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FOWLER CHAPTER 2 LECTURE 3 ELECTRICAL QUANTITIES AND UNITS CHAPTER 2 ELECTRICAL QUANTITIES AND UNITS P-17 COULOMB: BASE UNIT OF CHARGE= CHARGE SYMBOL =Q ABBREVIATION =C P-22 AMPERE 6.25 1018 ELECTRONS CURRENT= Q/t CHARGE/TIME 1A 1C s ONE COULOMB/SEC OR 6.25 1018 ELECTRONS/ SEC ELECTRIC CURRENT (I): MOVEMENT OF CHARGED PARTICLES IN A SPECIFY DIRECTION CAN BE +OR- LIKE CHARGES REPEL,UNLIKE CHARGES ATTRACK. + CHARGE CAN BE + ION - CHARGE IS AN ELECTRON THESE CHARGES CAN MOVE THRU SOLIDS,LIQUIDS OR GASES. IN OUR MOST COMMON CONDUCTOR COPPER, ONLY ELECTRONS CAN MOVE. P-18 SYMBOL I WAS CHOSEN BECAUSE IT WAS THOUGH TO REPRESENT THE INTENSITY OF ELECTRICITY IN A WIRE. CURRENT MOVES AT LESS THAN THE SPEED OF LIGHT IN ANY WIRE. WHEN A ELECTRIC FIELD IS PLACED ACROSS A WIRE. SAY FROM A BATTERY , VALENCE ELCTRONS MOVE FROM ATOM TO ATOM, CREATING + AND- IONS WHICH UNDER THE INFLUENCE OF THE ELECTRIC FIELD ALLOWS CURRENT TO FLOW. Basic Electricity - What is an amp? http://www.youtube.com/watch?v=8gvJzrjwjds CURRENT FLOWING IN THE SAME DIRECTION ALL THE TIME IS KNOWN AS DIRECT CURRENT + VOLTAGE TIME 0 _ P-19 CURRENTS IN GASES IN GASES UNDER THE INFULENCE OF A STRONG ELECTRIC FIELD + AND – CHARGES ARE FORMED i.e. + IONS AND ELECTRONS. THIS IS CALLED IONIZATION. METAL PLATE METAL PLATE + + - - + + - - + - + + NEON FILLED TUBE IN A NEAR VACUUM. NEON ATOM HAS 8 VALENCE ELECTRONS IN ITS OUTER RING CURRENTS IN LIQUIDS P-19 AGAIN + AND – CHARGES SERVE AS CURRENT CARRIES. UNLIKE GASES,IN LIQUIDS BOTH + AND - CURRENT CARRIES ARE IONS. - ARE Cl IONS + ARE Na IONS METAL PLATES + + + - + + + - - NaCl SOLUTION - + A LIQUID THAT CAN CARRY CURRENT IS CALLED AN ELECTROLYTE (e.g. SEA WATER,SULFURIC ACID IN CAR BATTERIES.) P-21 CURRENTS IN VACUUM IN THIS CASE AN ELECTRON TUBE IS USED AS AN EXAMPLE. + CATHODES ARE COATED WITH THORIUM,BARIUM OR STRONTIUM + - THERMIONIC EMISSION: EMITTION OF ELECTRONS FROM A HEATED PLATE. AS THE PLATE IS HEATED IT EMITS ELECTRONS WHICH ARE DRAWN TO THE POSITIVE PLATE BY THE ELECTRIC FIELD CREATED BY THE BATTERY. ON A SIMPLE SCALE, THIS IS HOW A CRT WORKS. ELECTRON MOVEMENT IN A TUBE FILAMENT CLOSE UP VIEW OF A TUBE FILAMENT P-23 VOLTAGE=EMF(ELCTROMOTIVE FORCE)=POTENTIAL DIFFERENCE (All mean the same thing) KINETIC VS. POTENTIAL ENERGY KINETIC: ENERGY OF MOTION OF AN OBJECT. POTENTIAL ENERGY: STORED ENERGY OF AN OBJECT AT REST. STATIC ELECTRICITY IS POTENTIAL ENGERY (PE) STATIC SPARK CONVERTS PE OF THIS SPARK INTO HEAT AND LIGHT IN ELECTRICITY, THE PE IS DUE TO THE ELECTRIC FIELD AND THE CHARGE SEPARATION BETWEEN TWO OBJECTS. YOU TUBE:A cell phone and static electricity: Bill Nye The Science Guy on Static Electricity: www.youtube.com/watch?v=gct1BmKNvU0 www.youtube.com/watch?v=Z-77IzaXGcg A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved in modern Van de Graaff generators