Brushed DC motor

Current in a wire, across a magnetic field, gets pushed sideways. Push one side of a loop up and the other down, and it turns.

• Force = current × length × field. It always points across the field, here straight up or down.
• Every half turn the commutator swaps which end of the coil touches +. The push keeps turning the same way.
• A spinning coil is also a generator. Its back-EMF fights the battery, so the current falls as speed rises.
• At the gap, the brushes touch nothing: no current. Real motors use 3+ coils so there's no dead spot.