Motor, Electromagnetic Induction and Electric Generator

The One Thing

Electric motors convert electrical energy into mechanical motion through the force on a current-carrying conductor in a magnetic field. Electric generators perform the reverse conversion: mechanical energy is converted into electrical energy through electromagnetic induction.

Reactions, Processes and Experiments

Key Terms

• Magnetic force on a current-carrying conductor: A conductor carrying current in a magnetic field experiences a force. The force is greatest when the conductor is perpendicular to the magnetic field.
• Motor principle: A current-carrying coil placed in a magnetic field experiences a turning effect, causing the coil to rotate.
• Electric motor: A device that converts electrical energy into mechanical energy by rotating a current-carrying coil in a magnetic field.
• Armature: The rotating coil of an electric motor or generator, usually placed between the poles of a magnet.
• Split-ring commutator: A device in a simple motor that reverses the direction of current in the coil after every half rotation, allowing continuous rotation in the same direction.
• Carbon brushes: Stationary conducting contacts that supply current to the rotating coil through the split-ring commutator.
• Electromagnetic induction: The phenomenon in which a changing magnetic field produces an induced current in a nearby coil or conductor.
• Induced current: Current produced in a closed conducting circuit because of a changing magnetic field linked with it.
• Fleming's left-hand rule: If the thumb, forefinger, and middle finger of the left hand are held mutually perpendicular, the forefinger shows the magnetic field direction, the middle finger shows current direction, and the thumb shows force or motion direction.
• Fleming's right-hand rule: If the thumb, forefinger, and middle finger of the right hand are held mutually perpendicular, the forefinger shows the magnetic field direction, the thumb shows conductor motion, and the middle finger shows induced current direction.
• Electric generator: A device that converts mechanical energy into electrical energy by rotating a coil in a magnetic field.
• Slip rings: Continuous conducting rings connected to the rotating coil of an alternating-current generator, allowing current to pass to the external circuit.
• Direct-current generator: A generator using a split-ring commutator to produce current that flows in one direction in the external circuit.
• Alternating-current generator: A generator using slip rings to produce current whose direction changes periodically.
• Lenz's law: The induced current flows in a direction that opposes the change in magnetic flux producing it.
• Magnetic flux: A measure of the magnetic field passing through a given area. A change in flux can induce current in a conducting coil.
• Force equation: The force on a current-carrying conductor is given by F = BIL sin theta, where B is magnetic field strength, I is current, L is conductor length, and theta is the angle between the conductor and magnetic field.
• Michael Faraday: The scientist credited with discovering electromagnetic induction in 1831.
• Law of conservation of energy: Energy is transformed, not created or destroyed; this principle applies to both motors and generators.

Easily Confused

• Motor and generator: A motor converts electrical energy into mechanical energy, whereas a generator converts mechanical energy into electrical energy.
• Fleming's left-hand rule and Fleming's right-hand rule: Fleming's left-hand rule determines force or motion in a motor; Fleming's right-hand rule determines induced current in a generator.
• Split-ring commutator and slip rings: A split-ring commutator reverses current every half rotation and is used in motors and direct-current generators; slip rings maintain continuous connections and are used in alternating-current generators.
• Induced current and continuous current without changing flux: Induced current requires a change in magnetic flux; a stationary magnet and stationary coil do not produce continuous induced current.
• Reversing one direction and reversing both directions in a motor: Reversing either the current or the magnetic field reverses rotation, whereas reversing both simultaneously does not reverse the rotational direction.
• Maximum and zero magnetic force: The force is maximum when the conductor is perpendicular to the field, because sin 90 degrees = 1, and zero when it is parallel, because sin 0 degrees = 0.

What Gets Asked

• Calculate or interpret magnetic force using F = BIL sin theta. Marks are lost by using the wrong angle or failing to identify that the force is maximum at perpendicular orientation and zero at parallel orientation.
• Explain the construction and operation of a simple motor. Required features include the rectangular coil, strong magnetic field, split-ring commutator, carbon brushes, battery or source, and axle; omitting the commutator’s role in reversing current after every half turn loses marks.
• Determine the direction of force or motion in a motor using Fleming's left-hand rule. The common error is confusing the roles of the forefinger, middle finger, and thumb.
• Describe electromagnetic induction and identify when induced current is produced. A changing magnetic flux is essential; a stationary magnet and stationary coil do not produce continuous induced current.
• Compare alternating-current and direct-current generators. The distinction depends on the current-transfer device: slip rings produce periodically changing current, whereas a split-ring commutator produces current in one direction in the external circuit.
• Determine the direction of induced current in a generator using Fleming's right-hand rule and Lenz's law. Marks are lost by using the left-hand rule or by giving a current direction that does not oppose the change in magnetic flux.