can reach 5 megavolts. ELECTROSTATIC ATTRACTION BETWEEN 2 CHARGED SPHERE’S electric flux animation http://www.youtube.com/watch?feature=fvwp&v=THybl_QGssw&NR=1 http://phet.colorado.edu/en/simulation/balloons TAKE ELECTROSTATIC PRECAUTIONS WHEN WORKING ON ANY ELECTRONIC EQUIPTMENT P-24 ELECTRIC FIELD LINES SPHERE WITH EXCESS POSITIVE CHARGES - SPHERE WITH EXCESS NEGATIVE CHARGES AS ELECTRON’S MOVE ACROSS THE ELECTRIC FIELD WORK IS DONE, ENERGY IS LOSS MOVING FROM – TO + THE PE IS DUE TO THE ELECTRIC FIELD AND ELECTRIC CHARGE BETWEEN TWO OBJECTS. THIS FIELD CREATES A POTENTIAL DIFFERENCE SHOW MU29 1300:1630 http://www.youtube.com/watch?v=PeBPPudNpAw P-25 UNIT OF VOLTAGE (V) VOLT WE NEED A WAY TO INDICATE THE P.E. DIFFERENT BETWEEN THE TERMINALS OF OUR BATTERY. ENERGY IS TRANSFORMED BY CHARGES(ELECTRICAL) ACROSS THE BATTERY TERMINAL. SINCE THE BASE UNIT OF ENERGY IS THE JOULE AND THE BASE UNIT OF CHARGE IS THE COULOMB; WE CAN DEFINE VOLTAGE AS JOULE/COULOMB V=JOULE/COULOMB V= ENERGY/CHARGE= W/Q OR W=VQ W(WORK OR ENERGY)= VQ EXAMPLE: D CELL BATTERIES ARE RATED AT 1.5V A FLASHLIGHT WITH 2D CELLS = 3 V WHAT DOES THIS MEAN? WHEN THE FLASHLIGHT IS ON , WITH A POTENTIAL DIFFERENT OF 3V V = J/C 3 JOULES OF ENERGY IS CARRIED BY 1 COULOMB OF CHARGE, WHICH ARE CONVERTED TO HEAT AND LIGHT. BATTERY IS DISCHARGING P-26 BATTERIES BATTERY IS CHARGING + ־ ־ DIRECTION OF ELECTRON FLOW + ELCTRON FLOW IS REVERSED THERE IS A P.E. DIFFERENT BETWEEN THE + AND – TERMINALS OF THE BATTERY. WHEN CURRENT FLOWS WORK IS DONE. P. E. IS CONVERTED INTO ANOTHER FORM OF ENERGY, IN THIS CASE AS HEAT IN THE RESISTOR. P-25 POLARITY: HELPS TO INDICATE WHICH DIRECTION CURRENT IS FLOWING. CAN BE + OR - WHEN TERMINALS ARE IDENTIFIED ON A ELECTRONIC DEVICE, IT IS SAID TO BE POLARIZED. POLARITY MUST BE CORRECT FOR THE DEVICE TO WORK. IF THE DEVICE IS CONNECTED BACKWARD, IT IS SAID TO BE IN A STATE OF REVERSE POLARITY AND NO CURRENT WILL FLOW. The Mechanical Universe - 31 – Voltage, Energy and Force 6:00-8:00,12:15-14:00 http://www.youtube.com/watch?v=jnWc_6PqJ6I GEOTHERMAL REMEMBER: ONE FORM OF ENERGY MUST BE CONVERTED TO ANOTHER TO CREATE VOLTAGE.EXAMPLE:POWER PLANT GENERATORS SOURCES OF VOLTAGE COAL CONVERT MECHANICAL ENERGY INTO ELECTRICAL ENERGY NUCLEAR SOLAR HYDROELECTRIC P-26 OTHER WAYS TO GENERATE ELECTRICITY CRYSTALS (VIA THE PIEZOELECTRIC EFFECT) BY VARYING PRESSURE APPLIED TO CRYSTALS CAN CREATE ELECTRICITY. USES:PHONOGRAPH PICKUPS, MICROPHONES IN MIRCOPHONES, SOUND ENERGY IS CONVERED TO MECHANICAL ENERGY, WHICH IS APPIELD TO A CYRSTAL TO CREATE ELECTRICITY THERMOCOUPLE(SEEBECK EFFECT): THE DIFFERENT IN TEMPERTURE BETWEEN TWO MATERIALS(USUALLY METALS OR SEMICONDUCTORS) CAN BE CONVERTED INTO ELECTRICITY. YOU TUBE: Thermocouples http://www.youtube.com/watch?v=gtO0kQ-PT_0 P-26 RESISTANCE: OPPOSITION TO CURRENT FLOW IN A MATERIAL. ELECTRICAL ENERGY IS CONVERTED