Physics 42 Lab: Static Electricity

Charge can neither be created nor destroyed: it can only be moved around! The purpose of the lab is to play with static electricity by moving charge around. You will charge objects by friction, induction and polarization and determine the charge with an electroscope. You will draw cute pictures showing each step and how the charges distribute to result in a final charge. The following cartoon sketch is an example of showing how a metallic sphere is charged by induction using a ground. This is the type of sketch you will draw today for each activity, describing what is happening for each step and summarizing your results.

I will grade your lab based on neatness and clear explanations!

Sample: Charging a metal sphere by induction using a negatively charged rod (a) A neutral metallic sphere, with equal numbers of positive and negative charges. (b) The electrons on the neutral sphere are redistributed when a charged negatively charge rubber rod is placed near the sphere. (c) When the sphere is grounded, some of its electrons leave through the ground wire. (d) When the ground connection is removed while leaving the rod close to the sphere, the sphere has excess positive charge that is nonuniformly distributed. (e) When the rod is removed, the remaining electrons redistribute uniformly and there is a net uniform distribution of positive charge on the sphere. Summary: Charging a neutral metal sphere by induction using a negative rod results in a positive charged sphere! **************************************************************************************** PARTS LIST: 1 Electroscope, 2 Conducting Spheres, Electrometer and cage with cables, wrist grounding wire, grounding wire, proof plane, assorted rods and fabrics *****************************************************************************************

WARNING: GROUND EVERTHING INCLUDING YOURSELF TO REMOVE EXCESS CHARGE AFTER EVERY PROCEDURE!!!!

Part I Creating Charge by Friction: Here you will rub different rods made of different materials with various fabrics to transfer charge and then use the proof plane and electrometer to determine the net charge on the rod. Put your results in the data table.

1. Attach the wire bucket to the electrometer as shown by your instructor. 2. Ground yourself and the electrometer. 3. Zero the electrometer. Set the Voltage to 100 V. Be careful not to peg the needle on the electrometer! 4. Grab one of the rods (glass, nylon,…) and rub it with one of the materials (silk, fur, satin, cotton, ….). 5. Dip the rod into the wire bucket (make sure not to let the rod touch it) and note the sign of the charge

you have created on the rod. Record your findings in Table 1 of your data sheet. 6. Repeat the above procedure with different rod-material combinations to complete Table 1.

Part II Charging by Polarization. Using two metal spheres and charged rods, you will move charge by polarization and determine the charge on the sides of the spheres using the proof plane and the electroscope. Sketch and explain each step as shown above.

1. Ground yourself and the electrometer. Set the Voltage on the electrometer to 3V (most sensitive). 2. Be careful not to peg the needle on the electrometer! Don’t bring the rod close to the bucket! 3. Ground both spheres temporarily to remove all charges from both spheres. 4. Choose a rod-material combination that can produces negative charge on the rod (see your Table 1). 5. Charge one sphere to its maximum capacity (rub the rod and touch the sphere with it several times). 6. Confirm the fact that you have negative charge on sphere using the proof plane and electrometer. 7. Ground yourself and the second sphere far away from first sphere. 8. Bring the second sphere next to the first sphere without touching it, leaving just enough space between

to fit the proof plane in between the spheres. 9. Using the proof plane, remove some charge from the side of the second sphere facing the fist sphere

and identify the polarity of that charge. 10. Zero the electrometer and discharge the proof plane. Repeat this step and identify the charge on side B

(side farthest away from sphere 1). Record your findings. 11. Create sketches with brief explanations for each step as done above. 12. Repeat for a positively charged rod.

Part III Charge by Induction. You will charge a metal sphere by induction using a ground (your finger) and determine the charge on the sphere using the proof plane and the electroscope. Sketch and explain each step as shown above.

1. Ground your self, the electrometer and the spheres to remove any charges. 2. Choose a rod-material combination that produces a negative charge on the rod. 3. Carry out the procedure as shown in the sample above. Verify the charge of the sphere with the proof

plane and electrometer and write a brief statement to this effect. You do NOT have to sketch this because it is already done for you above.

4. Ground your self, the electrometer and the spheres to remove any charges. 5. Choose a rod-material combination that produces a positive charge on the rod and repeat the charging

by induction, but this time for a positively charged rod. 6. Create sketches for the positive rod with brief explanations for each step as done above.

Part IV Charging an Electroscope. You will charge an electroscope using both contact and induction and determine the charge on the electroscope using the proof plane and the electroscope. Sketch and explain each step as shown above.

1. Ground your self, the electrometer and the electroscope to remove any charges.

2. Charge the electroscope by contact with a positive rod as shown in the image above. Verify the charge with the proof plane and electrometer. Write a brief sentence as to whether you did in fact verify this procedure. You do NOT have to sketch this because it is already done for you above.

3. Repeat for a negatively charged rod and verify the charges. Sketch and explain each step. What is the charge on top ball and on the gold leafs?

4. Charge the electroscope using induction with a ground using a negatively charged rod and create sketches with brief explanations for each step. What is the charge on top ball and on the gold leafs?

Charging Summary Fill in the blanks for these questions on your data sheet. When charging a metal sphere with contact using a positive rod, the final charge is ________________. When charging a metal sphere with contact using a negative rod, the final charge is _______________. When charging a metal sphere with induction using a positive rod, the final charge is ______________. When charging a metal sphere with induction using a negative rod, the final charge is ______________. Charging by contact gives the same or opposite charge as the rod? _______________ Charging by induction gives the same or opposite charge as the rod? _______________ More Fun Determine the charge on the Van de Graff Generator. Explain how you did it. Part V. Electric Field Hockey. Go to : http://webphysics.davidson.edu/Course_Material/Py230L/estatics/electrostatics.htm The objective is to shoot the positively charged puck into the goal by placing charges that produce electric forces and fields. Conceptually apply the inverse square law and the rule that opposite charges attract and like repel to reason where the charges should be placed. You can change the sign and magnitude of the charges. Start with “PRACTICE” and then do the games. Print out successful games of game 1 ,2 & 3. Use PRINT SCREEN to capture your successes and paste them into a word document. Shrink them so they fit on one page. Include this in your lab report.

Have Fun!

EXTRA CHARGES