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Sci-101 Introduction to Physics Chap. 9 Static and Current Electricity Dr. Alexander Dzyubenko http://www.csub.edu/~ adzyubenko © A.B. Dzyubenko 2003, 2004 Electric Force and Charge • A universal force, like gravity • Varies inversely as the square of the distance, like gravity • Billions and billions time stronger than gravity repel attract • There are two types of charges “+” and “-”, unlike gravity 2 L8: Electricity Atomic structure Every atom is composed of a positively charged nucleus surrounded by negatively charged electrons The electrons of all atoms are identical: same charge, same mass, same everything Protons and neutrons compose the nucleus Neutrons have Protons (+) are about 2000 slightly more mass more massive than than protons electrons (-) 3 L8: Electricity Atomic structure cont Normally an atom has many electrons and equal number of protons => neutral as a whole Hydrogen atom: one proton, one electron When an atom loses one or several electrons, it has a positive net charge; called a positive ion - An atom gains one or several electrons, it has a negative net charge; called a negative ion 4 L8: Electricity Making neutral objects charged Material objects are made of atoms Ordinarily have equal numbers of electrons and protons => neutral as a whole The electrons farthest from the nucleus can be stripped away Easily? It depends on a substance! Electrons are transferred from the fur to the rod 5 L8: Electricity Conservation of Charge Whenever something is charged, no electrons are created or destroyed The conservation of charge is a significant principle in physics 6 L8: Electricity Coulomb’s Law The force between two charges varies directly as the product of their charges and inversely as the square of the distance between them 7 L8: Electricity Coulomb’s Law cont The unit of charge, Coulomb (C) Equals the amount of charge that passes through a 100 W bulb in about 1 s The proportionality constant, k, is similar to the gravitational constant G k = 9x109 Nm2/C2 8 L8: Electricity Gravitational vs Electric Forces The gravitational force holds the satellite in orbit The electric force holds the electron in orbit Qualitatively, very similar Quantitatively? 9 L8: Electricity http://www.people.vcu.edu/~rgowdy/mod/077/xmp.htm L8: Electricity 10 Charge polarization Charge an inflated balloon by rubbing it on your hair Place it again a wall and it sticks … Negatively charged balloon polarizes atoms in the wall 11 L8: Electricity Electric Fields Electric (like gravitational) forces act between charged objects that are not in contact There exists a force field that influences distant charges (masses) Electric field near Electric field, E, is a vector a negative charge Field (or force) lines are used Direction? That of to represent E the force on a positive charge 12 L8: Electricity Electric field near two charged metallic plates. A capacitor. Examples Electric field near a negative charge. Electric field of a dipole, a pair of “+” and “–” charges. Net charge=0. 13 L8: Electricity Electrical Potential Energy A charged object has electrical potential energy by virtue of its location in an electric field (a) gravitational PE (b) PE of charged particles 14 L8: Electricity Electric Potential Energy, cont Work done on the spring goes to the increase of its mechanical PE Work done on the charge goes to its electrical PE 15 L8: Electricity Electric Potential Electric potential energy Electric potential amount of charge Unit of energy: Joule (J) Unit of charge: Coulomb (C) Unit of Electric Potential: Joule/Coulomb = Volt 1 V = (1 J)/(1 C) Terms electric potential and voltage are used 16 L8: Electricity Electric Potential and PE Whose energy is larger? Whose potential is larger? Two different charges at same distance from the charged dome 17 L8: Electricity Voltage Sources The ends of an electrical circuit are at different electric potentials (+) Charges in the conductor flow from high to low potentials the flow ceases when To attain a sustained the difference in flow of charge, we need a pressures ceases battery or generator The difference in Electrical “pumps” pressures is maintained with the pump L8: Electricity 18 Batteries A common automobile battery provides 12 Volts We say there is a 12-volt voltage drop in the circuit 12 Joules of energy are supplied to each 1 Coulomb of charge that is made to flow in the current 19 L8: Electricity Electric current Is the flow of charged particles In metals, electrons are flowing charged particles These are called conduction electrons Positively charged nuclei (protons+ neutrons) are locked in fixed positions The unit of electric current is Ampere (A): it is a rate of flow of 1 Coulomb per second through a wire 20 L8: Electricity Electric current, cont The current-carrying wire is not electrically charged The charges flow through a circuit when voltage is applied across a circuit 21 L8: Electricity Direct and Alternating Currents DC current: charges flow in one direction AC current may be accomplished by periodically switching the sign at the terminals Commercial US standard: 60 cycles per second or 60 Hertz (Hz) European standard: 50 Hz 22 L8: Electricity Electrical Resistance The longer the pipeline, the larger the resistance The longer the wire, the larger the resistance 23 L8: Electricity Ohm’s Law Current = Voltage / Resistance For a given circuit (Resistance), current is linearly proportional to voltage Good conductors (like metals) obey Ohm’s Law Not all substances obey Ohm’s Law Units for Resistance: Ohms, Amperes = Volts / Ohms 1 A = 1 V/ 1 24 L8: Electricity Quick Quiz How much current flows through a lamp that has a resistance of 60 when 12 Volts is impressed across it? Current = Voltage / Resistance Amperes = Volts / Ohms Current = (12 Volts) / (60 ) = 5 A 25 L8: Electricity Electric Shock Which causes electric shock in human body – current or voltage? Current! The resistance of the human body ranges from about 100 (when soaked with salty water) to 500,000 (when the skin is very dry) 0.001 A can be felt 0.05 A is painful 0.01 A causes involuntary muscle contractions 0.07 A probably fatal if lasts > 1 s 26 L8: Electricity Electric Circuits A circuit, any path through which electrons can flow Usually contains more than just one device Devices connected to each other Connections in series Connections in parallel Conduction electrons surge to and fro in the filament: AC voltage => AC current 27 L8: Electricity Connection in Series Electric current has just one path L8: Electricity Resistance of the circuit = SUM of all resistances 28 Connection in Parallel Each device operates independently but at same voltage The total current= SUM of all currents 29 L8: Electricity Appliances are connected in parallel Circuits that carry more that a safe amount of current are said to be overload A fuse is connected in series to prevent a possible overload 30 L8: Electricity Electric Power The moving charges do work (electric heaters and ovens, electric motors, …) The rate at which work is done is called electric power Power = Current x Voltage 1 Watt = 1 Ampere x 1 Volt Quiz: A 60 W lamp operates on a 120-V line. Current? Answer: Current = 60 W / 120 V = 0.5 A 31 L8: Electricity