INTO HEAT. CONDUCTORS: OFFER LITTLE RESISTANCE TO CURRENT FLOW. WHY? DO TO THE EASE AS WHICH VALANCE ELECTRONS CAN BE RELEASED FROM THE OUTER SHELL OF AN ATOM. GOOD CONDUCTORS: SILVER,GOLD,ALUMINIUM,COPPER. BAD CONDUCTORS: RUBBER,WOOD,CERAMIC MATERIAL, GLASS INSULATORS HAVE HIGH RESISTANCE TO CURRENT FLOW. A BAD CONDUCTOR IS A GOOD INSULATOR. MOST INSULATORS ARE COMPOUNDS OF TWO OR MORE ELEMENTS. THEYSHARE ELECTRONS BY COVALENT BONDING.(MAKES IT HARDER FOR ELECTRONS TO BE FREED.) GLASS INSULATORS METAL CONDUCTORS UNIT OF RESISTANCE IS THE OHM Ω 1Ω AMOUNT OF RESISTANCE IN A MATERIAL WITH 1 VOLT APPLIED WITH A CURRENT FLOW OF 1 AMP. 4 FACTORS THAT EFFECT RESISTANCE 1. TYPE OF MATERIAL 2. LENGTH OF OBJECT 3. CROSS SECTIONAL AREA 4. TEMPERATURE OF OBJECT R L A http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html AS TEMPERATURE. INCREASES SO DOES THE RESISTANCE TEMP. COEFFICENT THE CHANGE IN RESISTANCE OF A MATERIAL AS THE TEMPERATURE INCREASES OR DECREASES. MOST MATERIALS HAVE + TEMP. COEFF. i.e. AS TEMP ↑ SO DOES THE RESISTANCE. SOME HAVE – TEMP. COEFF. (EXAMPLE CARBON) TEMP COEFFICENT IS EXPRESSED AS THE CHANGE OF RESISTANCE(IN OHMS) PER PPM (PART PER MILLION) /C° RESISTIVITY:THE RESISTANCE OF A MATERIAL AS DEFINED BY A CERTAIN SIZE OF THE MATERIAL. CAN BE 1cm³, 1m³, 1ft³ THE RESISTIVITY UNIT IS THE Ω·cm. SEE APPENDIX E P.479 THE LOWER THE RESISTIVITY THE BETTER CONDUCTOR IT IS. R L A RESISTIVITY x LENGTH/CROSS SECTIONAL AREA RESISTORS: USED IN CIRCUITS TO CONTROL THE FLOW OF CURRENT. RANGE<1Ω TO>1 MILLION Ω’S RSISITANCE IN CIRCUIT WIRES IS SO SMALL THAT IT DOSEN’T AFFECT THE PERFORMANCE OF THE CIRCUIT. A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. How a Toaster Works https://www.youtube.com/watch?v=WYcw_DcZsak Most heating elements use Nichrome 80/20 (80% nickel, 20% chromium) wire, ribbon, straight or coiled. Nichrome 80/20 is an ideal material, because it has relatively high resistance and forms an adherent layer of chromium oxide when it is heated for the first time. Material beneath this layer will not oxidize, preventing the wire from breaking or burning out. Used in common items such as toasters and hair dryers, furnaces for industrial heating, floor heating, roof heating, pathway heating to melt snow, dryers, etc. Knife into toaster FAIL https://www.youtube.com/watch?v=So7I70Z0uCU P-30 POWER/ENERGY: IS ABILITY TO DO WORK, A GAGE OF HOW ENERGY IS CONVERTED FROM ONE FORM TO ANOTHER. POWER CAN ALSO BE THOUGHT OF AS THE RATE OF USING ENERGY OR WORK. HOW DO WE MEASURE THIS IN A PRACTICAL WAY? LIKE THIS: HOW MUCH ENERGY IS BEGING USED IN A GIVEN TIME. UNIT OF POWER IS THE WATT (W) WATT = 1 JOULE/ SECOND = 1 J/S POWER (P) =W/t = ENERGY/ TIME WATTSECOND = 1 J/ S·S =1 J JOULE IS A TINY AMOUNT OF ENERGY. POWER COMPANIES USES KILOWATT-HOUR INSTEAD OF WATTSECOND OLD ANALOG POWER METER NEW DIGITAL POWER METER