physics lab report

Electrostatic Charging Equipment list

Qty Items Parts Numbers

1 Charge Sensor CI-6555 1 Charge Producers and Proof Planes ES-9057A 1 Faraday Ice Pail ES-9024A

Introduction

The purpose of this activity is to investigate the nature of charging an object by contact as compared to

charging an object by induction. You will also determine the polarity of two charge ‘producers’ and

measure the amount of charge on each.

Background

Electric charge is one of the fundamental properties of matter.

Electrostatics is the study of electric charges and their

characteristics. For example, like charges tend to repel and unlike

charges attract. An object is electrically neutral most of the time;

that is, it has a balance of positive and negative electric charges.

The positive charges (+) come from the proton, while the

negative charges (-) are a result from the electrons. Rubbing

different materials together, contact with a charged object, and

charging by induction are the three ways to create an imbalance

of electric charge – sometimes called static electricity. Static

electricity is a charge and the unit of charge is the coulomb with

its SI symbol, q. Any positive or negative charge, q, that can be

detected can be written as q = ne where n = ±1, ±2, ±3, … in

which e, the elementary charge, has the value of 1.602 x 10-

19C. Frequently in experiments milliCoulombs (mC),

microCoulombs (μC), nanoCoulombs (nC) and even picoCoulombs (pC) are used.

As mentioned above, opposite charges always attract and like charges tend to repel. At an elemental level,

like charges always repel (electrons repel electrons, protons repel protons), but for macroscopic objects,

non-symmetric charge distribution can result in an overall attraction between two objects that carry the

same type of overall charge (positive or negative). Non-symmetrical charge distribution always results in

an attraction between a charged object and an electrically neutral (overall) object. Looking at the three

types of charging mentioned earlier, we can look at how the charges are distributed in each case:

1. Charging by rubbing: when two initially neutral non-conducting objects are rubbed together, one of

them will generally bind electrons more strongly than the other and take electrons from the other. The law

of conservation of charge requires that the total amount of electrons be conserved. That is, electrons only

move from one object to another, but no new electrons are created, nor do they disappear. Overall, the

two objects when considered together still have zero net charge.

2. Charging by contact: when a charged object is touched to a neutral (or less charged) object, repulsive

forces between the like charges result in some of the charge transferring to the less charged object so the

like charges will be further apart. This effect is much larger for conducting objects.

3. Charging by induction: the protons and electrons inside any object respond to electric forces of

attraction or repulsion. When an object is placed near a charged object, the charged object will exert

opposite forces on the protons and the electrons inside the other object, forcing them to move apart from

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each other. One side of the object will become more positive than it was initially. The other side will

become more negative, as electrons migrate internally. This condition is called polarization, a word that

refers to the object having “poles,” or opposite sides with different electrical states, even though the

object as a whole may still neutral. If a conductor is touched to the polarized object, some of the charge

will transfer to the conductor. If the conductor is then removed, the object now carries a net charge

different from its initial charge.

Historically, Michael Faraday used a metal ice pail as a conducting object to study how charges

distributed themselves with a charged object was brought inside the pail. The ‘ice pail’ had a lid with a

small opening through which he lowered a positively-charged metal ball into the pail without touching it

to the pail. Negative charges in the pail moved to the inner surface of the pail leaving positive charges on

the outside.

If the charged ball touches the inside of the ice pail, electrons would flow into the ball exactly

neutralizing the ball. This would leave the pail with a net positive charge residing on the outer surface of

the pail.

To experimentally investigate electrostatics, some charge-detecting or measuring device is needed. A

common instrument for this purpose is the electroscope, a device with two thin gold leaves vertically

suspended from a common point. When a charged object is brought near the electroscope, the gold leaves

separate, roughly indicating the magnitude of the charge.

Although there are many different versions of the electroscope, all such instruments depend upon the

repulsion of like charges to produce an output or reading. Unfortunately, such devices are relatively

insensitive (large amounts of charge are needed to make the gold leaves separate), and the device does not

have a quantitative reading.

The Charge Sensor is an ‘electronic electroscope’. In addition to providing a quantitative measurement,

the Charge Sensor is more sensitive and indicates polarity directly. Assuming there is no residual charges

and no charge leakage in the experiment, this instrument should provide accurate results. Corrections to 64 Electrostatic Charging Lab

Setup 1. Connect the Charge Sensor to the 850 Interface to any Analog Input available as shown below.

2. Start PASCO Capstone in the computer, and set the “Gain Select Switch” on the Charge Sensor to 5X.

3. Set up hardware by going to the left side menu and selecting Hardware Setup. Make sure the 850 Interface is selected and add the Charge Sensor by clicking on the Add Sensor/Instrument button.

Make sure Charge Sensor is added to the proper Analog Input in the diagram.

4. Set the sample rate to 10 Hz in PASCO Capstone at the lower bottom menu.

5. Drag a graph and meter displays from the Displays menu located at the right side. Set the “y” and “x” axes by left clicking on <Select Measurements> button and select Charge (µC) for the y-axis

and Time (s) for the x-axis.

6. Connect the alligator clips of the sensor’s cable assembly to the inner (longer wire inside – red band) and outer baskets (shorter wire outside – black band) of the Faraday Ice Pail as shown in

the diagram below.

Preparing to Record Data

Before starting any experiment using

the ‘Faraday Ice Pail’, the pail must be

momentarily grounded. To ground the

pail, touch the inner pail and the shield

at the same time with the finger of one

hand as shown in the image to the left.

