The Motor Effect and the Generator Effect Electromagnetism The Motor Effect and the Generator Effect

The Motor Effect 1. Current carrying wires in external magnetic fields When a current carrying conductor is in the presence of a magnetic field then a magnetic force will be induced onto the conductor. This is the principle behind an electric motor.

Moving charges experience a force upon entering a magnetic field Moving charges experience a force upon entering a magnetic field. Wires contain electrons (charged particles). If these charges are in motion (current flowing) then the wire experiences a force.

The size of the force is dependent upon the size of the fixed magnetic field and the amount of current in the wire. Hey! We’ve seen this formula before. The only difference is that a single length of wire is replaced with a loop of wire to create a motor.

Example: In the diagram below, a 5.0 cm wire experiences a 0.023 N force up out of the page in a 1.25 T magnetic field. What is the magnitude and direction of the current in the conductor?

How a Motor Works -the magnetic force on each side of the loop will rotate it onto the vertical position The loop will remain in the vertical position

SO: A major problem must be solved: Maintaining the same direction of rotation for the loop of wire To solve this problem, consider the diagram on the following slide. A loop of wire (called an armature) is placed within an external magnetic field.

An electric current sent through the armature will produce a force on each side of the armature as shown in the diagram. The forces result in rotation of the armature. If the armature is allowed to rotate far enough (180°) sides A and B will have switched places.

The result is that current flow will also be reversed from what it was. The result is that the armature flops back and forth, eventually coming to a stop in a position where the forces balance. This would not be a very useful motor. To maintain continuous rotation in the same direction, the electron flow must be reversed in the loop every half turn

For the loop to rotate continuously, a split ring commutator is used which reverses the electron flow every 180 degrees.

Commutator

NOTE: The input in a motor is electrical energy and the output is mechanical energy. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html#c1 http://www.walter-fendt.de/ph14e/electricmotor.htm http://www.edumedia-sciences.com/en/a182-dc-motor

Electromagnetic Induction (The Generator Effect) Electromagnetic induction – the generation or production of a current in a conductor moved through a magnetic field Discovered by English scientist Michael Faraday in the 1800’s (also by an American Joseph Henry)

When a wire passes through changing lines of magnetic field strength, a force is exerted onto the valence electron within the conductor The result is the production of an induced voltage as electrons move from one end to the other in a conductor, thus acting as a power source.

Key Element We must have a changing magnetic field achieved by varying the distance of the conductor from the source of the magnetic field ( moving the wire or the magnets creating the field) The input is mechanical energy and the output is electrical energy which is the opposite of the motor effect.

Galvanometer

Generator

Potential Difference The potential difference is created as a result of the 3rd LHR. Conductor motion: thumb •Magnetic field: fingers •Potential difference: palm (the direction the force is acting on the electrons) Induced current in the wire is directed out of the plane of the page.

When a conductor is moved through B, a force acts onto the electrons moving them to one end of the wire when electrons are forced to one end of the wire, a potential difference is produced Work done to the electrons increases their electric potential energy