TRANSISTORS: Are basic components of electrical circuits.They are made from semiconductor materials and have three electrodes called the base,the collector and the emitter.
There are two transistor: NPN and PNP.

1. When no electrons are flowing through the base,then no electrons can pass from the collector to the emitter.The transistor is in cut off.
2. When many electrons are flowing through the base,the route between the collector and the emitter will be completely open.The transistor is in saturation.
3. When the flow of electrons through the base is between the cut off and saturation levels,it will be proportional to the flow of electrons between the collector and the emitter.The transistor is in the active region.

Transistor are the basic components of the microprocessors and memory circuits that are used in computers.The following pictures illustre the concept of transistor gain.











8.2.BASIC DEVICES MADE WITH ELECTRONIC COMPONENTS:
We can build various devices with the electronic components discussed in the previous section.Here are two examples:

TIMERS: A timer is a device that operates for a certain period of time and then shuts itself off automatically. We can use the circuit on the right as an example:

1. At first,the LED is off,since there is no current flowing through the base.The transistor is in cut off.When we push the botton,current flows through the base,the transistor is activated,and the LED turn on.The capacitor also begins to charge itself.
2. When we released the botton,the LED continues to produce light for some time,using current from the capacitor.When the capacitor runs down,the transistor cuts off and the LED turns off.

INTEGRATED CIRCUITS: Integrated circuits consist of miniature electronic components,such as transistor,resistors and capacitors.For example,the following UL-N2803A integrated circuit contains eight pairs of Darlington transistors.

CAPACITORS: Are components that can store an electrical charge.
The value of a capacitors indicates the charge in volts that it can store.This is measured in farads.











DIODES:  Is an electronic component made from semiconductor materials.It only allows electric current to flow in one direction.A diode has two electrodes: an anode and a cathode.
A LED only gives off light when an electric current flows throungh it.

8.ELECTRONICS:
Electronics involves the study of circuits and compenents that modify the intensity, direction or properties of electric currents.

8.1.ELECTRONIC COMPONENTS:
In this section,we will analyse the most common electronic components.

FIXED RESISTANCE OR RESISTOR: A fixed resistance or resistor opposes the flow of electric currents.Its value,which we measure in ohms,is indicated by a code of colours and numbers.








The first two stripes are replaced by two numbers,according to their color.The third stripe indicates the number of zeroes that we must add to the previous figure.The fourth stripe indicates the tolerance or the tolerance or the máximum deviation from the theoretical value.











VARIABLE RESISTANCE OR POTENTIOMETER: The value of a variable resistance or potentiometer can be adjusted between zero and the máximum value specified by the manufacturer.








RESISTANCE THAT DEPENDS ON A PHYSICAL FACTOR:

• Resistance that depends on temperature is called a thermistor.There are two types of thermisors:

1. Negative temperature coefficient:The resistance decreases as the temperature rises.
2. Positive temperature coefficient:The resistance increases as the temperature rises.

• LDR:Resistance that varies according to the amount of light received.The resistance decreases as the amount of light increases.These devices,like potentiometers,are often used in security systems,where they are parts of sensors.

7.ELECTROMAGNETIC CONTROL SYSTEMS:
An electromagnetic control system activites the various parts of a machine,at the right momento and for the right amount of time,ensuring that the machine funtions 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 as the running of a motor or the illumination of a light bulb.

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

• Normally closed.
• Normally open.

6.2.ELECTRIC MOTORS:
An electric motors is a divice 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:
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 contract.When the electricity stops flowing,the moveable contact goes back to its original position.

6.ELECTROMAGNETIC MECHANISMS:
Electromagnetic mechanisms are devides that can convert movement into electricity or vice versa.In other words,they 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:Dynamos and alternators.

DYNAMOS:Consists of a magnet and rotary coil.The coil is located between the two poles of the magnet. The ends of the coil have two semi-circular conductors,which form the commulator.These conductors are attached to carbón brushes, which in turn are conneted to electrical wires.When electricity is applied to the coil,it rotates and begins to generate direct current in the coil.









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

5.EFFECTS OF ELECTRIC CURRENT:

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*R*t.


5.2.LIGHT:
There are various ways that electricy can be used to produce light.

Incandescent bulbs:When an electric current passes through the metallic filament of a light bulb,it produces light.The phenomenon is called incandescence.

Fluorescent tubes:Inside a fluorescent tube,there is a metallic filament,normally made of 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.

Light-emitting diodes:A light-emitting diode has layers of semiconductor materials.












5.3.ELECTROMAGNETIC EFFECTS:
In this experiment,the electric circuit has created a magnetic field.This effect can be used to produce movement.
The scientist Michael Faraday discovered to opposite effect.He noticed that electricity could be generated by using a manget and an electrical conductor.This principle allows us to build dynamos and alternators.













5.4.SOUND:
We can transform electric current into sound by using electromechanical devides,such as bells and buzzers.Some of these devides are based on the piezoelectric effect,or the ability of some materials to change when electricy is applied to them.