A current-carrying wire perpendicular to a magnetic field experiences a force — and the force is perpendicular to both the current and the field lines. Fleming's left-hand rule reads off which way it acts.
Use your left hand. Point your first finger along the field lines. Point your second finger along the current, at a right angle to your first finger. Stick your thumb out at a right angle to both fingers. Your thumb points along the force.
Given any two of field, current and force, the rule finds the third: line up the two fingers you know, and read the answer off the third.
The magnetic field points straight down. The current runs north. Which way does the force act on the wire?
Point your first finger straight down, along the field.
Point your second finger north, along the current.
Your thumb now points west.
The force on the wire acts west.
A wire's current runs east. The magnetic field points straight down. Which way does the force push?
A wire's current runs south. The magnetic field points straight down. Which way does the force push?
A wire's current runs straight up. The magnetic field points north. Which way does the force push?
A wire's current runs west. The force on it pushes straight up. Which way do the magnetic field lines point?
The magnetic field points south. The force on a wire pushes straight down. Which way does the current run?