Electromagnetic Induction Too
Fleming’s Right Hand Rule
Michael Faraday found that moving a conductor in a magnetic field (or by moving the
magnet field near a stationary conductor) created a voltage. The wire must be part of an
electrical circuit. Otherwise the electrons have no place to go. If the ends are attached to
e.g. a light bulb, the circuit is complete and electrical current is created.
The direction of the current can be found from Fleming's Right-hand Rule. (the lefthand rule is used for motors and motion produced by a magnetic field).
The right-hand rule is used for generators and current generated by a motion. Using the
right-hand, the thumb is in the direction of the motion, the first finger points in the
direction of the field and the second finger points in the direction of the current.
The voltage induced in the wire can be calculated by:
Induced voltage brought about by a magnetic flux change is of great importance in the
electric power system. A generator converts mechanical energy to electrical energy by
the induction of current in a coil which is rotated within a magnetic field or by rotating a
magnet within a coil. During rotation the magnetic flux through the coil alters because
the plane of the coil is alternately parallel to the magnetic field lines and then
perpendicular to them.
Dynamos use magnets, a coil and movement to produce electricity in exactly the
opposite way to the motor. E.g. a Wind turbine
Wind turbines have huge blades mounted on a tall tower. The blades are connected to a
housing that contains gears linked to a generator. As the wind blows, it transfers some of
its kinetic energy to the blades, which turn and drive the generator.
Alternating Current
The process of producing an alternating current requires a conductor loop spinning in a
magnetic field. The conductor loop, or coil, cuts through lines of force in a magnetic field
to induce A.C. voltage at its terminals. Each complete turn of the loop is called a "cycle."
Alternating Current Generator
A generator is a device that turns rotary mechanical energy into electrical energy.
An alternating current generator, or alternator, produces an alternating current, which
means the polarity of the current changes direction repeatedly. This type of generator
requires a coil to cut across a magnetic field, and is attached to two slip rings connected
to brushes. The brushes deliver the current to and from the load destination, thus
completing the circuit.
During the first half turn, the coil cuts across the field near the magnet's North Pole.
Electrons go up the wire, and the lower slip ring becomes positively charged. When the
coil cuts near the south pole of the wire during the second half turn, the lower slip ring
becomes negatively charged, and electrons move down the wire.
Direct Current Generators
Simple D.C. generators contain several parts, including an armature (or rotor), a
commutator, brushes, and field winding. A variety of sources can supply mechanical
energy to the D.C. generator to turn its armature. The commutator changes the
alternating current (A.C.) into direct current as it flows through the armature.
The stationary brushes, which are graphite connectors on the generator, form contact
with opposite parts of the commutator. As the armature coil turns, it cuts across the
magnetic field, and current is induced. At the first half turn of the armature coil (clockwise
direction), the contacts between commutator and brushes are reversed, or to put it
another way, the first brush now contacts the opposite segment that it was touching
during the first half turn while the second brush contacts the segment opposite the one it
touched on the first half turn. By doing this, the brushes keep current going in one
direction, and deliver it to and from its destination.
Direct Current Motors
A simple DC motor is like a generator in reverse. It has a coil of wire that can rotate in a
magnetic field. The current in the coil is supplied via two brushes that make moving
contact with a split ring. The coil lies in a steady magnetic field. The forces exerted on
the current-carrying wires create a torque on the coil.
The two forces shown here are equal and opposite, but they are displaced vertically, so
they exert a torque. (The forces on the other two sides of the coil act along the same line
and so exert no torque.)
The split ring ensures this is a DC motor. When the plane of the rotating coil reaches
horizontal, the brushes will break contact (nothing is lost, because this is the point of
zero torque anyway--the forces act inwards). The angular momentum of the coil carries it
past this break point and the current then flows in the opposite direction, which reverses
the magnetic dipole. So, after passing the break point, the rotor continues to turn
anticlockwise.