The d.c. motor
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
The Motor Effect in Action A d.c. motor converts electrical energy to kinetic (rotational) energy using the motor effect. Basic Principle A current-carrying coil in a magnetic field experiences a turning effect (torque).
Structure of a Simple D.C. Motor FIG 4.5.21: Simple d.c. motor Rectangular coil (armature) between poles of permanent magnet (N on left, S on right). Coil mounted on axle. Split-ring commutator attached to coil ends.
Two carbon brushes press against commutator halves. Battery connected to brushes. Key Components Coil (armature): Current-carrying rectangular coil that rotates Permanent magnets: Provide the magnetic field Split-ring commutator: Reverses current direction every half turn Brushes: Carbon contacts that supply current to commutator How the D.C.
Motor Works Current flows through the coil via brushes and commutator One side of coil has current flowing up, other side down Using Fleming's LHR: opposite forces on opposite sides These forces create a turning effect (couple) Coil rotates until vertical position Commutator reverses current direction in coil Forces reverse, maintaining rotation in same direction Coil continues to rotate Why the Split-Ring Commutator?
Without the commutator, the coil would oscillate back and forth instead of rotating continuously. The commutator reverses the current every half turn, ensuring forces always act to maintain rotation in the same direction.
Increasing the Turning Effect The turning effect (speed and power) of a motor can be increased by: Factor Change Effect Current Increase Greater force on each side of coil Number of turns Increase More wires experiencing force Magnetic field Stronger magnets Greater force on current-carrying wire Coil area Larger Longer sides in field, greater torque arm Practical Motor Improvements Soft iron core: Placed inside coil to concentrate magnetic field Radial field: Curved pole pieces give constant force throughout rotation Multiple coils: Smoother rotation, more power
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