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Basic Electrostatics System

Model No. ES-9080

32

!

1.

Calculate 63% of the voltage of the source. Locate the position in the

graph where the voltage has reached this value. How long a time has
passed to reach 63% of the voltage of the source? This time is

RC

.

(Using the Smart Tool in Data Studio or the Smart Cursor Tool in

ScienceWorkshop

£

makes these measurements easy!)

2.

Compare the measured time constant from the graph with the

calculated from the known values of and

R

. Now, when a fully

charged capacitor is discharged through a resistor, the voltage
across (and the charge on) the capacitor decreases with time

according to the equation

. After a time

t = RC

(one

time constant), the voltage across the capacitor decreases to 37% its
maximum value.

3.

Determine how much is 37% of the total voltage and locate where

in the discharging plot this value has been reached. How long a
time since the start of discharging did it take to reach this value?
(Use the smart cursor tools!)

4.

Compare this measured

RC

constant with the known value.

Procedure 4B: Charging/Discharging Capacitors with Signal

Generator

When a positive square wave signal is applied to a capacitor in an RC
circuit, the capacitor periodically charges and discharges, as shown in
Figure 4.2. The period of a full charge-discharge equals the period of
the wave.

Note: The procedure listed here specifies values for R, C and the frequency of
the signal that work well together. If you decide to use any other R or C
value, you have to adjust the frequency of the wave. Notice that the voltage
has to remain constant for enough time to fully charge the capacitor before
the voltage goes to zero and the capacitor is discharged. A good estimate of
the time needed to fully charge a capacitor can be determined as t = RC[lnV

o

V

V

0

e

l RC

e

=

&'

&

Figure 4.2: Charging and Discharging with a Square Wave Signal

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Содержание ES-9080A

Страница 1: ...Instruction Manual Manual No 012 07227D Basic Electrostatics System Model No ES 9080A ...

Страница 2: ...ane 8 9 Faraday Ice Pail 10 Conductive Spheres 11 Resistor Capacitor Network Accessory 11 Electrometer Operation and Setup Requirements 12 13 Suggested Demonstrations 14 34 Demonstration 1 Faraday Ice Pail and Charge Production 14 17 Demonstration 2 Charge Distribution 18 20 Demonstration 3 Capacitance and Dielectrics 21 29 Demonstration 4 Charging and Discharging Capacitors 30 34 Technical Suppor...

Страница 3: ...tics Voltage Source 1 ES 9077 3 Basic Variable Capacitor 1 ES 9079 4 Charge Producers 2 and proof plane 1 ES 9075B 5 Faraday Ice Pail and Shield 1 ES 9042A 6 Conductive Spheres 2 ES 9059B Additional Equipment Required Any PASCO data acquisition device ScienceWorkshop 500 750 SCSI or 750 USB interface CI 6400 or CI 6450 or CI 7599 Resistor Capacitor Network ES 9053A A computer NA DataStudio or Scie...

Страница 4: ...includes a diagram of the suggested equipment setup A preferred setup would also use a computer with a ScienceWorkshop interface to display the readings from the Electrometer ES 9078 in a computer screen that all can easily see You can use an analog display for example to show the deflections of the needle or a digits display to show the voltage If a computer is not available set the demonstration...

Страница 5: ...oof plane See the Equipment Description section Residual charge may build up in the plastic insulator between the handle and disk of the proof plane and charge producers Make sure to ground these parts before any experiment The capacitance of the electrometer must be considered when calculating the magnitude of a charge from the voltage reading of the electrometer See Demonstration 3 for the proce...

Страница 6: ...ly designed exclusively for experiments in electrostatics It has outputs at 30 volts DC for capacitor plate experiments and 1 kV 2 kV and 3 kV outputs for the Faraday ice pail and conducting sphere experiments All of the voltage outputs except for the 30 volt output have a series resistance associated with them which limit the available short circuit output current to about 8 3 microamps The 30 vo...

Страница 7: ...g they will cease to be parallel and adjustments must be made On the back side of the fixed plate is a second smaller plate with three set screws By adjusting these three screws the two plates may be kept parallel Keep the clear acrylic plate supports clean to prevent charge leakage from the plates Charge Producers and Proof Plane ES 9057A The Charge Producers and the Proof Plane are electrostatic...

Страница 8: ...y on the conductive proof plane disk The charge producers are designed to be used with the ES 9078 Electrometer They do not produce sufficient charge for use with a standard electroscope The Proof Plane As shown in Figure 5 the proof plane is an aluminum covered conductive disk attached to an insulated handle The conductive disk material is carbon filled black polycarbonate about 103 with an alumi...

