Electric Charge and Pith Ball Lab Recap

1. Electric Charge and Pith Ball Lab Recap Here are the main concepts you should know from lab and class—week 1

2. Here are the main concepts you should have from lab 16.1: • Electricity is not created or destroyed, it moves from object to object • When an object has an equal number of protons and electrons it is neutral (not charged)

3. Physics and Chem Unite • different types of material have a likelihood of gaining or loosing electrons to acquire a more positive or negative charge depending on what type of material it is rubbed with-

4. Physics and Chem Unite • -think back to chemistry • certain materials on the periodic table are more likely to gain or lose electrons depending on where they are located.  • This concept holds true for electrostatics as well.

5. when an object gains or loses electrons when introduced or rubbed with a second material an object becomes charged (becomes an ion) • an object that loses electrons to another object becomes more positive

6. an object that tends to gain electrons becomes more negative • when object with the same charge are brought together they repel

7. when objects with different charges are brought together they share electricity and attract-they try to maintain equilibirium • when objects are rubbed together, the electricity moves from one object to the other and both become charged

8. when one object that is charged is touched with an object that is not charged, electric charge is exchanged and the objects might end up with no charge • there is a tendency for electrical fluid to even out so that all objects have natural amounts

9. Pith Ball Concept Sum Up • materials that hold most loosely to electrons---most likely to gain a positive charge when rubbed • metals, glass, wool, fur, silk, cotton (cotton is special)

10. when cotton is rubbed with rubber it becomes positively charged, when rubbed with glass it becomes negatively charged (transitional)--changes what charge it induces or picks up based on what it is rubbed with

11. materials that hold tightly to electrons--most like to gain electrons from material rubbed creating a negative charge • wood, cork, rubber

12. Conductors: silver copper gold aluminum iron steel brass bronze mercury graphite dirty water concrete

13. Insulators: glass rubber oil asphalt fiberglass porcelain ceramic quartz (dry) cotton (dry) paper (dry) wood plastic air diamond pure water

14. It must be understood that not all conductive materials have the same level of conductivity, and not all insulators are equally resistant to electron motion.

15. REVIEW: • In conductive materials, the outer electrons in each atom can easily come or go, and are called free electrons. • In insulating materials, the outer electrons are not so free to move. • All metals are electrically conductive.

16. Dynamic electricity, or electric current, is the uniform motion of electrons through a conductor. Static electricity is an unmoving, accumulated charge formed by either an excess or deficiency of electrons in an object. • For electrons to flow continuously (indefinitely) through a conductor, there must be a complete, unbroken path for them to move both into and out of that conductor.

17. Pith Ball • A pith-ball electroscope, consists of a small ball of some lightweight nonconductive substance, originally pith, suspended by a silk thread from the hook of an insulated stand. • In order to test the presence and magnitude of a charge on an object, the object is brought near to the uncharged pith ball. If the object is charged, the pith ball will be attracted to it.

18. Polarization • This attraction occurs because of induced polarization of the atoms in the pith ball. The pith is a nonconductor, so the electrons are not free to leave their atoms and move about the ball, but they can move a little within the atoms.

19. If, for example, a positively charged object is brought near the ball, the negative electrons in each atom will be attracted and move slightly toward the side of the atom nearer the object.

20. The positively charged nuclei will move slightly away. Since the negative charges are now nearer the object than the positive charges, their attraction is greater than the repulsion of the positive charges, resulting in a net attractive force.

21. This separation of charge is microscopic, but since there are so many atoms, the tiny forces add up to a large enough force to move a light pith ball.

22. The pith ball can be charged by touching it to a charged object. Then the ball can be used to distinguish the polarity of charge on other objects, because it will be repelled by objects charged with the same polarity or sign it has, but attracted to charges of the opposite polarity.

23. Often the electroscope will have a pair of suspended pith balls. This allows one to tell at a glance whether the pith balls are charged. If one of the pith balls is touched to a charged object, charging it, the second one will be attracted and touch it, communicating some of the charge.

24. Now both balls have the same polarity charge, so they repel each other, and hang in an inverted 'V' shape with the balls spread apart. The distance between the balls will give a rough idea of the magnitude of the charge.

25. Electroscope-demo in class notes • When the metal disc is touched with a conductive charged object the gold leaves spread apart in a 'V'. • This is because the charge on the object is conducted through the disc and metal rod to the leaves. • Since they receive the same sign charge they repel each other and thus diverge.

26. If the metal rod is touched with a finger, the charge is transferred through the human body into the earth and the leaves of electroscope close together

27. If a charged object is brought near the electroscope terminal, without touching it, the leaves also diverge, because charges in the disk with identical polarity to the charged object are repelled to the leaves.

28. If the electroscope is touched, and hence grounded, in this condition, the leaves close, but spread again if the charged object is moved away.

29. The electroscope becomes charged with polarity opposite to the polarity of the charge in the object. The