# Electrostatic homework questions

• harimakenji
In summary, the conversation discusses the behavior of charged objects in relation to neutral objects, specifically an electroscope and a stream of water. It explains the phenomena of the leaf rising and falling based on the charge of the object brought close to it, and how a stream of water can be attracted by a positively charged object due to electrostatic induction. The conversation also mentions the different ways to charge an object, including induction and conduction, and clarifies that the third method is electrostatic induction. Finally, it discusses the correct answer for a question about the flow of electrons between differently charged objects.
harimakenji

## Homework Statement

1. a. When a negatively charged object brought close to negatively charged electroscope, the leaf rises more
b. When a positive object is brought close to a positively charged electroscope, the leaf becomes closer.

Explain the above phenomena

2. Which of the following can attract a stream of water?
(i) a positively charged conductor
(ii) a positively charged insulator
(iii) a neutral conductor

a. 1 and 2
b. 1 and 3
c. 2 and 3
d. 1,2, and 3

3. I read there are 3 ways to charge an object. I know induction and conduction. What is the last one? Electromagnetic induction will do but I haven't covered that yet. Is there another method which is simpler?

4. Which of the following figures shows a current of electrons?

## The Attempt at a Solution

1. I understand part (a), the electrons repel each other more. But what about (b)? In my opinion, the leaf also rises more because the protons on outside object repel protons on electroscope. Why can the leaf become closer?

2. I am guessing the answer is (d). A positive conductor or isolator can attract the negative charge of water (maybe the negative ion) and neutral conductor can also do it. Not sure though

3. Do not know

4. I guess there is no right answer in the choices. Electrons will flow from more negative to more positive object. (a) and (c) have the same amount of charge so they are neutral compared to each other. for (b) the electrons should flow from left to right because left one is more negative. for (d) electrons should flow from right to left because the left one is more positive.
Right or wrong?

Thank you very much

can someone help me?

have not gotten help yet

harimakenji said:

## Homework Statement

1. a. When a negatively charged object brought close to negatively charged electroscope, the leaf rises more
b. When a positive object is brought close to a positively charged electroscope, the leaf becomes closer.

1. I understand part (a), the electrons repel each other more. But what about (b)? In my opinion, the leaf also rises more because the protons on outside object repel protons on electroscope. Why can the leaf become closer?

Are you sure that electroscope was not still negatively charged in the second case?

Your answer is correct, but the explanation is a bit foggy. The phenomenon is connected to electrostatic induction.
The rod of the electroscope and the leaf are of metal, a conductor, in which the negative electrons can move freely.

If the electroscope is negatively charged, it has excess electrons. If a negatively charged object is brought close to the terminal of the electroscope, it will repel the electrons in the terminal, so they move inside. As a result, the leaf will have more electrons as before, so it opens. A positively charged object would attract electrons, so they would move towards the terminal, leaving the leaf less negative, so it closes. But a positive object will open the leaf of a positively charded electroscope.

harimakenji said:

Explain the above phenomena

2. Which of the following can attract a stream of water?
(i) a positively charged conductor
(ii) a positively charged insulator
(iii) a neutral conductor

a. 1 and 2
b. 1 and 3
c. 2 and 3
d. 1,2, and 3

I am guessing the answer is (d). A positive conductor or isolator can attract the negative charge of water (maybe the negative ion) and neutral conductor can also do it. Not sure though

A neutral body has equal amount of positive and negative charges. A charge d body has some excess charge.
You can pick up light pieces op paper with a rubbed comb. The excess charge of the comb attracts the opposite charges in the neutral body and repels the like ones. The opposite charges get closer, where the attractive force is higher. The opposite charges got farther so they experience less force. So the attractive forces are higher than the repulsive ones, the neutral body will move towards the charged one.
But nothing happens if the comb has no excess charge.

The stream of water is neutral,has equal amount of positive and negative charges Its molecules are dipoles - the hydrogen atoms carry positive charge, the oxygen atom carries negative charge. The positively charged object brought close to the stream of water will make the molecules turn in such way that the oxygen atoms get closer to the object,and the hydrogen atoms will be farther. So the oxygen atoms experience higher attractive force than the repulsive force on the hydrogen atoms, all molecules experience a net attraction.

So what is the correct answer?

harimakenji said:
3. I read there are 3 ways to charge an object. I know induction and conduction. What is the last one? Electromagnetic induction will do but I haven't covered that yet. Is there another method which is simpler?

It is electrostatic induction. The charges in a neutral object rearrange inside it by the effect of a charged object, so as the opposite charges move towards the object, the like ones away from it.

Conduction means a flow of charges from one place to other.

harimakenji said:

4. I guess there is no right answer in the choices. Electrons will flow from more negative to more positive object. (a) and (c) have the same amount of charge so they are neutral compared to each other. for (b) the electrons should flow from left to right because left one is more negative. for (d) electrons should flow from right to left because the left one is more positive.

Right or wrong?

It is right, as the flow of electrons was the question.

ehild said:
Are you sure that electroscope was not still negatively charged in the second case?

