Opposite sides of a current-carrying coil experience opposite forces in a magnetic field. These forces provide a turning effect that rotates the coil. In a simple dc motor, a split-ring commutator reverses coil current each half-turn so the turning effect continues in the same rotational direction; brushes maintain electrical contact. Example: Explain why the forces on opposite sides differ in direction. Current flows oppositely along those sides in the same field. Use evidence. Apply the model.