Static Electricity Review

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Static Electricity Review

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Answers to Questions: All || #1-16 || #17-32 || #33-43

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Part A: Multiple Choice

1. Which of the following are true of static charges? Choose all that apply.

Like charges repel.
Like charges attract.
Opposite charges repel.
Opposite charges attract.
A positively charged object has lost electrons.
A positively charged object has gained protons.
A negatively charged object has lost protons.
A negatively charged object has gained electrons.

Answer: ADEH

A and D are true; they are the simple statement of our essential charge interactions, best remembered by the jingle: "opposites attract, likes repel."

B and C are false for these same reasons; they violate the basic statement of charge interactions.

E and H are true. For an object to become charged, it must either gain or lose electrons. Losing electrons results in more positive charge than negative charge, making the object charged positively. Gaining electrons results in more negative charge than positive charge, making the object charged negatively. Protons are tightly bound in the nucleus of atoms and can never be added nor removed from atoms by ordinary electrostatic methods. The same reasoning leads one to reject choice F and G as possible true statements; the suggest that protons can be added or removed.

Useful Web Links

The Structure of Matter || Neutral vs. Charged Objects || Charge Interactions

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2. Identify the following objects as being either

Answer: See table above

i. Protons are positive; electrons are negative; more protons than electrons would mean an overall positive charge.

ii. Neutrons are neutral and will not have an influence on the overall charge. So this object could be +, -, or neutral depending on the relative number of protons and electrons.

iii. If a neutral object loses some electrons, then it will possess more protons (positive charge) than electrons (negative charge); the object will have an overall + charge.

iv. If a neutral object gains some electrons, then it will possess more electrons (negative charge) than protons (positive charge); the object will have an overall - charge.

v. A negatively charged object would never be attracted to a - balloon. However, a + object would be attracted to a - balloon (opposites attract) and a neutral object would be attracted to a - balloon (since neutral objects are attracted to any charged object).

vi. An object which attracts neutral paper bits must be charged. If the object also attracts a - balloon, then it must have a + charge (opposites attract).

vii. If an object has a repulsive interaction with any other object, then it is definitely charged. Since like charges repel, the charge which it has is the same type of charge as the charge on the object which it repels. This object has a - charge.

viii. A neutral object will be attracted to any charged object, whether + or -. In this case, the object attracts both + balloons and - balloons. Only a neutral object could do this.

ix. Balloon A is either + (since it attracts a - object) or neutral (since neutral objects will also attract any charged object). If our object attracts balloon A, then it is either + or - (since such objects would attract balloon A if it were neutral) or even neutral (since it would attract balloon A if balloon A were +).

x. Balloon C must be - since it repels another - balloon (like charges repel). If our object attracts balloon C, then it is either + (since opposites attracts) or neutral (since neutral objects are attracted to any charged object).

xi. Balloon E must be + if it repels balloon F (like charges repel). If our object repels balloon E, then it to must have a + charge.

xii. By definition, the electric field vector points in the direction which a + test charge would be accelerated. So by extension, the E vector points towards - charges and away from + charges. This object must have a - charge.

xiii. Using the same reasoning as above, this object must have a + charge.

Useful Web Links

The Structure of Matter || Neutral vs. Charged Objects || Charge Interactions

3. Consider the following statements given below and determine if the charge of Object A is:

a. positive

b. negative

c. neutral

Useful Web Links

Charging by Friction || Charging by Induction || Charging by Conduction

4. A neutral plastic strip is rubbed with cotton and acquires a positive charge. Which of the following statements are true of the positively-charged strip?

It lost some electrons to the cotton during the charging process.
It lost all of its electrons to the cotton during the charging process.
It has the opposite charge as the cotton.
It would now be repelled by the piece of cotton which was used to charge it.
It gained protons during the rubbing process.
As a material, plastic has a greater affinity for electrons than cotton.
It could exert either a repulsive or attractive influence upon neutral paper bits.
It has an excess of protons compared to the number of electrons.
It could be used to charge an electroscope negatively by the process of induction.
It lost negative electrons and gained positive electrons during the charging process.
It lost neutrons during the charging process (or at the very least, its neutrons became ineffective).

