Final answer:
The magnetic force felt by a wire carrying current south in the Earth's magnetic field will be directed upwards, as determined by the left-hand rule for negatively charged carriers.
Step-by-step explanation:
When a north-south oriented wire carries current in the south direction and is placed in the Earth's magnetic field, which also has a north-south orientation, we can determine the direction of the magnetic force felt by the wire using the left-hand rule (since force and field involve the direction of conventional current, we use the left hand for negatively charged carriers like electrons in a wire). According to the left-hand rule, when you point your thumb in the direction of the electron current (or opposite to the conventional current) and your first finger in the direction of the magnetic field, your second finger will point in the direction of the force acting on the current-carrying wire. Given that electrons in the wire are moving north (since conventional current is heading south) and the Earth's magnetic field has a horizontal component pointing north, the left-hand rule would suggest that the force on the wire will be directed upwards.
This application of the left-hand rule is grounded in the physics topic known as electromagnetism, where electric currents can generate magnetic fields and be influenced by them as well, leading to a force that obeys the left-hand rule for negatively charged carriers.