Currents make magnetic fields, and those fields then push on other currents. Two wires side by side will either pull together or spring apart, depending only on the direction of their currents.
A straight wire makes a field of concentric circles; a flat coil makes a field like a short bar magnet; a solenoid makes a strong, nearly uniform field inside. A ferrous core greatly increases the flux density. Because each wire sits in the other's field, parallel currents in the same direction attract and in opposite directions repel.
Choose the direction and size of each current and the separation. Same-direction currents attract, opposite-direction currents repel, and the force per unit length grows with each current and falls as the wires move apart.
Know the field shape of each arrangement, and what iron does.
"Same direction attracts" is the opposite of two bar magnets, so do not guess from poles. If you reverse both currents, the force is unchanged (still attraction or still repulsion), because their relative direction is the same. A soft iron core increases the field; it does not change its shape. The field inside a long solenoid is uniform, drawn as parallel, equally spaced lines.
Four quick checks on fields made by currents. Each correct answer earns XP and lights this skill on your star map.
Two long straight parallel wires carry current in the same direction. The force between them is:
Placing a soft iron core inside a current-carrying solenoid:
The magnetic field well inside a long current-carrying solenoid is best described as:
Two parallel wires attract each other. If the current in both wires is reversed, the wires now:
When sketching, label the direction of each field with the grip rule, and show the force arrows perpendicular to the wires. Remember the contrast with magnets: like currents attract, while like poles repel. The flat coil and solenoid both behave like bar magnets, so identify their north and south faces from the current direction.
This skill is now lit on your star map. Keep the chain going.