Transcript Slide 1
The last revolution happened ~ 60 years ago, when vacuum tubes were replaced by semiconductor devices 1918 1960s Vacuum tubes show high performance but are too big Vacuum tubes can survive a nearby nuclear explosion What is a vacuum tube? What vacuum tubes are for Diode: a valve for electrons + gate _ Triode: a switch or an amplifier First digital computers ENIAC (1946) weighed 30 tons, occupied 1800 square feet and had 17,468 vacuum tubes. It could make 5000 additions per second First computer bug was a real bug Harvard Mark II computer, 1947 1954-1963: SAGE Air Defense Project • 2 32-bit computers • Each contains 55,000 vacuum tubes, weighs 250 tons, and consumes 3 Megawatt • Tracks 300 flights • Total cost: $60 billion (double the price of Manhattan Project!) • Performance equivalent to $8 calculator built on transistors! Dangers of forecasting Computers in the future may weigh no more than 1.5 tons. (Popular Mechanics, 1949) 1940's - IBM Chairman Thomas Watson predicts that "there is a world market for maybe five computers". 1950's - There are 10 computers in the U.S. in 1951. The first commercial magnetic hard-disk drive and the first microchip are introduced. Transistors are first used in radios. ENIAC (1946) weighed 30 tons, occupied 1800 square feet and had 17,468 vacuum tubes. It could make 5000 additions per second 1960's-70's - K. Olson, president, chairman and founder of DEC, maintains that "there is no reason why anyone would want a computer in their home." The first microprocessor, 'floppy' disks, and personal computers are all introduced. Integrated circuits are Semiconductor Revolution “One should not work on semiconductors, that is a filthy mess; who knows whether they really exist.” Wofgang Pauli 1931 Transistor invention: 1947 John Bardeen, Walter Brattain, and William Shockley AT&T Bell Labs Nobel Prize in Physics 1956 energy Semiconductor or dielectric metal Conduction band Band gap Valence band The coolest thing about semiconductors: they can be doped Even tiny fraction of impurities (0.00001) dramatically increases the ability to conduct current (conductivity) Energy diagram of doped semiconductors Free electrons are supplied to the empty conduction band Holes are left in the full valence band Both electrons and holes can now conduct current! P-n junction P-type neutral N-type _ + neutral Forward bias _ + Reverse bias _ + Field-effect transistor _ + _ + Voltage on gate controls the current of electrons from source to drain. FET can be made very small using nanotechnology! Commons.wikimedia.org Current, Ohm’s Law, Etc. dQ i dt V Ohm ' s Law : R ; R Const (independent of V ) i l R A j E E j The Continuity Equation for Steady State Currents Currents and current densities are constant in time – steady state. The flux of j out of any closed surface must be zero. j dS 0 Drift velocity j v nq For steady state situation j d S 0 E dr 0 1.Kirchhoff’s junction rule: The algebraic sum of the currents into any junction is zero. 2.Kirchhoff’s loop rule: The algebraic sum of the potential differences in any loop must be zero. Resistors in series: R R1 R2 Resistors in parallel: 1 1 1 R R1 R2 Circuit with capacitors Joule’s Law 2 V P Vi i R R 2 Why does a light bulb burn out when you switch the light on and never when you turn it off? The amount of current i drawn by a given device is determined by its power input P P Vi i V The current in a 100-W light bulb is P 100 W i 0.83 A V 120 V V 120 V R 144 i 0.83 A A kitchen circuit An 1800-W toaster, a 1.3-kW electric frying pan, and a 100-W lamp are plugged into the same 20-A, 120-V circuit. a) What current is drawn by each device, and what is the resistance of each device? b) Will this combination blow the fuse? To prevent the fuse from blowing, you replace the fuse with one rated at 40 A. Is this a reasonable thing to do? Pi R 2 Wheatstone bridge Charles Wheatstone British scientist and inventor 1802-1875 Samuel Hunter Christie, British scientist and mathematician Wheatstone English concertina Electromotive force The Magnetic Field The force F on a charge q moving with a velocity F q( E v B) v The magnitude of the force F qvB sin [ B] Newtons /(Coulomb meter / sec) 1T (tesla) 1w / m 1Newton / C m / s 10 gauss 2 4 4 BEarth 1Gauss 10 T left-hand rule right-hand rule