Use of ～のか (～んですか) in questions not seeking a yes/no answer


I would like to know if there is a shift in nuance in questions such as these:

> 誰が参加したんですか。 vs. 誰が参加しましたか。
>
> いつ着いたんですか。 vs. いつ着きましたか。

I wish to limit discussion to only non-yes/no questions (so questions asking
Who? When? Where? Why? and so on). I have read [a
paper](http://www.lang.nagoya-u.ac.jp/nichigen/0-kyouiku/seminar/pdf/027-2.pdf)
(PDF) which states that adding ～のか indicates that the asker's feelings of
wanting to know the answer are stronger, and that it puts more of a burden of
responding on the askee. Can anyone confirm or contradict this?




の/ん often indicate that the speaker is attempting to explain or account for
some fact. This can connect the question to a previous statement made by the
addressee.

For example:

> A: (Wow, some of the people who participated in the tournament were really
> good!)  
>  B: 誰が参加したんですか。

Here B would like to know which people inspired A to make that statement.

> A: (I got to this party so early that the host looked at me funny.)  
>  B: いつ着いたんですか。

B wants to know how early A was to the party, thus explaining why the host
looked at her funny.




の/ん often indicate that the speaker is attempting to explain or account for
some fact. This can connect the question to a previous statement made by the
addressee.

For example:

> A: (Wow, some of the people who participated in the tournament were really
> good!)  
>  B: 誰が参加したんですか。

Here B would like to know which people inspired A to make that statement.

> A: (I got to this party so early that the host looked at me funny.)  
>  B: いつ着いたんですか。

B wants to know how early A was to the party, thus explaining why the host
looked at her funny.

