## Unit 8

1. In your book, Hewitt claims that the mass of about 2000 electrons would equal the mass of 1 proton. Use the values of mass given above to determine if Hewitt is correct.

The ratio of mass of one proton to the mass of one electron should give us the answer:

The actual number is 1836, but Hewitt is close with his estimate of 2000.

2. Determine the charge of 1000 electrons.

-1.6 x 10-16 Coulombs

3. Determine the charge of 1 X 109 protons.

(1x109)(1.6x10-19 Coulombs)=1.6 x 10-10 Coulombs

4. Determine the number of electrons in one coulomb.

6.24x1018 electrons

5. Determine the charge of one mole of hydrogen nuclei in coulombs.

(6.02x1023)(1.6x10-19 coulombs)=1.6 coulombs

6. A neutral glass rod is rubbed with a neutral silk cloth. The silk has a larger electron affinity and 500,000 electrons are transferred.

a. Determine the charge on the glass rod.

(500,000 electrons)(-1.6x10-19 coulombs)=-8.0x10-14 coulombs

b. Determine the charge on the silk.

(500,000 electrons)(-1.6x10-19 coulombs)(-1)=8.0x10-14 coulombs

c. Did any protons get transferred during this rubbing?

No. Protons are never transferred.

d. Describe this type of charging.

Conduction.

7. If two objects attract they are both charged. True or False. Explain.

False. Only one object needs to be charged because one charged object can polarize a neutral object which will cause an attraction.

8. A balloon is rubbed against Lichti's head again and held next to a negatively charged pith ball. Will the pith ball attract or repel? Explain.

Repel. When the balloon rubs against Lichti's head, it gains electron causing it to become negative. When two negatively charged objects are held next to one another, the repel.

A B C

9. Which letter above contains objects that will attract? A B C

B and C

Explain.

In B, the positive charge would pull the electrons in the neutral atom towards it. The negative charge caused by this would attract the two charges together.

In C, the both the negative and the positive charge would attract to the other charge because the negative has excess electrons and the positive needs electrons.

10. Which letter above contains objects that will repel? A B C

A

Explain.

The two negative charges both have excess electrons and since electrons repel one another, the two charges will repel.

11. Determine the force that would exist between two 1 Coulomb charges separated by 1 meter of distance. Determine this force in Newtons and pounds.

To do this we will use coulombs law.

12. Determine the force between two 5 micro-coulomb charges separated by 3 cm.

13. If the force between two charges is 10 N and the charge of one charge is tripled while the distance between the two charges is doubled, what force now exists?

Pluggin these numbers into coulombs law and dividing by the orginal equation should give us the answer. However, since everything else cancels. We are left with the ratio:

After solving for that ratio, multiplying by the original force will give us the final force. So .5x10=5 N. So 5 N is the answer.

14. By what factor will the force between two charges change if the distance between those charges is cut in half while both charges are doubled?

The same idea is used with different numbers plugged in.

So 80 N is the answer.

15. Refer to Challenge #5 above to answer this question. In challenge #5 you determined the number of coulombs in one mole of hydrogen nuclei. If you placed one mole of hydrogen protons (nuclei) 1600 meters (1 mile) away from another mole of hydrogen protons (nuclei), determine the force with which these two moles repel.

First two neutral cans are positioned side by side touching one another.

Secondly, a negatively charged balloon will be brought near can 1.

Thirdly, the two cans are separated using their insulating bases while keeping the negative balloon near can 1.

Lastly, we will leave the cans separated and remove the balloon.

16. Indicate the charge distribution for each can in Step 2 by placing a (+) on the can with excess positive charge, and a (-) on the can with excess negative charge.

The left can is negative and the right can is positive.

17. If 1 X 105 electrons are transferred, determine the charge in coulombs for can 1 and can 2.

Can 1 1.6x10-14 Can 2 ____-1.6x10-14______

18. Indicate the charge distribution for each can in Steps 3 and 4 by placing a (+) on the can with excess positive charge, and a (-) on the can with excess negative charge.

Can 2 should be positive and can 1 should be negative in both steps.

_a.__19. When a neutral conductor is touched by a negative conductor it:

a. gains electrons and becomes negative.

b. looses protons and becomes negative.

c. creates its own negative charge using the energy from the negative conductor's presence.

Use the following situation where a positively charge electrophorous aluminum plate is touched to a neutral can for challenges 20 - 22.

20. When the can is touched by the positively charged aluminum plate, explain if electrons or protons flow and in which direction electrons or protons will flow.

Yes, electrons will flow from the can to the plate since they are both conductiors and allow electrons to flow freely.

21. After the can and aluminum plate touch will they tend to attract or repel?

They will repel since the can lost some of its electrons causing it to become positive and the plate was already positive.

22. After the can is touched by the aluminum plate it is touched by a charge tester as shown below. Which side of the charge tester lights up?

Explain.

The left side of the charge tester will light up. This is because the electrons are flowing from the hand to the can and since the hand is near the left side, that side will light up.

23. As charge flows through the neon-bulb charge tester, the can is grounded or neutralized. Are these charges that flow positive protons, negative electrons or both? Explain.

The only charges that ever flow are electrons. Since the can is positive, it needs electrons to become neutral and therefore the electrons flow from the hand to the can.

24. Although no charge is shown on the bar above the overall neutral electroscope, it has a negative charge.

Explain.

Since the top part of the electroscope is positive, even though it is negative overall, something must have pushed the electrons down to the bottom. Since like charges repel, bar must have the same charge as electrons, which is negative.

25. If the neutral electroscope above is touched by the charged bar, what will be the charge of the electroscope? Explain

The electroscope will become negative. Electrons will flow from the bar making the top of the electroscope neutral. However, when the bar is taken away, the electrons that were pushed to the bottom will come back up to the top causing there to be excess electrons and making the electroscope negative overall.

Glenbrook South Physics Team - Tom Henderson, John Lewis, Neil Schmidgall, Dave Smith, Suzanne Webb & Brian Wegley
4000 West Lake Ave
Glenview, IL 60025 - 1200
(847)729-2000
Page Maintained by: Brian Wegley

Last Updated: October 8, 1997