# IB High Level Physics Question

## Main Question or Discussion Point

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

## Answers and Replies

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mfb
Mentor
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.

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.

mfb
Mentor
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).

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.

mfb
Mentor
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.

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?

mfb
Mentor
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.

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.