NOTE: You may need to redo the

grounding and/or zeroing of the Ice Pail

during the experiment. It is very easy to

transfer charge to the ice pail by

touching it or even getting too close to

it with a charged object. It may even

acquire a charge sitting on the table for

a while. To see how sensitive the system is, stick a finger down the axis of the inner cylinder (without

touching the cylinder.) Now rub your fingers through your hair, or on your shirt, or shuffle your shoes on

the floor and try sticking your finger back into the Ice Pail. See any difference? What happens if you

touch the Ice Pail? What’s the moral about where you put your hands during the experiment? Redo the

grounding of the Ice Pail immediately before each procedure.

Procedure Part 1: Determine the Polarity of the Charge Producers

7. Ground the ‘Ice Pail’ and press the ‘ZERO’ button on the Charge Sensor to discharge the sensor. Press Record in Capstone. Then insert, without touching the walls, the white charge producer into

the inner pail then stop recording. Record their values for Question 1. Save the graph. Then repeat

process for the dark charge producer, and then repeat process for both charge producers at the

same time.

8. Briskly rub the black and white Charge Producers together several times.

9. Click ‘Record’ in Capstone to start recording data.

a. Without touching the ‘Ice Pail’, lower the white Charge Producer as close to the bottom of the ‘Ice Pail’ as possible without touching the walls. Watch the Meter and Graph displays.

b. Remove the white Charge Producer and then lower the black Charge Producer into the ‘Ice Pail’. Watch the results. Remove the black Charge Producer. After a few moments, stop

recording data and save the graph.

Part 2: Measure the Charge on the White Charge Producer.

10. Ground the ‘Ice Pail’ and press the ‘ZERO’ button on the Charge Sensor to discharge the sensor.

11. Briskly rub the black and white surfaces of the Charge Producers together several times.

12. Start recording data. Record the zero value on line 1.

a. Lower the white Charge Producer into the bottom of the ‘Ice Pail’. Record this value on line 2.

b. Quickly rub the surface of the white Charge Producer against the inner pail Watch the Meter and Graph displays. Record this value on line 3.

c. Remove the white Charge Producer from the inner pail. Record this value on line 4.

13. After a few moments, stop recording data and save graph.

Measure the Charge on the Dark Charge Producer

14. Ground the ‘Ice Pail’ and press the ‘ZERO’ button on the Charge Sensor to discharge the sensor.

15. Briskly rub the black and white surfaces of the Charge Producers together several times.

16. Start recording data. Record the zero value on line 1.

a. Lower the black Charge Producer into the bottom of the ‘Ice Pail’. Record this value on line 2.

b. Quickly rub the surface of the black Charge Producer against the inner pail Watch the Meter and Graph displays. Record this value on line 3.

c. Remove the Charge Producer from the inner pail. Record this value on line 4.

17. After a few moments, stop recording data and save graph.

Part 3: Charge the ‘Ice Pail’ by Induction

18. Ground the ‘Ice Pail’ and press the ‘ZERO’ button on the Charge Sensor to discharge the sensor.

19. Briskly rub the black and white surfaces of the Charge Producers together several times.

20. Start recording data. Record the zero value on line 1.

a. Without touching the ‘Ice Pail’ with the Charge Producer, lower the white Charge Producer into the bottom of the ‘Ice Pail’. Record this value on line 2.

b. While the white Charge Producer is still inside the inner pail, use the finger of one hand to momentarily ground the ‘Ice Pail’. Watch the results, the charge should go to approximately zero.

Record this value on line 3.

c. After you ground the ‘Ice Pail’, remove your hand and then remove the white Charge Producer.

21. After a few moments, record this value on line 4 and stop recording data. Save graph.

22. Ground the ‘Ice Pail’ and zero the sensor and repeat the procedure using the black Charge Producer and save graph.

Lab Report: Electrostatic Charging Name: ________________________________________________________________ Graphs: Indicate on the graphs each step of the experiment, i.e. charge producers ‘far’ away from experiment;

charge producer inducing current, pail grounded, charge producer in contact with pail, etc. You can

annotate directly on the graph by using the large red A: on the graph display, or physically write on the

graphs once printed out.

Analysis Questions 1. From Part 1, what charge and polarity (what sign) are the two Charge Producers? Where did the

electrons go (or come from)?

2. (Part 1) After the rubbing process, what is the overall net charge of the two wands?

3. (Part 1) What evidence is there that the rubbing gave each wand an electrical charge?

4. (Part 2) After charging by contact, what was the charge of the white charge producer and the basket? Was it

the same polarity?

5. (Part 2) After charging by contact, what was the charge of the dark charge producer and the basket? Was it

the same polarity?

6. (Part 3) After charging by induction, what was the charge of the white charge producer and the basket? Was

it the same polarity?

7. (Part 3) After charging by induction, what was the charge of the dark charge producer and the basket? Was

it the same polarity?

8. How does the process of charging by contact differ from the process of charging by induction?

9. How many electrons are inside of the pail if the pail has a charge of 50 μC?

10. Assuming you have 6.24 x 1014 electrons and the surface area of the pail is 0.2 m2, what is the charge

density (C/m2)?

11. Previously experiments were conducted where two pith balls were used to look at static electricity.

Looking at one pith ball as an example, shown below, show the mathematical proof that in this equation

the sin can be replaced with a tan. (Hint: Small angle approximation.)

𝐹𝑐 = 𝑚𝑔 sin 𝛽 ≅ 𝑚𝑔 tan𝛽 = 𝑚𝑔𝑥

𝑙