Some remarks
In our (daily) use of Arabic language we consider both nouns al-Fuad الفؤاد and al-Qalb القلب as synonyms, but this isn't the case in the Qur'an and sunnah (see for example in this sahih hadith).
Note that al-Qalb could also refer to anything which is inside of something else or the centre of it. It is also often used referring to the mind (this use is present in the Qur'an like in 7:179 and 22:46).
Also note that the Qur'an always mentioned al-Fuad after as-Sma' السمع (the hearing) and al-Bassar البصر (the sight) (see for example 17:36) while al-Qalb is usually mentioned before them (7:179, 22:46, 24:37, 45:23 and 50:37) one exception is (6:46).
From the Qur'an we may conclude that al-Qalb has the upper hand in on our thoughts in all our cases, while the mind can get lost.
Now to your example
The verse you've posted is one of the best examples. As one would ask himself how could the heart (Fuad) became empty? ... the verse indicates that something happened to al-Fuad, while al-Qalb kept safe, so there's clearly a distinction between both:
The scholars basically answered this saying:
والفؤاد مستعمل في معنى العقل واللب .
As may read in ibn Ashur's at-Tahrir wa tanwir: This means al-Fuad was used to refer to the brain (mind) and the core.
As to the meaning of emptiness here there are the following explanation:
- Her mind got empty from the revelation or ilham that Allah gave her.
- empty from every worldly matter except the remembrance of her son Musa ().
- Her mind got lost out of fear or astonishment.
Imam al-Qurtobi -in his tafsir- added the following after quoting these interpretations (My own translation take it carefully):
ونحوه قوله تعالى : وأفئدتهم هواء أي جوف لا عقول لها كما تقدم في سورة ( إبراهيم ) وذلك أن القلوب مراكز العقول ; ألا ترى إلى قوله تعالى : فتكون لهم قلوب يعقلون بها
And this is like Allah the Exalted said: " and their hearts are void. (14:43)" which means that their brains have become like a hole without a mind as we already discussed in surat Ibrahim and this is because al-Qulub القلوب (plural of al-Qalb) are the centre of the minds, don't you read Allahs words: "So have they not ... have hearts by which to reason 22:46"?
Finally (some statements without good backup):
Some say that al-Fuad is the cover of al-Qalb, others said al-Qalb refers to the mind and al-Fu'ad to the affection and awareness. And others said al-Qalb is the heart and al-Fuad is the same but with the attribute of being able to blaze (react having an affection etc.)
Note: here's a long reaserch paper in Arabic on this topic.
