged capacitor holds enough charge to light a light bulb when discharged. Low voltage (low electric potential) but LARGE amount of charge – Dangerous!