Страница 9: ... plane becomes part of the conductive surface If the effect on the shape of the surface is significant the sampling of the charge density will not be accurate Therefore always touch the proof plane to the conductor in such a way as to minimize the distortion of the shape of the surface Figure 7 shows the recommended method for using the proof plane to sample charge on a conductive sphere in sample...

Страница 10: ...0 cm in diameter and 15 cm high When a charged object is placed inside the pail but without touching it a charge of the same magnitude is induced on the outside of the pail See Figure 9 An electrometer connected between the pail and the shield will detect a potential difference The greater the charge the greater the potential difference So even though the electrometer will give readings of voltage...

Страница 11: ...e wired into a switching network which permits the components to be connected in various configurations The RC Network is designed for the study of RC time constants and for investigating capacitors connected in series and parallel the RC time constants range from 25 to 200 seconds A third capacitor 0 22 PF can be selected in series or parallel with the other two capacitors On the front of the RC ...

Страница 12: ...tor of a Wimshurst machine 3 Never touch the input leads until you have grounded yourself to an earth ground A person walking across a rug on a cool dry day can easily acquire a potential of several thousand volts Setup 1 Before turning on the electrometer check to ensure that the meter reads zero If not turn the Mechanical Zero Adjust screw located just below the electrometer face until it does 2...

Страница 13: ...There may still be stray charges within the electrometer circuitry 3 For good results it is essential that the electrometer be connected to an earth ground a water pipe or the ground wire from a 120 VAC socket Only an earth ground provides a sufficient drain for excess charges that may build up during an experiment It is also helpful if the experimenter is grounded Just touch one hand to a good ea...

Страница 14: ...he leads to show how the meter needle deflects in the opposite direction Explain how this can tell us the type of charge in the ice pail Equipment Setup Introduction The purpose of this demonstration is to investigate the relation between the charge induced on the ice pail by a charged object placed in the pail and the charge of the object This demonstration is also useful for investigating the na...

Страница 15: ...Performing tests or experiments with an ungrounded pail could cause possible electrical shock or injury Procedure 1A Charging by Induction vs Charging by Contact 1 Connect the electrometer to the Faraday Ice Pail as shown in Figure 1 1 Make sure to ground the electrometer and the ice pail The electrometer should read zero when grounded indicating there is no charge in the ice pail Press the Zero b...

Страница 16: ...ing If the handle never touched the pail the reading must be zero Question Why was there a potential difference between the pail and the shield only while the charged object was inside 6 Push the Zero button to remove any residual charge Now insert the object again but let it touch the ice pail Make sure your students know and see that you are touching the ice pail with the charged disk this time ...

Страница 17: ...dles 4 Insert both charge producers into the ice pail and rub them together inside the pail Note the electrometer reading Do not let the charge producers touch the pail 5 Remove one charge producer and note the electrometer reading Replace the charge producer and remove the other Note the electrometer readings Using the magnitude and polarity of the measurements comment on conservation of charge E...

Страница 18: ...rface the charge has the same magnitude and sign This occurs for uniform charge distribution An important aspect of measuring charge distributions is charge conservation The proof plane removes some charge from the surface it samples If the proof plane is grounded after each measurement the surface will be depleted of charge with consecutive measurements However by not grounding the proof plane an...

Страница 19: ... apart Connect one of the spheres to the Electrostatic Voltage Source ES 9077 providing 2000 VDC The voltage source is to be grounded to the same earth ground as the shield and the electrometer The connected sphere will be used as a charging body 3 Momentarily ground the other sphere to remove any residual charge from it 4 Start the demonstration by sampling and recording the charge at several dif...

Страница 20: ... the charge density in the inner and outer surfaces Notice that charge is always on the outside 2 To show how the surface shape affects charge density try touching two charged proof planes together so that they are symmetrical around their point of contact Measure the charge on each Next touch them in an asymmetrical manner and measure the charge in each Does one have more charge than the other Wh...

Страница 21: ...this demonstration For all experiments the electrometer can be thought of as an infinite impedance voltmeter in parallel with a capacitor as shown in Figure 3 1 The capacitor CE represents the internal capacitance of the electrometer plus the capacitance of the leads Whenever you want quantitative measurements of charge voltage or capacitance you need to consider the effect of the internal capacit...

Страница 22: ...r CE The total capacitance becomes C CE The known capacitor will discharge across the electrometer and a voltage VE will be read Since the total charge in the system is still just the charge of the known capacitor we know that CV C CE VE 1 Obtain a low leakage polypropylene or air dielectric capacitor of known value C around 30 pF 2 Charge the capacitor with a known voltage V not higher than 100 V...

