Can someone explain me this problem? (Circuit analysis using superposition)

[Alexmanh]

I do not really understand why we don't include 7Ω resistor in step 1, and I need some explanation with step 2.

 

[Orodruin]

In 1 it is irrelevant for the sought current (the potential difference is set by the source. In the other two it is parallel with a zero resistance (again, known potential difference - zero).

 

[rude man]

In step 1 the 7 ohm resistor does not change the effects of the 18V voltage source on the rest of the circuit so it can be ignored. The voltage is 18V regardless of the value of the (7ohm) resistor.. You are computing the effects of the 18V source by itself.
In step 2 you are shorting the 18 and 24V voltage sources and computing the effects of the current source by itself.
Step 3, same idea as step 1 except this time the 4 ohm resistor is in series with the shorted 24V source so it must be included.
Note the fact that all three sources are mutually independent.

 

[jocke]

This is a strange exercise! Who did it?
The voltage over the 4 ohm resistor must be 24 V.
So the the current -I2 is 24/4=6 A.
Why is that? A current source cannot have voltage, i.e it is zero.
In practical life this is not possible.
If the target is to teach circuit analysis, I don't know if it is a good idea
with such examples.

 

[rude man]

This is a strange exercise! Who did it?
The voltage over the 4 ohm resistor must be 24 V.
So the the current -I2 is 24/4=6 A.
Why is that? A current source cannot have voltage, i.e it is zero.
In practical life this is not possible.
If the target is to teach circuit analysis, I don't know if it is a good idea
with such examples.

A current source can have any voltage you choose around it. A current source has infinite impedance.
There is nothing "strange" about the circuit. Learn about Thevenin and Norton equivalent circuits.