Answer: ACHI

a. During charging by friction, electrons are transferred from one object to the other object. The object which acquires a + charge is the object which loses the electrons.

b. While some electrons are lost, not all electrons lost.

c. Charging by friction results in two objects with the opposite type of charge.

d. If the two rubbed objects are brought near, then they will attract (rather than repel) since they are charged oppositely.

e. Protons are never gained or lost during ordinary electrostatic experiments. They are tightly bound in the nuclei of atoms; it would require an atom-smasher to induce protons to move.

f. The object which has the greater electron affinity is the one which acquires the - charge during the friction charging process.

g. A neutral object like paper could only attract a charged object like the plastic; repulsion is only observed of two charged objects having the same type of charge.

h. Since it has lost electrons, the plastic will have an excess of protons.

i. If a positively-charged object is used to charge an electroscope by induction, then the electroscope would acquire a - charge. Induction charging results in objects with opposite types of charge.

j. There is no such thing as positive electrons (at least not for our purposes).

k. As you may have noticed, talk of neutrons in a unit on electrostatics is largely absent. Neutrons have nothing to do with the discussion.

Useful Web Links

Charging by Friction || Charging by Induction || Charging by Conduction

5. A positively-charged glass rod is touched to the plate of a neutral electroscope. Upon contact, the electroscope becomes charged and the needle deflects. Which of the following statements are true of the charged electroscope?

The electroscope is now charged positively.
The electroscope and the glass rod now have the same type of charge.
The electroscope was charged by the method of induction.
The electroscope gained protons during the charging process.
The electroscope gained electrons during the charging process.
The electroscope lost all of its electrons during the charging process.
During the charging process, some electrons left the electroscope and entered the glass rod.
The number of electrons present in the electroscope is less than the number of protons.
The electroscope needle will deflect even more if the glass rod is brought near it again.
The electroscope needle would slowly approach the neutral position if a negatively-charged balloon is brought near.

Answer: ABGHIJ

a. Charging by contact places the same type of charge on the object being charged. So if a positively charged rod is used to charge the electroscope, the electroscope becomes charged positively.

b. Same as part a.

c. Induction charging would involve bringing the glass rod near to the electroscope without making contact. The method described above is the contact charging method.

d. Protons are never gained or lost during ordinary electrostatic experiments. They are tightly bound in the nuclei of atoms; it would require an atom-smasher to induce protons to move.

e. The glass rod, being charged positively, would attract electrons which were present upon the electroscope. These electrons would be transferred to the glass rod. The electroscope would lose electrons, not gain them. In fact, that is why it becomes charged positively.

f. While the electroscope does lose electrons, it does not lose all its electrons.

g. See the explanation in part e above.

h. Having lost electrons, there are now more protons in the electroscope than there are electrons.

i. The electroscope and the glass rod are both positively-charged. The presence of the glass rod near the positively-charged electroscope would induce the needle to deflect even more rather than cause it to move towards the neutral position.

j. Since the electroscope is now charged positively, bringing a negatively charged object nearby would cause the needle to return to the neutral position.

Useful Web Links

Charging by Conduction

6. A negatively-charged balloon is brought near to (without touching) a neutral electroscope. With the negatively-charged balloon held near, the electroscope is momentarily touched by a hand (ground). The balloon is then removed and the needle deflects, thus showing a charge. Which of the following statements are true of the charged electroscope?

The electroscope was charged by the method of induction.
The charge on the electroscope is the same type of the charge on the balloon.
The electroscope has more protons than electrons.
The electroscope gained protons during the charging process.
The electroscope gained electrons during the charging process.
The electroscope lost all of its electrons during the charging process.
The electroscope lost some electrons during the charging process.
During the charging process, protons moved from the electroscope to the balloon.
During the charging process, electrons moved from the electroscope to the balloon.
During the charging process, electrons moved from the electroscope to the hand (ground).
During the charging process, electrons moved from the hand (ground) to the electroscope.

