大学物理下课件27-Circuits

1、大学物理下课件大学物理下课件27-Circuits27-CircuitsWhat Is A Circuit?An electric circuits is a combinations of resistors, capacitors, and batteries which are connected by wires.emfemf ( electromotive force) : a mechanism to pump charges, or to do work on the charge carriers to maintain a potential difference betwe

2、en a pair of terminals dqdW :emfExample of emfBattery: a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energyE l e c t r i c g e n e r a t o r : a d e v i c e t h a t converts mechanical energy to electrical energySolar cell: an electrical

3、 device that converts the energy of light directly into electricity by the photovoltaic effectFuel cell : a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent.Calculating the Current in a Single-Loop CircuitTo calc

4、ulate the current in energy method:idtdqdW :charge on the doneWork The work done on the charge equals to the thermal energy appear in the resistor:Rdtiidt2iRRiTo calculate the current in potential method:Loop rule: the algebraic sum of changes in potential encountered in a complete traversal of any

5、loop of a circuit must be zeroComplete loop with clockwise from point a:aaViRViRRiOther Single-Loop Circuit Internal resistance0iRirRriOther Single-Loop Circuit Resistance in series0321iRiRiR321RRRi0eqiReqRi31321jjeqRRRRRFor complete loop:For equivalent resistance Req:Potential Difference between Tw

6、o PointsbaVirVirVVabRrirRRVVab?:ProblemabVVV12,0 . 4,0 . 2:ExampleRrV0 . 8abVVPotential, Power, and emfPotential difference across a real battery:irVGrounding a circuit means that the potential is defined to be zero at the grounding point in the circuit.Power, potential and emf:remfPPriiiriiVP2riPiP

7、remf2, whereV80baVVV80abVVMultiloop Circuit312iiiKirchhoffs junction rule: The sum of the currents entering any junction must be equal to the sum of the currents leaving that junctionBasic tools for solving complex circuits - loop rule: based on the conservation of energy - junction rule: based on t

8、he conservation of charge0 :b fromdirection ck countercloin loop hand-Left33111RiRi0 :b fromdirection ck countercloin loop hand-Right22332RiRi0 :b fromdirection ck countercloin badcb loop Big222111RiRiAt junction b or d:Sample Problem - Multiloop circuitProblem: R1 = R2, 1 = 2. Find the currents i1,

9、 i2, and i3.033111RiRiSolution:0222111RiRiJunction d: 312iii132iiBig loop badcb in counterclock direction from b: Left-hand loop in counterclock direction from b: 31112RRi311122RRii31113222RRii12iiResistance in Parallel CircuitWhen a potential difference V is applied across resistances connected in

10、parallel, the resistances all have that same potential difference V.Resistances connected in parallel can be replaced with an equivalent resistance Req that has the same potential difference V and the same total current i as the actual resistanceseqRVVRRRRVRVRViiii3213213211113132111111jjeqRRRRRSeri

11、es and Parallel Resistors and CapacitorsResistanceCapacitanceSeriesParallelSeriesParallelSame current through all resistorsSame potential difference across all resistorsSame charge on all capacitorsSame potential difference across all capacitorsnjjeqRR1njjeqRR111njjeqCC111njjeqCC1Sample Problem - Ma

12、ny real batteries in series and in parallel in an electric fishProblem: Electric fish are able to generate current with biological cells called electro plaques (血小板）血小板）, which are physiological emf devices. The electro plaques in the type of electric fish known as a South American eel （鳗鱼）（鳗鱼）are a

13、rranged in 140 rows, each row stretching horizontally along the body and each containing 5000 electro plaques. The arrangement is suggested in Figure; each electro plaque has an emf of 0.15 V and an internal resistance r of 0.25 . The water surrounding the eel completes a circuit between the two end

14、s of the electro plaque array, one end at the animals head and the other near its tail. (a) If the water surrounding the eel has resistance Rw = 800, how much current can the eel produce in the water? (b) How much current irow travels through each row?Sample Problem - Many real batteries in series a

15、nd in parallel in an electric fishSolution:V750V15. 050005000rrow25. 0V,15. 0rr125025. 050005000rRrow(a)rowjroweqRRR114011140193. 81401250140roweqRRA93. 080093. 8V750weqrowRRi(b)rowjrowiii1401401A106 . 61403iirowSample Problem - Multiloop circuit and simultaneous loop equationsProblem: The figure sh

16、ows a circuit whose elements have the following values: 1=3.0V, 2=6.0V, R1=2.0, R2=4.0. The three batteries are ideal batteries. Find the magnitude and direction of the current in each of the three branches.Solution:Junction rule: 213iiiLeft-hand loop: start at point b, and arbitrarily traverse the

17、loop in the clockwise direction, 022311111RiRiRiRight-hand loop: start at point b, and arbitrarily traverse the loop in the counterclockwise direction, 022312212RiRiRiA25. 0,A25. 0,A50. 021321iiiiiThe Ammeter and the VoltmeterAmmeter : An instrument used to measure currents- The resistance RA of the

18、 ammeter must be very much smaller than other resistances in the circuit. Otherwise, the very presence of the meter will change the current to be measured.- Series connectedVoltmeter : A meter used to measure potential differences- The resistance RV of a voltmeter must be very much larger than the r

19、esistance of any circuit element across which the voltmeter is connected. Otherwise, the meter itself becomes an important circuit element and alters the potential difference that is to be measured.- Parallel connectedRC Circuit Charging a CapacitorCharging a capacitor: close the switch S on point a

20、 RC series circuit0 :clockwiseCViR0,CqdtdqRCqVdtdqiCRCteCqqt10, 0RCteRdtdqiThe exponential base e = 2.718 63. 0 :constant timeCapacitive CqRCRC Circuit Discharging a CapacitorDischarging a capacitor: close the switch S on point b RC series circuit00CViR0,CqdtdqRCqVdtdqiCRCteqqCVqt000, 0RCteRCqdtdqi0

21、Sample Problem - Discharging an RCcircuit to avoid a fire in a race car pit stopProblem: As a car rolls along pavement, electrons move from the pavement first onto the tires and then onto the car body. The car stores this excess charge and the associated electric potential energy as if the car body

22、were one plate of a capacitor and the pavement were the other plate (Figure). When the car stops, it discharges its excess charge and energy through the tires, just as a capacitor can discharge through a resistor. If a conducting object comes within a few centimeters of the car before the car is discharged, the remaining energy can be suddenly transferred to a spark between the car and the object. Suppose the cond