7. Electromagnetic control systems.

An electromagnetic control system activates the various parts of a machine, at the right moment and for the right amount of time, ensuring that the machine functions properly.

7.1. Cam switch controller.
The device on the side of the pulley in the picture above is called a cam. The shape of this device allows us to control the moment and duration of an activity, such us the running of a motor or the illumination of a light bulb.

7.2. Limit swtiches.
The picture below shows an electrical control system for a water tank. The battery provides power for the pump, which moves water from the lower tank to the upper tank. is full, a limit switch turns off the pump.

The switch is activated when the float rises to a certain level. When the water level goes down, the switch returns to its original position and the pump turns on again. There are two types of limit switch:

6. Electromagnetic mechanisms.

Electromagnectic mechanisms are devices that can convert movement into electricity or vice versa. In other words, the use electromagnetic phenomena to produce electricity or convert it into mechanical energy.

6.1. Electromagnetic generators.
Electromagnetic generators transform mechanical energy into electricity. There are two types of generators, depending on the type of current that is produce. Generators that produce direct current are called dynamos, and those that produce alternating current are called alternators.

DYNAMOS.
A dynamo consists of a magnet and a rotary coil. The cail is located between the two poles of magnet. The ends of the coil have two semi-circular conductors, which form the commutator. When electricity is applied to the coil, it rotates and begins to generate direct current in the col.


ALTERNATORS.
A simple alternator is almost identical to a dynamo, except for the commutator, which consists of two metallic rings connected to carbon brushes. Instead of dirrect current this produces alternating current.

6.2. Electric motors.

An electric motor is a device that can transform electrical energy into movement. It uses the forces of attraction and repulsion between a magnet and an electrically charged wire.


6.3. Relays.
A relay is an electromagnetic component that works as a switch. When electricity passes through the coil, it acts like a magnet. The coil attracts a moveable metal contact towards another fixed contact. When the electricity stops flowing, the moveable contact goes back to its original position.
Relays may have a single circuit with one moveable contact. The may also have two or four circuits, in wich case they are called two- or four-pole relays.

5. Effects of electric current.

The movement of electrons through conductive materials produces effects that have useful applications.

5.1. Heat.
The energy that an electric current produces as heat is called the Joule Effect. It is expressed by the following formula:
                 E = I² x R x t

5.2. Light.
There are various ways that electricity can be used to produce light.

-Incandescent bulbs.
When an electric current passes through the metallic filament of a light bulb, it produces light.


-Fluorescent tubes.
There is a metallic filament, normally made og tungsten. There is also an inert gas, such as argon and a small amount of mercury. When an electric current passes through the filament, electrons are emitted into the inert gas. These react with the mercury, creating invisible, ultraviolet light. Fluorescent tubes contain a number of toxic substances, such as phosphor and mercury.

-Light-emitting diodes (LED).
LED has layers of semiconductor materials. The n-type layer has extra electronns with negatively charged particles. In contrast, the p-type layer has holes where there aren't enough electrons. When electricity is applied to the LED, the electrons and holes cross over into the active layer, where they combine and produce photons, or particles of light.



5.3. Electromagnetic effects.
The scientist Michael Faraday discovered the opposite effect. He noticed that electricity could be generated by using a magnet and an electrical conductor. This principle allows us to build dynamos and alternators.


5.4 Sound.
We can transform elctric current into sound by using electromechanical devices, such as bells and buzzers. Some of these devices are based on the piezoelectric effect, or the ability of some materials to changue shape when electricity is applied to them.