1. Sep 20, 2010

### willdefender

1.If you charge a pocket comb by rubbing it with a silk scarf, how can you determine if the comb is positively or negatively charged?

6.Contrast the net charge on a conductor to the "free charges" in the conductor.

7.A charged rod placed near an unchanged metal object can attract(or repel) electrictrons. There are a great many electrons in the metal, yet only some of them move. Why not all of them?

12. What experimental observations mentioned in the text rule out the possibility that the numerator in Coulomb's law contains the sum (Q1 and Q2)rather than the product Q1Q2?

20.Show, using the three rules for field lines, that the electric field lines starting or ending on a single point charge must be symmetrically spaced around the charge.

THREE RULES:
A.The field lines indicate the direction of the electric field;the field points in the direction tangent to the field line at any point;
B.The lines are drawn so that the magnitude of the electric field, E, is proportional to the number of lines crossing unit area perpendicular to the lines. The closer the lines, the stronger the field;
C.Electric field lines start on positive charges and end on negative charges; and the number starting or ending is proportional to the magnitude of the charge.

23.Consider a small positive test charge located on an electric field line at some point, such as point P. Is the direction of the velocity and/or acceleration of the test charge alone this line? Discuss.

24.We wish to determine the electric field at a point near a positively charged metal sphere(a good conductor).We do so by bringing a small test charge, q0, to this point and measure the force F0 on it. Will F0/q0 be greater than, less than, or equal to, the electric field E as it was at that point before the test charge was present?

2. Sep 20, 2010

### zhermes

Everyone on the forums is happy to help, but they won't simply answer homework problems for you. For each of the above problems please describe your attempts at the solution, or particular points that you are confused about so we can help as efficiently as possible.

3. Sep 22, 2010

### willdefender

1.If you charge a pocket comb by rubbing it with a silk scarf, how can you determine if the comb is positively or negatively charged?
----Our teacher suggest to use the electroscope which has some charged on the leaves originally.And then put the comb near the metal of the electroscope but not touch it.He said in this way we can test it. But I didn't understand the process.

6.Contrast the net charge on a conductor to the "free charges" in the conductor.
----I think, as for the net charge on a conductor, we can consider the conductor as a whole, then, net charge is what the whole shows, positive or negative and how many of them.
As for free charges, I think it is the charge that can move in the conductor freely, such as the valence electrons.

7.A charged rod placed near an unchanged metal object can attract(or repel) electrictrons. There are a great many electrons in the metal, yet only some of them move. Why not all of them?
----I think only some of them move because only the valence electros have larger possibility to move. But our teacher said that some of them move because the electrons have reached a dynamic balance before all of them move.

12. What experimental observations mentioned in the text rule out the possibility that the numerator in Coulomb's law contains the sum (Q1 and Q2)rather than the product Q1Q2?
----I don't make out what this question means.

20.Show, using the three rules for field lines, that the electric field lines starting or ending on a single point charge must be symmetrically spaced around the charge.

THREE RULES:
A.The field lines indicate the direction of the electric field;the field points in the direction tangent to the field line at any point;
B.The lines are drawn so that the magnitude of the electric field, E, is proportional to the number of lines crossing unit area perpendicular to the lines. The closer the lines, the stronger the field;
C.Electric field lines start on positive charges and end on negative charges; and the number starting or ending is proportional to the magnitude of the charge.
----I think A tells us about the direction of the lines, we learn the dirction of the field lines produced by a single point is either towards or outwards the point.

23.Consider a small positive test charge located on an electric field line at some point, such as point P. Is the direction of the velocity and/or acceleration of the test charge alone this line? Discuss.
----I think both of them are not on that line but on the diretion of the line.

24.We wish to determine the electric field at a point near a positively charged metal sphere(a good conductor).We do so by bringing a small test charge, q0, to this point and measure the force F0 on it. Will F0/q0 be greater than, less than, or equal to, the electric field E as it was at that point before the test charge was present?
----It depends. If the point charge is positive(the same with the test charge),it will be less.If it is negative, it will be larger.

4. Sep 22, 2010

### willdefender

Thank you for your suggestions! This is my first time discussing questions here. I hope the ones below will satisfy the requires here!