Answer: ACGJ

a. This is a perfect description of the induction charging method.

b. Induction charging places a charge on an object which is opposite of the type of charge on the object used to charge it. Thus, the electroscope acquires a positive charge.

c. Since the electroscope is positively charged, there are more protons than electrons on the electroscope.

d. Protons are never gained or lost during ordinary electrostatic experiments. They are tightly bound in the nuclei of atoms; it would require an atom-smasher to induce protons to move.

e. When the - balloon was brought near, electrons in the electroscope move to a location far from the balloon (like-charged objects repel). When touched by the hand (ground), the electrons move even further, entering the hand and the person. So in the induction charging process, the electroscope lost electrons.

f. While the electroscope loses electrons, it does not lose all of its electrons.

g. See explanation to step e above.

h. Protons are never gained or lost during ordinary electrostatic experiments. They are tightly bound in the nuclei of atoms; it would require an atom-smasher to induce protons to move.

i. Electrons cannot move from the balloon to the electroscope because there is no available conducting pathway. The air separates the two objects and serves as an insulator, thus preventing the movement of electrons between them.

j. See explanation to step e above.

k. See explanation to step e above.

Useful Web Links

Charging by Induction

7. The outer shell electrons in metals are not tightly bound to the nuclei of their atoms. They are free to roam throughout the material, moving from atom to atom. These materials are good ____.

conductors
insulators
for nothing

Answer: A

Metals are great conductors. That is, electrons are free to move across the surface of metals, migrating from atoms to atoms. Being conductors, metals can serve as pathways for the movement of electrons from atom to atom, molecule to molecule, and object to object. All of this is due to the fact that the outer electrons (also known in some chemistry circles as valence electrons) are not tightly bound to the nuclei of their atoms.

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Conductors and Insulators

For Questions #8 and #9, consider the following situation. Connor Duct (Con to his friends) takes a positively charged rubber rod and touches a metal sphere on an insulated stand as shown at the right. Draw the direction of electron flow.

8. The charge on the metal sphere will end up ___.

neutral
negative
positive

Answer: C

This is an example of charging by contact. When a neutral object is charged by contact, the object acquires the same type of charge as the object used to charge it. In this case, the sphere acquires the same type of charge as the rubber rod - a positive charge. (As for how this happens, see the next question.)

Useful Web Links

Charging by Conduction

9. The sphere acquires this charge because ____.

electrons move from the rubber rod to the sphere
electrons move from the sphere to the rubber rod
protons move from the rubber rod to the sphere
protons move from the sphere to the rubber rod
the rubber rod creates a charge on the sphere

Answer: B

Once touched, the sphere and the rubber rod transfer electrons between them. Since the rubber rod is + and electrons are -, the rubber rod attracts electrons from the sphere towards itself. Some of these electrons move into the rubber rod, thus reducing the amount of + charge upon it. The sphere in turn becomes charged positively since it has lost electrons.

Useful Web Links

Charging by Conduction

10. If a positively charged plate is brought near the top of a positively-charged electroscope, then the deflected needle will ____.

not move at all
be deflected more
be deflected less

Answer: B

A positively charged electroscope has protons and electrons; only there would be more protons compared to the number of electrons. There is an excess of + charge ("protons without partners") uniformly distributed over the electroscope. Bringing another + object nearby, will draw electrons out of the needle of the electroscope up into the plate. This would leave and even greater excess of charge in the needle of the electroscope. As such, the needle would be seen to deflect even more.

Useful Web Links

Charge Interactions || Charging by Induction

11. If an electroscope, charged up with excess negative charge, is touched and grounded, then it will end up with ___ charge.

no
a negative
a positive

Answer: A

When a charged object is grounded, there is a movement of electrons between the object and the ground until the object has a balance of charge. In this case, electrons move from the electroscope to the ground until the number of electrons equals the number of protons.

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Grounding - the Removal of a Charge

12. A negatively charged balloon is brought near a metal can that rests on a wood table. The side of the can opposite the balloon is momentarily touched. The can is then _______.

positively charged
negatively charged
uncharged

Answer: A

This is an example of charging by induction. In all such cases, the object being charged acquires a charge that is opposite in type to the object used to charge it. Bringing the - balloon near the can will chase electrons in the can to the far side of the can. When touched on the far side, electrons are chased further, exiting the can and migrating into the finger and the person. Having lost electrons, the can now has an excess of positive charge. In this example, the finger and person serve as the ground - a seemingly infinite reservoir or holding place of charge.

