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Transcript
Electric Current: Charges in Motion • Lightning was found to be an electric discharge- a gigantic spark but until 1800 not much usefulness • Allesandro Volta (1745 – 1827) invented the electric battery- from this discovery a new era opened, Today's electrical technology is based on electric current Volta disproves Galvani’s animal electricity and invents the 1st battery • Scientists recognized that batteries produce electricity by transforming chemical energy into electrical • electrodes- 2 dissimilar metals (one can be carbon). These are immersed in a solution known as the… • Electrolyte: today’s dry cell – this is a powdery paste • Battery refers to multiple cells Continued… • Terminals: two electrodes that stick out of the electrolyte • A chemical reaction occurs that causes electrons to flow from the negative terminal to the positive. When the chemical reaction stops, we say the battery is “dead” Electric Current • A circuit is produced when a continuous path connects the terminals of a battery + - SIGN FOR BATTERY Image 18-6 Pg. 530 Formula I = ΔQ Δt Current (amperes)= Charge(coulombs) Time (seconds) Sample Problems • A steady current of 2.5 amps flows in a wire for 4 minutes. a) How much charge passed through a point in the circuit? b) How many electrons would this be? Answer! A) ΔQ = I Δt 2.5C/s (240sec) = 600C B) Charge of 1 electron is 1.6x10-19C so 600C would be… 600/ 1.6 x 10-19C = 3.8 x 1021 electrons Current Flow • Electrons actually flow from negative terminal to positive; when we are speaking of conventional current, that is from positive to negative. In liquids and gasses, both positive and negative charges (ions) can move. OHMS LAW • Current is proportional to voltage • water analogy: – 2 pipes Which one will let the most water flow?? OHMS LAW • Resistance is opposite to the flow of electricity • R OHMS (Ω) • I = V/R • Current (amps)= Voltage (volts) Resistance (ohms) • V = IR Sample Problem: • A small flashlight bulb draws 300mA from its 1.5 volt battery a) What is the resistance of the bulb? b)If the voltage dropped to 1.2V, how much would the current change? ANSWER! A) R = V/I 1.5V/ .3A = 5Ω B) If resistance stays constant I = V/R 1.2V/ 5Ω .24A a drop of 60mA All Appliances Have Resistance • Hair dryer, TV’s, Lightbulbs, electric motors, ect. • In many electronic devices resistors are used to control current. They can be less than 1 ohm to millions of ohms • Symbol for resistance: Pg. 534 Pg. 534 Pg. 534 Resistivity • Resistance can change for a substance Low High Thick Cross Section Thin Short Length Long Chart pg. 535 R = P(L/A) R = resistance (Ω) L = length (m) A = cross section area (m2) P = Resistivity (coefficient for substance) Temperature Affects Resistivity • Resistance increases with temperature • At super low temperatures, resistance becomes zero- super conducting Electric Power • Electric energy is useful because it can easily be converted into other forms of energy. • Motors transform electricity into mechanical work, heaters, stoves, toasters; all transform it to thermal energy, light bulbs convert it to heat and light! Formulas P = QV/t OR P = IV Power (watts)=Current (Voltage) BY SUBSTITUTION P= IV = I (IR) = I2R =(V/R)V = V2/R Sample • Calculate the resistance of a 40W headlight designed for a 12V automobile = R= 2 V /P 2 (12V) / 40W = 3.6Ω Sample • An electric heater draws 15A on a 120V line. How much power does it use and how much does it cost per month (30 days) if it operates 3 hours per day and the electric company charges 10.5 cents per kWh? P = IV = 15A(120V) = 1800Watts 1.8 kW 30 days (3hr/ day) = 90hrs 1.8 kW (90hrs)($.105)= $17 Sample • A typical lighting bolt can transfer 109J of energy across a potential difference of 5x107 Volts during a time interval .2 seconds. Calculate charge transferred, current, average power over the time 9J interval. 9 I = Q/t P = 10 Q = 10 J 5x107V = 20C 20C .2sec =100A .2sec = 5x109W 5 Giga Watts Power in Household Circuits • SAFETY: – Thickness in wires – Fuses – Circuit Breakers – Shorts – Fires – GFCI outlet near a water source in your home Will a Fuse Blow? Find the total current drawn by all the devices Sum of Power = 100 + 1800+ 350 + 1200 Voltage = 120V I = P/V 3450W/ 120= 28.8A this would blow a 20 amp fuse! AC DC • Batteries move electrons in one direction DC • Electric generators at power plants produce alternating current AC • This type changes the direction of electron flow many times per second (In US 60Hz – In Europe 50Hz) • The AC voltage is sinusoidal Electrons Moving in a Wire • When we flip a light switch- there's light! • Do electrons move fast in a wire? • - NO it only appears that way Add Water Over Flow Water Electron drift speed is .05mm/sec or 5½ hrs to go 1m CHAPT. 19 DC Circuits • Electric circuits are everywhere- cars, homes, schools, businesses, radios, TVs and computers Basic Symbols Battery Capacitor Resistor Wire with negligible resistance Switch Resistance in Series and Parallel Series: one path Parallel: multiple paths Series Circuit • Rtot=R1+R2+R3+… • The Current through each resistance is the same: I tot=I1=I2=… • The Voltage across each resistance splits up proportionally among the R’s. Vtot=V1+V2+V3… Parallel Circuit • Current from the source splits into separate branches Wiring in businesses and houses are like this- that way if one bulb burns out, the rest stay on! Parallel Circuit Current splits up among the resistance so they add… Itot = I1 + I2 + I3 • Voltage is the same for each resistor Vtot=V1=V2=V3 • But I = V/R • SO resistors in parallel add this way: 1/Rtot = 1/R1 + 1/R2 + 1/R3 Example • Two 8Ω resistors in parallel and have a Rtot of 4Ω 1/Rtot = 1/8Ω + 1/8Ω = 2/8Ω = 1/4Ω Rtot = 4Ω Water analogy Dual water pipe Water pipes in parallel- analogy to electric currents in parallel Example 19-1 EMF • Electro Motive Force (not a true force- not measured in Newtons) • This is the potential difference across the terminals of a battery (generator) when NO current flows into an external circuit. • Battery Voltage isn't constant (Start your car, headlights dim) Battery can’t supply current fast enough to maintain the full EMF EMF • Batteries have their own internal resistance designated r • Terminal Voltage is what we measure across the terminals of a battery when no circuit flows Terminal Voltage = Vab = EMF - Ir Kirchoff’s Rules • Deals with complicated circuits • Junction Rule (conservation of charge): At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction. • Loop Rule (conservation of energy): The sum of the changes in potential around any closed path of a circuit must be zero. Electric Hazards • The severity of electric shock depends on the magnitude of the current. Also how long it acts and through what part of the body it passes. • Heart and Brain are most critical • Electric current heats up tissue and causes burns Electric Hazards • The severity of shock depends on the effective resistance of a body. Living tissue has quite low resistance (cells contain ions) and is a good conductor. However, the outer layer of skin, when dry, offers a high resistance 104 to 106 ohms. • When the skin is wet, resistance can be 103 Ohms or less, this allows for more current to flow and a more severe shock Grounding A) An electric oven operating normally with 2 prong plug B) Short to the case with ungrounded case: shock! C) Short to the case with the case grounded with Third prong. Circuit Breakers Birds and squirrels don’t get shocked?