IB High Level Physics Question

In summary, the conversation discusses a question about a physics test involving a positively charged rod being attracted to a nearby stationary rod. The possible nature of the stationary rod is debated, with two options being a charged rod or an insulator. The speaker believes that the answer should be "I and II" instead of "I and III" as stated on the answer sheet. The conversation then delves into a discussion about the behavior of charged objects and the role of insulators in inducing and maintaining charge. The summary concludes that the conversation ultimately confirms that the correct answer is indeed "I and II."
  • #1
paulfr
193
3
IB High Level Physics Test Question

I have encountered a question whose answer I disagree with.
I think the answer is I and II , not the I and III on the answer sheet.

What do you think ?

=================
A positively charged rod R is suspended by insulating string.
When a stationary rod S is placed nearby, rod R is attracted towards it.

Consider the following statements regarding the possible nature of the rod S.
I. Rod S is charged.
II. Rod S is an uncharged insulator.
III. Rod S is an uncharged conductor.

Which statement(s) can explain the attraction of rod R to rod S?
A. I only
B. II only
C. III only
D. I and III only

=====================
Clearly a negatively charged Rod S can attract Rod R.
So "I" is ok.

But the conductor can not maintain the induced charge differential
needed to attract Rod R since any charge concentration / voltage
will cause a current eliminating charge build up.

However, an insulator can maintain an induced charge differential.

Did the International Bacalaureate HL Test writers get it wrong
or am I missing something ?

Thanks
 
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  • #2
The insulator cannot get an induced charge - it is insulating.

The conductor will maintain the induced charge, as only this induced charge leads to zero field inside. The separated charges are the equilibrium, if there is an external field (=satisfied here).

I and III are right.
 
  • #3
Thanks for the reply.

I think an insulator can have an induced charge.

Have you ever seen a charged comb pick up bits of paper ?
Paper is not a conductor but there is an induced charge and it is
maintained because it is an insulator.
Balloons, wool, amber and other insulators are used routinely in
electrostatic demonstrations.

A positively charged object will cause electrons to collect
near the positive field. The question is whether they will
stay there. It seems to me a conductor will not allow this.
But an insulator will since its resistance is high and thus
discharge time constant will be very long.
 
  • #4
paulfr said:
I think an insulator can have an induced charge.
If it gets in direct contact with something, it can get a tiny amount of charge. Afterwards, it has a net charge. Both effects do not happen here. In addition, all real-life objects are not perfect insulators.

A positively charged object will cause electrons to collect
near the positive field. The question is whether they will
stay there. It seems to me a conductor will not allow this.
A conductors enforces this.
But an insulator will since its resistance is high and thus
discharge time constant will be very long.
It is the opposite, charging times will be very long (for real insulators).
 
  • #5
II and III are definite yes, both are by definition negatively charged with respect to R. I is a maybe, if S is not positively charged relative to R then there will be attraction.
 
  • #6
S is uncharged (has no net charge) in II and III.

"Which statement(s) can explain" -> so I is a definite yes, a net charge can explain an attraction.
 
  • #7
mfb said:
The insulator cannot get an induced charge - it is insulating.

I always thought that sticking a balloon to the wall with static electricity was a demo of inducing a charge in the wall. Albeit without moving electrons though the insulator but by polarizing the particles at the surface. Is my understanding of this flawed?
 
  • #8
I always thought that sticking a balloon to the wall with static electricity was a demo of inducing a charge in the wall.
With a direct contact. And the balloon is not a perfect insulator.
Polarization is a small effect compared to some net charges, I think.
 
  • #9
MalachiK said:
I always thought that sticking a balloon to the wall with static electricity was a demo of inducing a charge in the wall. Is my understanding of this flawed?

No. It is correct IMO.

Paper [an insulator] bits stick to a charged plastic comb because
charge can be induced to redistribute in the paper and remain
so for a period of time depending on its resistivity.
Same for the wall holding the balloon.
 

What is IB High Level Physics Question?

IB High Level Physics Question refers to the type of questions that are asked in the International Baccalaureate (IB) High Level Physics course. These questions are designed to test students' understanding and application of physics concepts at a higher level.

What topics are covered in IB High Level Physics Question?

The topics covered in IB High Level Physics Question include mechanics, thermal physics, waves, electricity and magnetism, circular motion and gravitation, atomic, nuclear and particle physics, and energy production.

How are IB High Level Physics Questions different from regular physics questions?

IB High Level Physics Questions are designed to assess students' understanding of physics concepts in a more in-depth and complex manner. These questions require students to apply their knowledge to solve complex problems and think critically about the concepts.

How can I prepare for IB High Level Physics Question?

To prepare for IB High Level Physics Question, it is important to have a strong understanding of the fundamental concepts and formulas in physics. Practice solving complex problems and familiarize yourself with the format of IB High Level Physics Questions.

What is the best way to approach IB High Level Physics Question?

The best way to approach IB High Level Physics Question is to carefully read and understand the question, identify the relevant concepts and formulas, and then proceed to solve the problem step by step. It is important to show all your working and use appropriate units in your answers.

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