Basic facts about electrical gadgets

Basic facts about electrical gadgets Dr. Don Franceschetti Physics 1010 Last Lecture !!!!! Remember basic facts about atoms  There is a positive nucleus of protons and neutron. The orbital electrons balance the charge and provide chemical bonding.   Rubbing can transfer electrons from an insulator. Electrons can run around inside a conductor (metal).  Electric Charge  Electric charges can be positive or negative. Bulk matter is positively charged if it has fewer electrons than protons. Bulk matter is negatively charged if it has more electrons than protons. Electrons get transferred when dissimilar materials are rubbed against each other. This is static electricity.    Attraction and Repulsion  Tricks for storing charge  Use parallel metal plates (capacitor or Leyden jar) separated by a thin insulator.  Excess charge arranges itself to screen out electric field inside conductors (van de Graff) The Leyden Jar  Was the first device to store static charge today. Basis for the capacitor today. Funny factoid. Ben Franklin tried to electrocute a turkey with a Leyden Jar. Nearly killed himself instead.   A Capacitor  And the battery  Acts like a pump to set up a certain potential energy difference per electron. Changing the plate spacing changes the amount of charge – how your computer keyboard works.  The Coulomb  1 coulomb (C) is a lot of charge. Elementary charge is 1.6x10-19C. Charge on protons in 1 g. of hydrogen is 96,500 C.   Induced Polarization  Water has a permanent dipole  Electric Potential  Moving electrically charged objects subject to electrical forces changes their potential energy.  Electric potential = Electric potential energy/charge  volt = joule/coulomb Electric Current is     The flow of electric charge In a metal, the flow of conduction electrons Measured in Amperes (amps, A) Remember:   Coulomb in the unit of charge Elementary charge is 1.6x10-19 C, this is the quantum of charge Amps, Watts, Ohms, Volts (AVOW)  Amps measure current flowing past a point in a circuit. Amp=Coulomb/sec Watts measure electrical power supplied by a source. Watt=Joule/sec Ohms measure resistance to current flow. Ω = Volt/Amp. Mho=Amp/Volt, now called Siemens, Volt measures potential energy difference. Volt=Joule/Coulomb    Field Concept  We imagine each charge as surrounded by an electric field. Field points away from positive charges and towards negative. Batteries set up an electric field in the conductors they are connected to.  Electric Current  An overall slow drift of electrons caused by an electric field. Speed may be km/s but drift is cm/sec. Voltage sources  Create a difference in electrical potential across the ends of a conductor. Electrical potential (voltage) is potential energy/charge 1 volt = 1 joule/coulomb      Battery converts chemical energy in to electrical—a source of d.c. Photocell converts light energy into electrical– a source of d.c. Thermocouple converts heat energy into electrical–a source of d.c. Generator converts mechanical energy into electricity– a source of a.c. This battery is a lemon!  Something fishy about this battery..  You can think of a battery as a pump  Simple circuit  Pump analogy again…  Normal conductors  Offer a resistance to electric current—turn electrical energy into heat. Obey Ohm’s Law V=IR. But superconductors offer no resistance at all! R measured in ohms     1 ohm = 1 volt/ampere Electric Shock  Skin resistance is   High when dry Low when wet Especially through the heart  Current can kill you  More about bioelectricity  Nerves carry electrical impulses across their membranes allowing message speeds of 100m/s. Some fish use electrical fields to detect prey. A few use electric discharges to kill them.  More about bioelectricity  Electrocardiogram (ECG, EKG) records small voltages due to heart activity. Current through heart may be needed to synchronize muscle fiber contractions (fibrillation). Electroencephalogram (EEG) records small voltages due to brain activity.   Electroconvulsive therapy a.k.a electroshock Based on observations that febrile seizures were often followed by improvement in schizophrenic patients. Mechanism not fully understood. Still in use! May be replaced by TCM (transcranial magnetic stimulation). Electric Power   P=IV, watts=amps x volts Using V=IR, power loss in a resistance is P=I2R 2  P=V /R The higher the voltage and the smaller the current, the less energy is lost getting from the source to the destination  Direct vs. Alternating  Direct is simple to understand, but hard to transform from low voltage to high to reduce power loss. Alternating is easy to transform, but hard to describe with equations. Edison loved DC, Westinghouse and Tesla loved AC and Westinghouse won (fortunately).   