هذا تيار كهربائي.

It is a nominal sentence in Arabic, meaning a sentence without a verb in the present tense.

A simple breakdown is:

• هذا = this
• تيار = current
• كهربائي = electric / electrical

So the structure is:

• هذا = the topic / thing being pointed out
• تيار كهربائي = what it is

In traditional grammar, هذا is the mubtadaʾ and تيار كهربائي is the khabar.

In Arabic, the verb to be is usually not expressed in the present tense.

So where English says:

• This is an electric current

Arabic simply says:

• هذا تيار كهربائي

This is completely normal.
If you wanted past or future, then Arabic would use a verb, for example:

• كان هذا تيارًا كهربائيًا = This was an electric current

Because تيار is a masculine singular noun.

In MSA Arabic:

• هذا = this for masculine singular
• هذه = this for feminine singular

Since تيار is masculine, you use هذا.

For example:

• هذا كتاب = this is a book
• هذه سيارة = this is a car

Because in Arabic, adjectives normally come after the noun they describe.

So:

• تيار كهربائي = literally current electric
• natural English = electric current

This is the normal noun + adjective order in Arabic.

More examples:

• كتاب جديد = a new book
• بيت كبير = a big house

Because the sentence means This is an electric current, not This is the electric current.

Arabic often uses this pattern:

• هذا + indefinite noun = This is a ...

So:

• هذا تيار كهربائي = This is an electric current

If you wanted this electric current as a noun phrase, you would normally say:

• هذا التيار الكهربائي

That means this electric current / this electrical current, where both the noun and adjective are definite.

So the difference is:

• هذا تيار كهربائي = This is an electric current
• هذا التيار الكهربائي = this electric current

No. It is not an iḍāfa.

It is a noun + adjective phrase:

• تيار = noun
• كهربائي = adjective describing that noun

A quick way to tell is that an adjective agrees with the noun in:

• definiteness
• gender
• number
• case

Here both are indefinite, masculine, singular.

An iḍāfa would be more like current of electricity, and it would be built differently.

So تيار كهربائي means electric current, not current of electricity.

In this sentence, كهربائي functions as an adjective meaning electric / electrical.

It follows the noun and matches it in form, which is exactly what adjectives do in Arabic.

Compare:

• تيار كهربائي = electric current
• جهاز كهربائي = electrical device
• طاقة كهربائية = electrical energy

Notice how the adjective changes to match the noun:

• كهربائي with a masculine noun
• كهربائية with a feminine noun

That agreement is a strong sign that it is an adjective.

In fully vocalized MSA, it would be:

• هٰذا تَيّارٌ كَهْرَبائِيٌّ

The two final words are usually in the nominative here, so they take -un in full formal pronunciation:

• تَيّارٌ
• كَهْرَبائِيٌّ

In normal printed Arabic, these short vowels are usually omitted, so you commonly just see:

• هذا تيار كهربائي

That does not mean the grammar is different; the endings are just unwritten.

A careful MSA pronunciation is approximately:

• hādhā tayyārun kahrabāʾiyyun

In ordinary learning situations, many people simplify the case endings and say something like:

• hādhā tayyār kahrabāʾī

A few pronunciation notes:

• ذ in هذا sounds like th in this
• ء in كهربائي is a glottal stop, a small break in the sound
• the final -ī in كهربائي is long

Because it must agree with تيار, and تيار is masculine.

Arabic adjectives agree with the noun they describe in:

• gender
• number
• definiteness
• case

So:

• تيار كهربائي = masculine noun + masculine adjective
• طاقة كهربائية = feminine noun + feminine adjective

The adjective changes according to the noun, not according to the English translation.

Usually, no. On its own, هذا تيار كهربائي is most naturally understood as a full sentence:

• This is an electric current

If you want the noun phrase this electric current, Arabic normally makes the noun phrase definite:

• هذا التيار الكهربائي

So for learners, this is a useful distinction:

• demonstrative + indefinite noun often gives a full sentence: this is a ...
• demonstrative + definite noun gives a noun phrase: this ...

That pattern appears very often in MSA.