Your answer is correct, but the explanation is a bit foggy. The phenomenon is connected to electrostatic induction.
The rod of the electroscope and the leaf are of metal, a conductor, in which the negative electrons can move freely.

If the electroscope is negatively charged, it has excess electrons. If a negatively charged object is brought close to the terminal of the electroscope, it will repel the electrons in the terminal, so they move inside. As a result, the leaf will have more electrons as before, so it opens. A positively charged object would attract electrons, so they would move towards the terminal, leaving the leaf less negative, so it closes. But a positive object will open the leaf of a positively charded electroscope.

I just checked the question again and it is true the electroscope is positively charged in the 2nd case. So I think we are agree that the leaf will open more in 2nd case and there is something wrong with the question

A neutral body has equal amount of positive and negative charges. A charge d body has some excess charge.
You can pick up light pieces op paper with a rubbed comb. The excess charge of the comb attracts the opposite charges in the neutral body and repels the like ones. The opposite charges get closer, where the attractive force is higher. The opposite charges got farther so they experience less force. So the attractive forces are higher than the repulsive ones, the neutral body will move towards the charged one.
But nothing happens if the comb has no excess charge.

The stream of water is neutral,has equal amount of positive and negative charges Its molecules are dipoles - the hydrogen atoms carry positive charge, the oxygen atom carries negative charge. The positively charged object brought close to the stream of water will make the molecules turn in such way that the oxygen atoms get closer to the object,and the hydrogen atoms will be farther. So the oxygen atoms experience higher attractive force than the repulsive force on the hydrogen atoms, all molecules experience a net attraction.

So what is the correct answer?

The correct answer is (a) 1 and 2 because neutral conductor will not cause attraction or repulsion to the water molecule.

It is electrostatic induction. The charges in a neutral object rearrange inside it by the effect of a charged object, so as the opposite charges move towards the object, the like ones away from it.

Conduction means a flow of charges from one place to other.

Sorry for not stating it clearly. I know 2 of the 3 ways, which are conduction and induction. By 'induction' I mean electrostatic induction, just as you explained above.

What is the last way to charge an object (besides conduction, electrostatic induction, and electromagnetic induction)?

I am so happy to have a reply. Thank you very much for the reply

You can not charge an object by electrostatic induction, because it means only rearranging the charges. BUT: if you have two pieces in contact, and put a charged body close, and take the pieces apart, both of them become charged.
If you mean the induced voltage or current on the effect of changing magnetic flux, this will not result in excess charge.
But you can make a body charged by rubbing it. Or you can put a metal into a dilute acid or salt solution and it gets excess charge. That is as batteries work. When two different metals are brought into contact, some electrons will go from one to the other. Or you can produce excess charge with light in solar cells (photoelectric effect)... and there can be some more ways.

ehild

ehild said:
You can not charge an object by electrostatic induction, because it means only rearranging the charges. BUT: if you have two pieces in contact, and put a charged body close, and take the pieces apart, both of them become charged.
If you mean the induced voltage or current on the effect of changing magnetic flux, this will not result in excess charge.
But you can make a body charged by rubbing it. Or you can put a metal into a dilute acid or salt solution and it gets excess charge. That is as batteries work. When two different metals are brought into contact, some electrons will go from one to the other. Or you can produce excess charge with light in solar cells (photoelectric effect)... and there can be some more ways.

ehild

Is rubbing can be said the same as conduction? Or are they different because conduction is just touching and rubbing involves more physical contact than just touching the objects?

harimakenji said:
Is rubbing can be said the same as conduction? Or are they different because conduction is just touching and rubbing involves more physical contact than just touching the objects?

No, rubbing is not the same as conduction. Electric conduction is a flow of charge through a transmitting medium (conductor) under the effect of an electric field. Charge can flow from one piece of material to an other one if they are in electric contact. Rubbing and other contact phenomena between two pieces of different media means charge transfer confined basically at the interface.

ehild

ehild said:
No, rubbing is not the same as conduction. Electric conduction is a flow of charge through a transmitting medium (conductor) under the effect of an electric field. Charge can flow from one piece of material to an other one if they are in electric contact. Rubbing and other contact phenomena between two pieces of different media means charge transfer confined basically at the interface.

ehild

OK. Thank you very much for your time, passion, and explanation.

## 1. What is electrostatics?

Electrostatics is the study of electric charges at rest and the forces and fields associated with these charges.

## 2. What are some applications of electrostatics?

Electrostatics has many practical applications, such as in electronic devices, air purifiers, inkjet printers, and electrostatic precipitators.

## 3. How do I calculate the electric field of a point charge?

The electric field of a point charge can be calculated using the formula E = kq/r^2, where k is the Coulomb's constant, q is the charge of the point charge, and r is the distance from the point charge.

## 4. What is Coulomb's law?

Coulomb's law states that the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

## 5. How do I determine the direction of the electric field at a given point?

The direction of the electric field at a given point is the direction in which a positive test charge would experience a force if placed at that point. The direction is always away from positive charges and towards negative charges.

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