Страница 23: ...he spheres is connected to the voltage source set at 2000 VDC Take care to place the capacitor sufficiently far away from the sphere and the voltage source to prevent it from being charged by induction 2 Press the ZERO button to remove any residual charge from the electrometer and the plates of the capacitor 3 Set the plate separation to about 2 mm Use a proof plane to transfer charge from the cha...

Страница 24: ...gate the charge density at various points on the plates both on the inner and the outer surfaces How does the charge density vary over the plate 3 Choose a point near the center of one capacitor plate and measure charge density in this area at different plate separations Keep in mind whether you are increasing or decreasing the capacitance by moving the plates How does the charge vary with capacit...

Страница 25: ...ations How does the potential vary with capacitance NOTE An alternative method is to charge one of the spheres and then transfer some charge to the capacitor The charge however will not be as high Procedure 3C Dielectric Coefficients The dielectric coefficient is the dimensionless factor by which the capacitance increases relative to the value of capacitance before the dielectric when a dielectric...

Страница 26: ... them to about 4 5 full scale Record the voltage reading of the electrometer Vi 4 Carefully increase the separation of the plates until it is enough to insert the dielectric without forcing it It should be enough so that you can simply lean the dielectric sheet against the stationary plate Make sure the dielectric you are using is free of residual charge before inserting it 5 After inserting the d...

Страница 27: ...this initial system is given by After inserting the dielectric Let be the new charge on the capacitor plates the capacitance is now Let be the new charge on the internal capacitance of the electrometer Since there is no dielectric in its value is still the same Let be the new reading of the electrometer The total charge in the system after inserting the dielectric is given by Now the total amount ...

Страница 28: ...xamine the effect of placing capacitors in series and in parallel You will need two capacitors of known value between 200 400 µF to ignore the internal capacitance of the electrometer a DC voltage source the electrometer some cables and a double throw switch Material Vacuum 1 Air 1 00059 Polystyrene 2 6 Paper 3 7 Pyrex 4 7 Mica 5 4 Porcelain 6 5 Ccp Cp CE Vi V f CpVi CpVf Ccp Cp e N N HAd HOAd Ccp...

Страница 29: ...own values of capacitance determine the amount of charge in each of them Q1 and Q2 7 Questions Can you find a relation between V1 V2 and the voltage of the source How does Q1 and Q2 relate to the original charge on C1 3D 2 Capacitors in Parallel 1 Make sure all capacitors are uncharged before connecting them to the circuit 2 Set up the parallel circuit as shown in Figure 3 7b 3 Set the voltage sou...

Страница 30: ... the internal capacitance of the electrometer needs not be considered You can adjust the resistance value for a convenient RC constant There are two variations of the same activity presented here The first uses a direct DC voltage source and results are obtained in a voltage vs time graph Use higher resistances 10 90 k for this method The second uses a signal generator with a square wave to charge...

Страница 31: ... screen 5 When the charge in the capacitor has reached the value of the source 30 VDC flip the switch to position B to begin discharging the capacitor Observe the behavior of the voltage in the screen 6 You may want to try the experiment with different values of R and notice the differences in charging time Analysis When a capacitor is charged through a resistor from a DC power supply the charge o...

Страница 32: ...ing plot this value has been reached How long a time since the start of discharging did it take to reach this value Use the smart cursor tools 4 Compare this measured RC constant with the known value Procedure 4B Charging Discharging Capacitors with Signal Generator When a positive square wave signal is applied to a capacitor in an RC circuit the capacitor periodically charges and discharges as sh...

Страница 33: ... DataStudio or ScienceWorkshop software create a display of voltage vs time for the readings of the electrometer 3 Set the signal generator to produce a positive square wave of maximum around 4 volts and of frequency 0 45 Hz Set the signal generator to AUTO In this way the signal will turn on and off as you press Start or Stop to collect data 4 Start recording data Observe the behavior of the volt...

Страница 34: ...Basic Electrostatics System Model No ES 9080 34 Extra Things to Try 1 Check what is happening to the voltage across the resistor while the capacitor charges and discharges ...

Страница 35: ...nship for a period of one year from the date of shipment to the customer PASCO will repair or replace at its option any part of the product which is deemed to be defective in material or workmanship The warranty does not cover damage to the product caused by abuse or improper use Determination of whether a product failure is the result of a manufacturing defect or improper use by the customer shal...

Страница 36: ...upport For assistance with the Basic Electrostatics Systems equipment or any other PASCO products contact PASCO as follows Address PASCO scientific 10101 Foothills Blvd Roseville CA 95747 7100 Phone 916 786 3800 FAX 916 786 3292 Web www pasco com Email techsupp pasco com ...

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