PHY-2049A closed circuitPower in DC CircuitThe figure below gives the electrical potential V(x) along a copper wire carrying a uniform current, from a point at higher potential (x=0m) to a point at a lower potential (x=3m). The wire has a radius of 2.45 mm. What is the current in the wire?We have all we need….Let’s add resistors …….Series CombinationsSlide 8Parallel Combination??What’s This???Slide 11Power Source in a CircuitA REAL Power Source is NOT an ideal batteryA Physical (Real) BatteryBack to which is brighter?Slide 16Back to PotentialConsider a “circuit”.To rememberNEW LAWS PASSED BY THIS SESSION OF THE FLORIDUH LEGISLATURE.TWO resistors againA single “real” resistor can be modeled as follows:We start at a point in the circuit and travel around until we get back to where we started.Take a trip around this circuit.Circuit ReductionMultiple BatteriesReductionAnother Reduction ExampleSTART by assuming a DIRECTION for each CurrentThe Unthinkable ….RC CircuitClose the SwitchReally Close the SwitchThis is a differential equation.Slide 35Time ConstantResult q=CE(1-e-t/RC)q=CE(1-e-t/RC) and i=(CE/RC) e-t/RCDischarging a CapacitorSlide 40If the resistance is doubled what is the power dissipated by the circuit?PHY-2049Current & CircuitsFebruary ‘08A closed circuitHot, Hot HotPower in DC Circuit REIVRIPRIIRIIVPPowerVItQVt222W:PowerQVW:isbattery by the done workofamount The battery. by theresistor the throughpushed is Q charge a t, In timeThe figure below gives the electrical potential V(x) along a copper wire carrying a uniform current, from a point at higher potential (x=0m) to a point at a lower potential (x=3m). The wire has a radius of 2.45 mm. What is the current in the wire?copper12 volts 0 voltsWhat does the graph tell us??*The length of the wire is 3 meters.*The potential difference across the wire is 12 volts.*The wire is uniform.Let’s get rid of the mm radius and convert it to area in square meters:A=r2 = 3.14159 x 2.452 x 10-6 m2orA=1.9 x 10-5 m 2Material is Copper so resistivity is (from table) = 1.69 x 10-8 ohm metersWe have all we need….ma 49.41067.21012RVi:Law sOhm' From67.2 109.1 0.3m-ohm 1069.13658ohmsvoltsmxmxALRLet’s add resistors …….Series CombinationsR1 R2i iV1 V2ViiRseriesRgeneralRRRiRiRiRVVVandiRViRV)(:2121212211SERIES ResistorsThe rod in the figure is made of two materials. The figure is not drawn to scale. Each conductor has a square cross section 3.00 mm on a side. The first material has a resistivity of 4.00 × 10–3 Ω · m and is 25.0 cm long, while the second material has a resistivity of 6.00 × 10–3 Ω · m and is 40.0 cm long. What is the resistance between the ends of the rod?Parallel Combination??R1, I1R2, I2ViiRRgeneralRRRsoRVRVRViiiiRV11111..212121What’s This???In Fig. 28-39, find the equivalent resistance between points (a) F and H and [2.5] (b) F and G . [3.13](a) Find the equivalent resistance between points a and b in Figure P28.6. (b) A potential difference of 34.0 V is applied between points a and b. Calculate the current in each resistor.Power Source in a CircuitThe ideal battery does work on charges moving them (inside) from a lower potential to one that is V higher.A REAL Power Sourceis NOT an ideal batteryVE or Emf is an idealized device that does an amount of work E to move a unit charge from one side to another.By the way …. this is called a circuit!Internal ResistanceA Physical (Real) BatteryInternal ResistanceRrEmfiBack to which is brighter?Back to PotentialRepresents a charge in spaceChange in potential as one circuitsthis complete circuit is ZERO!Consider a “circuit”.This trip around the circuit is the same as a path through space.THE CHANGE IN POTENTIAL FROM “a” AROUND THE CIRCUIT AND BACK TO “a” is ZERO!!To rememberIn a real circuit, we can neglect the resistance of the wires compared to the resistors.We can therefore consider a wire in a circuit to be an equipotential – the change in potential over its length is slight compared to that in a resistorA resistor allows current to flow from a high potential to a lower potential.The energy needed to do this is supplied by the battery. VqW NEW LAWS PASSED BY THIS SESSION OF THE FLORIDUH LEGISLATURE.LOOP EQUATIONThe sum of the voltage drops (or rises) as one completely travels through a circuit loop is zero.Sometimes known as Kirchoff’s loop equation.NODE EQUATIONThe sum of the currents entering (or leaving) a node in a circuit is ZEROTWO resistors againiR1 R2V1 V2Vjj2121RRResistors SERIESfor GeneralRRRoriRiRiRVA single “real” resistor can be modeledas follows:RabVpositionADD ENOUGH RESISTORS, MAKING THEM SMALLERAND YOU MODEL A CONTINUOUS VOLTAGE DROP.We start at a point in the circuit and travel around until we get back to where we started.If the potential rises … well it is a rise.If it falls it is a fall OR a negative rise.We can traverse the circuit adding each rise or drop in potential.The sum of all the rises around the loop is zero. A drop is a negative rise.The sum of all the drops around a circuit is zero. A rise is a negative drop.Your choice … rises or drops. But you must remain consistent.Take a trip around this circuit.Consider voltage DROPS:-E +ir +iR = 0orE=ir + iRriseCircuit Reductioni=E/ReqMultiple BatteriesReductionComputes iAnother Reduction ExamplePARALLEL12121600503012011RRSTART by assuming a DIRECTION for each CurrentLet’s write the equations.The Unthinkable ….RC CircuitInitially, no current through the circuitClose switch at (a) and current begins to flow until the capacitor is fully charged.If capacitor is charged and switch is switched to (b) discharge will follow.Close the SwitchI need to use E for ENote RC = (Volts/Amp)(Coul/Volt) = Coul/(Coul/sec) = (1/sec)Really Close the SwitchI need to use E for ERERCqdtdqorECqdtdqRCqiREdtdqi since0Equation LoopNote RC = (Volts/Amp)(Coul/Volt) = Coul/(Coul/sec) = (1/sec)This is a differential equation.To solve we need what is called a particular solution as well as a general solution.We often do this by creative
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