More on AC  Frequency: 60 Hertz is most common in US. In some places 25 hertz, 30 hertz or 50 hertz is used. Root-mean square voltage is 120 volts in US, made lightbulbs glow as brightly as a gas lamp. Europe uses mostly 220 volts. Buy a converter!    Electromagnetic induction  Faraday found that changing the magnetic field inside a coil created (induced) a voltage.  Rotating a coil in a magnetic field accomplishes the same thing. Basis of electric generator and most of 20th Century. Converting AC to DC is Easy  Use a diode, changes ac to pulsating dc. Capacitors and coils ―smooth out‖ the current.  What do you get from the electric company?  Electrical Energy measured in Kilowatt hours. You supply the electrons.  Nikola Tesla 1856-1943  Simplest circuits  Series – current flows through each element in sequence. Break circuit anywhere and all current stops Parallel – current flows through several parallel paths. House wiring is in parallel.  Fuses or circuit breakers used to prevent overload. A parallel circuit in the making  House wiring is usually in parallel. But fuse or circuit breaker is in series.  Edison’s bright idea  ―Success has many fathers.‖ Magnetic Forces  Exist between moving charges in addition to electrostatic force. Permanent magnets exert magnetic forces because charge motion is synchronized on atomic level. Only a few elements (Fe, Co, Ni, Gd) and a few alloys, like Alnico can be made into permanent magnets  Permanent magnets have poles  North pole is really ―north-seeking pole‖ South pole is really ―south-seeking pole‖ So the Earth’s N pole is really a S pole and vice versa. Like poles repel each other. Unlike poles attract each other. Don’t confuse N and S with + and -     Magnetic Fields  Point from North Pole to South Pole. Exert force only on moving charges and currents. Force is perpendicular to both field direction and velocity. Exert torques on permanent magnets, making them line up with firld   Earth has a magnetic field.  Magnetic compass  Also shoes dip. Magnetic Force acts on Currents or Moving Charges  Parallel currents attract. Anti parallel currents repel. Torque on current loop makes it line up with magnetic field.    Basis for galvanometer Magnetic field  Directed from north pole to south pole of permanent magnet. Directed in circles around current carrying wire. Adds (like vectors) from different forces. B = (maximum value of F)/qv on charge q moving at speed v .    Current carrying wired have a magnetic field.  On a moving charged particle  Magnetic force is at right angles to both field and velocity. In uniform magnetic field, path is circular or helical. Used in mass spectrometer and cyclotron to separate particles by mass or speed.   Ferromagnetic materials  Force between neighboring atoms lines up electron currents (Currents due to electron spin and orbital angular momentum)   Unmagnetized material is composed of microscopic domains, each a tiny magnet. Magnetizing occurs because domains grow or shrink. Basis of much recording/data storage technology Magnetism disappears above Curie temperature.   Permanent magnets  Electromagnets  Current flowing through a solenoid (coil) creates stronger magnetic field. Inserting an iron core makes field much stronger Magnetism can be turned on or off.   Magnetic Earth  Earth is a big magnet. Can’t be permanent magnet – too hot inside. Must be a dynamo. Layers of some rocks record evidence for changes in magnetic field direction. More than 20 reversals in past 5 million years. May be due to collisions with asteroids.    Magnetism and the solar system  Van Allen belts, charged particles trapped in Earth’s magnetic field. Sun has complex magnetic field which changes over a 22 year cycle. Tied to sunspot activity.   Other planets have magnetic fields too. Useful in modeling their innards. biomagnetism  Magnetite (a magnetic iron oxide) granules found in:        Bacteria Pigeon brains Bees Wasps Monarch butterflies Sea turtles and fish 1992, discovered in human brains! Electromagnetic induction  Faraday found that changing the magnetic field inside a coil created (induced) a voltage.  Rotating a coil in a magnetic field accomplishes the same thing. Basis of electric generator and most of 20th Century. Magnetic levitation  Maxwell’s Equations  Summarized all that had been discovered about electricity and magnetism. Showed that self-sustaining electric and magnetic fields would travel through space at exactly the speed of light. Light is an electromagnetic wave. Other forms of electromagnetic waves exist (radio, microwave, x-ray, etc). Led to Einstein’s discovery of relativity to explain how speed of light always measured to be the same, even by observers in relative motion!    