Intro to Electromagnetism: five conceptual questions

Click For Summary

Homework Help Overview

The discussion revolves around five conceptual questions related to electromagnetism, focusing on charge interactions between objects, including conductors and insulators. Participants are exploring the effects of charged objects on neutral conductors and the implications of grounding and charge induction.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants share their reasoning for each question, questioning the nature of charge interactions, particularly in scenarios involving grounding and the behavior of insulators versus conductors. There is a focus on whether certain assumptions about charge movement and object types are valid.

Discussion Status

Some participants have provided agreement on certain answers while expressing differing views on specific questions, particularly question E. There is an ongoing examination of the reasoning behind the answers, with some participants suggesting reconsideration of definitions and assumptions related to charge behavior.

Contextual Notes

Participants note potential ambiguities in the problem setup, such as the nature of the wire connecting the spheres and whether the objects in question are conductors or insulators, which may affect the outcomes of the scenarios presented.

EMJ
Messages
2
Reaction score
0
Hello! I'm working on an online assignment and am on my last chance to submit the right answer, so I figured I'd ask here for some help to see where my reasoning is wrong.

Problem:
For each of the five questions, I can choose one of four responses: Positive, Negative, Neutral, or Cannot Tell.

A. A positively charged ball is brought close to a neutral isolated conductor. The conductor is then grounded, while the ball is kept close. If the ground connection is first removed and then the ball is taken away, the conductor has a _______ charge.

B. A positively charged glass rod attracts an object suspended by a non-conducting thread. The charge on the object is _______ .

C. A negatively charged glass rod repels an object suspended by a non-conducting thread. The charge on the object is _______ .

D. A positive point charge is brought near the outside surface of a neutral conducting sphere and released. The initial position of the point charge is on the on the positive x-axis and the sphere is centered on the origin. The point charge moves in the _______ x direction. (If the point charge does not move, select "Cannot tell".)

E. Two neutral metal spheres, #1 and #2, are mounted on insulating supports. They are connected by a wire. A positively charged glass rod is brought near sphere #1. The wire connecting the spheres is removed and the charged rod is then taken away. Now the charge on sphere #1 is _______ .

My Latest Answers and Reasoning:

A. Negative
Neutral objects can still have induced charge. So, when the positive ball is introduced, the negative charge is attracted to it. When it is connected to the ground, electrons are donated from the ground into the initially neutral object, making it net negative. When the connection the ground in removed, the negative charge is stuck in the conducting object, making it negatively charged.

B. Cannot Tell
Attractions can be produced between oppositely charged objects, but also can be induced between a charged and neutral object. If the suspended object is negative, it would be attracted to the positively charged rod. If the suspended object is neutral, the charges can rearranges so that the electrons are pooled towards the direction of the rod, producing an attraction. Therefore, we cannot tell whether the suspended object is negative or neutral.

C. Positive
Unlike attractions, repulsions can only be caused between two of the same charges. This is because is an object is charged (say, positively) and another object is neutral, when the positively charged object approaches the neutral object, the negative charges in the neutral objects will pool towards the positive object. The negative charges in the neutral object will always be closer in distance to the positive object, than the neutral object's positive charges. Therefore induced charge can only produce an attraction.

D. Negative
I'm less sure about this one.
I feel like the neutral object's charges will rearrange so that its negative charges pool towards the positive point charge? However the fact that it's a point charge and not a charged object gets to me. Perhaps it's too insignificant compared to the charged object to cause an attraction or repulsion?

E. Can't Tell
Even less sure about this one.
I initially thought that sphere #1 would be negatively charged, because the positively charged rod would make the negative charge from sphere #1 and #2 pool in sphere #1 towards the rod (and the negative charges from sphere #2 would go to sphere #1 through the wire). When the wire is cut, the negative charges would remain stuck in sphere #1, giving it a negative charge.
When I was reviewing all my false submissions and didn't know what to change, I figured maybe it was a trick question because they don't specify the wire is conducting and is instead insulating? In which case, sphere #1 would have remained neutral... So the sphere could be negative or neutral.

My Previously Submitted (False) Attempts:
1.
A. Negative
B. Cannot Tell
C. Positive
D. Cannot Tell
E. Negative

2.
A. Negative
B. Cannot Tell
C. Positive
D. Negative
E. Negative

3.
A. Negative
B. Negative
C. Positive
D. Cannot Tell
E. Negative

4.
A. Negative
B. Negative
C. Positive
D. Negative
E. Negative

5.
A. Negative
B. Cannot Tell
C. Positive
D. Negative
E. Cannot Tell

I'd appreciate any help :smile: Thanks!
 
Physics news on Phys.org
I get the same answers as you except on E. The word "wire" normally implies that it is conducting.
 
You seem to think that
EMJ said:
If the suspended object is neutral, the charges can rearranges so that the electrons are pooled towards the direction of the rod, producing an attraction.
Is that always the case? I think not; it only happens with conductors in which charges are free to move about. It does not happen with insulators. So consider that anything referred to as an "object" could be either a conductor or an insulator. I agree with @berkeman about E but I disagree about C. Your reasoning in C is correct, but your most recent answer for C is inconsistent with that reasoning.
 
kuruman said:
I disagree about C. Your reasoning in C is correct, but your most recent answer for C is inconsistent with that reasoning.
Oops, I'm dyslexic apparently. Thanks for catching that @kuruman
 

Similar threads

  • · Replies 8 ·
Replies
8
Views
2K
Replies
25
Views
2K
  • · Replies 17 ·
Replies
17
Views
2K
  • · Replies 7 ·
Replies
7
Views
7K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 4 ·
Replies
4
Views
6K
  • · Replies 8 ·
Replies
8
Views
5K
  • · Replies 1 ·
Replies
1
Views
1K
Replies
23
Views
5K
Replies
6
Views
2K