Lorentz Force Law: The magnitude of the force on a straight wire of length carrying current in a uniform magnetic field is given by , where is the angle between the wire and the field lines.
Fleming's Left-Hand Rule: This mnemonic helps determine the direction of the force. The Thumb represents the direction of the Thrust (Force), the First Finger represents the Magnetic Field (North to South), and the Second Finger represents the Current (Positive to Negative).
Magnetic Flux Interaction: The force arises because the magnetic field produced by the current in the wire interacts with the external magnetic field, creating a region of high flux density on one side and low density on the other.
Torque Generation: Because the current flows in opposite directions on the two sides of the rectangular coil, the forces produced are also in opposite directions. This pair of forces creates a 'couple' or torque, causing the coil to rotate.
The Vertical Position: When the coil is vertical, the split-ring commutator loses contact with the brushes, momentarily cutting the current. However, the momentum of the rotating coil carries it past this dead spot until contact is regained.
Continuous Rotation: Once the coil passes the vertical point, the commutator segments swap brushes, reversing the current flow relative to the coil. This reversal ensures the force on the 'new' left side is always upward and the 'new' right side is always downward.
| Feature | DC Motor | AC Motor |
|---|---|---|
| Power Source | Direct Current (Battery/Cell) | Alternating Current (Mains) |
| Commutation | Uses a split-ring commutator to reverse current | Often uses slip rings or induction principles |
| Speed Control | Easily controlled by varying voltage | Typically controlled by varying frequency |
| Direction | Reversed by swapping battery terminals | Reversed by swapping phase connections |
Directional Checks: Always use your left hand for motors. A common mistake is using the right hand, which is reserved for generators (electromagnetic induction).
Identifying the Commutator: If a diagram shows a ring split into two halves, it is a DC motor. If the ring is solid and continuous, it is likely an AC device or a generator.
Factors of Strength: To increase the motor's power or speed, remember the 'BICN' rule: Increase B (Magnetic Field), I (Current), C (Coil turns), or use a soft iron N (Core) to concentrate the field.
Zero Torque Point: Remember that torque is zero when the coil is perpendicular to the magnetic field lines, as the forces act outwards/inwards along the same line rather than creating a turning effect.
