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ELECTROSTATICS

THE ELECTROMETER AND ELECTROSTATICS:

In the lecture, the topic of electrostatics has been introduced. In the laboratory, if one wishes to investigate the nature of electrostatics, a charge detecting device is needed. Traditionally an electroscope, an instrument with two gold leaves suspended from a common point which separate upon certain conditions is used. This device is simple, inexpensive, but only gives qualitative results. Quantitative results can only roughly be estimated by the amount the gold leaves spread. The electrometer is an expensive instrument (which does many things besides) that indicates the qualitative and quantitative results needed to investigate electrostatics thoroughly. In addition to indicating that there is some charge on a body, it actually tells what kind of charge (+ or -) and the amount. It is nearly 1000 times more sensitive than a standard gold-leaf electroscope and can measure charges as low as 10-11 coulombs.

As just mentioned, the electrometer is very expensive and is very sensitive. It can be damaged easily if certain precautions are not followed. Examine the electrometer. You can GROUND it by attaching a wire to the gas pipe at the lab station. This should be done for you already. If not, please check with your instructor before continuing. More instructions pertaining to the electrometer will be given in the PROCEDURES section of the handout.

In investigating electrostatics you will use the "FARADAY ICE PAIL". It works on the principle that a charge placed inside a conducting surface will induce an equal charge on the outside of that surface. For example, if we suspended a charged sphere inside a closed canister a charge equal to that on the sphere would appear on the outside of the can. Charge is still conserved, but the charge has migrated over the surface of the can to produce differing distributions of charge. For example, if one inserted a + charged sphere in the can, electrons or -charged particles would be attracted to the inside of the can leaving a positive charge on the outside of the can. The actual distribution has changed, but not necessarily the total quantity of charge. This allows you to measure charge by INDUCTION.

The "ice-pail" is a wire mesh about 8 cm in diameter mounted on an insulated support. To eliminate the problem of stray charges a wire mesh shield is placed around the pail. It will be grounded and should prevent stray charges from producing erroneous readings.

OBJECTIVES:

1. To investigate the nature of charging an object by contact as compared to charging by induction.
   
   
2. To investigate variations of charge densities.

APPARATUS:

• 1 Electrometer with cables
• 1 "ice pail" with shield
• 1 white faced charge producer
• 1 blue faced charge producer
• 1 "proof plane"

PROCEDURE:

Measuring electrostatic charge by induction

1. First set the Electrometer as follows:
   
   
   1. Connect the GROUND wire to a pipe at the lab table.
   2. Set the ON/OFF switch to OFF.
   3. Set the Meter switch (left side) to CTR 0.
   4. Set the Range switch to 100.
   5. Connect the BNC coaxial cable with two clips to the INPUT.
   
   
   
2. Connect the Red wire to the "pail" and the Black wire to the shield as shown below.
   
   

   Figure 1: Electrical Corrections to Faraday's Ice Pail

3. The two Charge Producers and the Proof Plane are similar. The charge producers have a white and blue face respectively and the proof plane is plain aluminum. Ground the charge producers by touching them to the pipe. The "pail" and shield will be grounded when you momentarily depress the Zero button to CHECK position.
   
   
4. Turn the Electrometer ON and Zero the needle. If you have trouble, ask your instructor for help. You are ready to measure charge!
   
   
5. Rub the two charge producers together and insert one of them into the ice pail, but DO NOT allow the wand to TOUCH the pail. Record the electrometer reading (Magnitude and Direction.) A charge is INDUCED on the inside pail that is equal but opposite to that on the charge producer. Keep this is mind as you make all measurements. The charge on the wand is proportional to the voltage reading of the electrometer. The exact values must be calculated through capacitance equations. That is not necessary at this point. The relative magnitudes and polarity are all that is needed in this experiment.
   
   
6. Remove the object and record the electrometer reading.

Measuring charge by contact

7. Reinsert the object, TOUCH it to the pail, and now remove it. Record the electrometer reading. Explain this phenomena in terms of how the induction process works.
   
   
8. Momentarily ground the ice-pail (hit the CHECK switch) and touch the object to the ice pail and note the electrometer reading. Does any charge remain on the object?
   
   
9. What can you conclude about the induced charge on the ice pail as compared to the charge on the object?
   
   
10. GROUND the charge producers.
    
    
11. Insert the charge producers into the pail and rub them together inside the pail. Note the reading.
    
    
12. Remove one charge producer and note the reading.
    
    
13. Replace it and remove the other. Note the reading.
    
    
14. GROUND the charge producers again. Repeat steps 11-13 with the white charge producer and the aluminum proof plane.
    
    
15. Do the same with the blue charge producer and the aluminum proof plane.
    
    
16. Construct a list of these 3 materials such that if a material lower on the list is rubbed with a material higher on the list, the higher material is always positive. This list is called an Electrostatic Series, i.e. order these 3 materials. You are determining which kind of material is more or less receptive (and in what direction) to the transfer of electrons.

ELECTROSTATIC INDUCTION

Investigating the variations of charge density (or charge distributions) involves sampling the charge on a surface with a proof plane, inserting it into the Faraday Ice Pail, and observing the relative charges recorded on the meter. Not all surfaces have a uniform charge density on them.

It is wise to touch the spheres with the flat side of the proof plane to get an accurate indication of the charge distribution. It is also recommended that the proof plane not be grounded between samplings or the charge will be depleted. In this case the charge will be conserved.

1. Place the 2 aluminum spheres perhaps 50 cm apart. Connect one to the GREEN terminal of the power supply and set the power supply to 1000 VDC. Connect the ground terminal of the power supply to ground. The sphere at 1000 VDC will provide a charging body.
   
   
2. The second sphere should be momentarily grounded to remove all charges. This is done by touching it to the GROUND pipe. Sample and record the charge density at several points on this (second) sphere. Give some thought as to what areas should be sampled.
   
   
3. Place the 1000 VDC sphere about 1 cm from the second sphere. Again, sample and record the charge distribution as before.
   
   
4. GROUND the second sphere.
   
   
5. Move the 1000 VDC sphere as in step 1 above, say 50 cm from the second sphere. Sample and record as before.
   
   
6. What produced the charge distributions at each step of the experiment? Did any charge remain on the second sphere even after it was grounded? Why?

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