Sci-101 Introduction to Physics Dr. Alexander Dzyubenko Chap. 9 Static and Current Electricity adzyubenko © A.B. Dzyubenko 2003, 2004

L8: Electricity 2 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 There are two types of charges “+” and “-”, unlike gravity repel attract

L8: Electricity 3 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 Protons (+) are about 2000 more massive than electrons (-) Neutrons have slightly more mass than protons

L8: Electricity 4 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 -

L8: Electricity 5 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

L8: Electricity 6 Conservation of Charge Whenever something is charged, no electrons are created or destroyed The conservation of charge is a significant principle in physics

L8: Electricity 7 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

L8: Electricity 8 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 = 9 x 10 9 N  m 2 /C 2

L8: Electricity 9 Gravitational vs Electric Forces The gravitational force holds the satellite in orbit The electric force holds the electron in orbit Qualitatively, very similar Quantitatively?

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L8: Electricity 11 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

L8: Electricity 12 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, E, is a vector Field (or force) lines are used to represent E Electric field near a negative charge Direction? That of the force on a positive charge

L8: Electricity 13 Electric field near a negative charge. Electric field of a dipole, a pair of “+” and “–” charges. Net charge=0. Electric field near two charged metallic plates. A capacitor. Examples

L8: Electricity 14 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

L8: Electricity 15 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

L8: Electricity 16 Electric Potential 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

L8: Electricity 17 Electric Potential and PE Two different charges at same distance from the charged dome Whose energy is larger? Whose potential is larger?

L8: Electricity 18 Voltage Sources The ends of an electrical circuit are at different electric potentials (+) Charges in the conductor flow from high to low potentials To attain a sustained flow of charge, we need a battery or generator Electrical “pumps” the flow ceases when the difference in pressures ceases The difference in pressures is maintained with the pump

L8: Electricity 19 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

L8: Electricity 20 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

L8: Electricity 21 Electric current, cont The current-carrying wire is not electrically charged The charges flow through a circuit when voltage is applied across a circuit

L8: Electricity 22 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

L8: Electricity 23 Electrical Resistance The longer the wire, the larger the resistance The longer the pipeline, the larger the resistance

L8: Electricity 24 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  Ohm’s Law

L8: Electricity 25 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

L8: Electricity 26 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) 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

L8: Electricity 27 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

L8: Electricity 28 Connection in Series Electric current has just one path Resistance of the circuit = SUM of all resistances

L8: Electricity 29 Connection in Parallel Each device operates independently but at same voltage The total current= SUM of all currents

L8: Electricity 30 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

L8: Electricity 31 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