Useful Web Links

Charging by Induction

13. Charge carriers in a metal are electrons rather than protons. This is due to the fact that electrons are __________.

loosely bound
lighter
far from a nucleus
all of the above
none of the above

Answer: A

Electrons are light and they are far from the nucleus, but neither of these features explain why they are the carriers of charge in electrostatic experiments. Electrons, unlike the protons, are not bound up in an inescapable condition within the atoms of metals. As such, they can move from atom to atom throughout the surface of the metal and can even from object to object. On the other hand, protons are tightly bound in the nucleus of an atom and are unable to escape by ordinary electrostatic means.

Useful Web Links

The Structure of Matter || Conductors and Insulators

14. Two like charges ________.

attract each other
repel each other
neutralize each other
have no effect on each other
must be neutrons

Answer: B

There are three charge interactions which should be known; this is one of them: Like charged objects repel. The other two: i) oppositely-charged objects attract, and ii) a neutral object and any charged object (whether + or -) will attract.

Useful Web Links

Charge Interactions

15. If you comb your hair and the comb becomes positively charged, then your hair becomes _______.

positively charged
negatively charged
uncharged

Answer: B

This is an example of charging by friction. Two neutral objects, when rubbed together, will transfer electrons between them. The object with the greatest electron affinity will acquire electrons from the other object; the two become charged oppositely. In this case, the hair acquires electrons from the comb, leaving the comb with a shortage of electrons and a + charge; the hair now has an excess of electrons and a - charge.

Useful Web Links

Charging by Friction

16. Which of the following charging methods can result in charging an object with a negative charge? Choose all that apply.

a. charging by friction

b. charging by contact

c. charging by induction

Answer: ABC

If object A is rubbed with another object having a lower electron affinity, then the atoms of object A will steal electrons from the other object and acquire a negative charge.

If object A is touched by a negatively-charged object, then it will become charged negatively.

If a positive charge object is used to charge object A by the method of induction, then object A will acquire a negative charge as it draws electrons from the ground.

Useful Web Links

Charging by Friction || Charging by Induction || Charging by Conduction

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Description of Object	a, b, or c?
i. An object possesses more protons than electrons.	A
ii. An object possesses more neutrons than electrons.	ABC
iii. A formerly neutral object that just lost some electrons.	A
iv. A formerly neutral object that just gained some electrons.	B
v. An object which attracts a negatively-charged balloon.	AC
vi. An object which attracts neutral paper bits and attracts a negatively-charged balloon.	A
vii. An object which attracts neutral attracts paper bits and repels a negatively-charged balloon.	B
viii. An object which attracts a negatively-charged balloon and attracts a positively-charged balloon.	C
ix. An object which attracts a charged balloon (balloon A) which is attracted to a negatively-charged balloon (balloon B).	ABC
x. An object which attracts a balloon (balloon C) which is repelled by a negatively-charged balloon (balloon D).	AC
xi. An object which repels a balloon (balloon E) which is repelled by a positively-charged balloon (balloon F).	A
xii. An object around which the electric field vector is directed inwards.	B
xiii. An object around which the electric field vector is directed outwards.	A

Description of Charging Method	a, b, or c?
i. Object A is charged by friction using animal fur. Animal fur has a greater electron affinity than object A.	A
ii. Object A is charged by contact using a negatively charged object.	B
iii. Object A is charged by induction using a positively charged object.	B
iv. Object A is used to charge an aluminum plate by the method of induction. The aluminum plate acquires a positive charge.	B
v. Object A is used to charge an aluminum plate by the method of induction. The aluminum plate acquires a negative charge.	A
vi. A rubber rod has a greater electron affinity than animal fur. The rubber rod is charged by friction with animal fur. The rubber rod is then used to charge Object A by the method of contact.	B
vii. A rubber rod has a greater electron affinity than animal fur. The rubber rod is charged by friction with animal fur. The rubber rod is then used to charge Object A by the method of induction.	A
viii. A rubber rod has a greater electron affinity than animal fur. The rubber rod is charged by friction with animal fur. The rubber rod is then used to charge an aluminum pop can by the process of induction. The pop can is then contacted to object A.	A