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Post IGCSE Physics Course Experimental Physics usingData Loggers and Computers14 Capacitor Electromagnetism .

Dr Andrew FrenchP5 6 Winchester CollegeLast updated April May 2017 Experimental setup discharge.

Fixed switchDC power supplyresistors forCapacitor Fixed 2 x 47 000 F resistors for.

wired in parallel i e so total dischargecapacitance of 94 000 F Two capacitors of capacitance 47 000 F wired in paralleli e a total capacitance of 94 000 F Measuring the resistance of the.

fixed resistors using a multimeter Charging a capacitor using a DC source1 Switch closed Current flows throughresistor and positive charge builds upon right capacitor plate An equal amount of.

negative charge builds up on left plate 2 Electrical field set up between capacitorplates as no current can flow Voltage V betweenthe plates is V Q C where Q is the total chargedeposited and C is the capacitance charge per unit.

3 As charge builds up on right plate potentialdifference between capacitor and source reduces This reduces the current flowing onto the plate VEventually the voltage V becomes V0 and hence nomore current can flow .

Dielectric Breakdown field3 Note the amount of charge which can be deposited strength Vm 1depends on the resulting electrical field strength Air 3 x 106between the plates Above the breakdown field Mineral oil 15 x 106strength current will flow between the plates.

Neoprene 16 x 106Water 65 x 106Mica 118 x 106 Charging a capacitor using a DC source 1 t V dV V dVdt .

RC 0 0 V V 0 V VQ CV V V IR I ln V V 0Ohm s law Definition ofcurrent t V V voltage ln V V ln V ln .

relationship RC V I C V V V R dt e RCR C V V 1 e RC.

Discharging a capacitor 1 t V dVdQ RC 0 V V 0Q CV V IR I t ln V VOhm s law Definition of RC 0.

current andrelationshipnegative since t V charge is ln V dV discharged RC V0 .

I C from platesV V0 e RCNote V V0 when t 0C R V V0 e RCSo RC is a characteristic time for.

charging or discharging a capacitor During charging phase MATLAB keytime m program is run and a button on thekeyboard is pressed when a fixed voltage is reached 1 2 3 4 5 6 7 8 9 5Vare an appropriate set keytime m stores the times of button presses to high precision It is essentially.

a stopwatch with a split time facility Once charging is complete pressing q and then return will stop the timerand export the timings into an Excel sheet The process is repeated for the discharging phase Charging and discharging is then repeated for the range of fixed resistors .

From the recorded data both the capacitance and the internal resistance of thecircuit can then be calculated DischargeV V0 e RCln V ln V0 .

Obtain RC from gradientof lnV vs t graphNote V0 is oftenCharge not zero .

V V0 V V0 1 e RC V V0 V0 V0 e RC V V e RC V V0 V Vt V V .

ln 0 RC V V Obtain RC from gradientof ln vs t graph Significant timing errors .

Might be worth repeatingCalculate capacitance from0 0941R 8 533 R R0 Cgradient of resistance vs RCtime graph and internal.

C 94 100 Fresistance R0 from intercept 8 533and gradient R0 90 7 Measuring the resistance of the fixed resistors using a multimeter Charging a capacitor using a DC source Switch 1. Switch closed. Current flows through resistor and positive charge builds up on right capacitor plate. An equal amount of negative charge builds up on left plate. 2. Electrical field set up between capacitor plates as no current ...

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