Context: There is an N-magnet and an S-magnet. Nobody and nothing connected the magnets, there was no agent (even there was no wind intervention), but they just connected naturally.
In this case, can I use passive voice as below?
Passive form: “Two magnets were connected.”
And as a result, do “two magnets were connected” and “two magnets (have) connected” have no semantic difference?
There is a class of verbs that have both transitive and intransitive forms, in which the object of the transitive form is the subject of the intransitive. These are sometimes called labile or ergative verbs. The usual example is "to open":
John opens the door.
The door opens.
The intransitive form implies there is no agent. The transitive form has an explicit agent.
The passive is formed from the transitive verb, and has an implied agent.
The door was opened.
This implies the existence of an unnamed agent.
But we have to be careful with this, because often there is an adjective that looks exactly like a past participle.
So we have "to break":
John broke the window.
The window broke. (the labile intransitive)
A broken window. (an adjective)
The window was broken (ambiguous, could be a passive, but could be an adjective)
In the last case the existence of the adjective means that we can't conclude that "somebody broke the window". The word "broken" could be understood as an adjective and that doesn't imply the existence of an agent.
In the case of "open" the adjective is "open", we say "An open door" or "The door was open" when we use the adjective, so the ambiguity doesn't exist.
Now, while "The window was broken" is syntactically ambiguous, it is not usually semantically ambiguous. The meaning is clear.
How about "connect"? Well it is like "break", because there is a valid as an adjective: "Two connected magnets". So the sentence
The two magnets are connected.
Can be understood as being [subject] [linking verb] [adjective complement] and no agent is implied.
To begin, a few small unrelated corrections: you are referring to the magnets in the singular (connected by an "and"), so you should not use "are". Also, when pronouncing the letters of the alphabet as letters, not using them consonants/vowels, check to see if the name of the letter starts with a vowel for a/an reasons. Also be careful of articles.
There is an N-magnet and an S-magnet. Nobody connected the magnets, but they just connected.
Now to your question. Yes, when using the passive voice, you can absolutely drop the agents. This is in fact the most common use of the passive voice, and is the main reason why modern English teachers of native speaking English students highly discourage the use of the passive voice. By not including the agents, it's easy to "lie by omission". It's common to use the passive voice in an untrustworthy way, and many people think it is sneaky.
In scientific contexts however, the passive voice is often very appropriate. No one caused these things to happen, they just happened. Saying "Nobody connected the magnets" is a very strange thing to say, as we wouldn't expect anyone to do so. They're magnets, after all.
Two magnets were connected.
This is fine.
It is perfectly normal not to specify an agent in a passive voice sentence. Here are two examples:
The toilets are cleaned daily.
My car was stolen yesterday.
In the first sentence, you don't want to specify the agent because it's not important to the reader who cleans the toilets. In the second sentence, you don't know who stole your car, so you can't specify the agent.
If you do need to specify an agent, you should really consider whether it would be better to use active voice.
Looking at the magnets, it sounds to me like they have been connected for a while and will remain so: their state is connected. We generally use present simple to describe states.
The magnets are connected North-to-South.
Note that, in this sentence, connected is an adjective that defines the state of the magnets, not a past participle.
If you want to describe the moment that the magnets come into contact you would way "the magnets connect" as a part of a sentence that describes the circumstances under which they come into contact. Note that the verb connect can be intransitive, so you don't need to specify an object. Here is an example:
The magnets connect when they come within 5mm of each other.
If you use passive voice, many readers would assume that somebody deliberately connected them, but it's not important who did it.
The magnets were connected
