8. ELECTRONS

 8.1. 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.





Variable resistance or ptentiometer.
The value of a variable resistance or potentiometer can be adjusted between zero and the maximum value specified by the manufacturer.





Resistance that depends on a physical factor.
The physical factors that effect resistance may be temperature or the amount of light:
  -Resistance that depends on temperature is called a thermistor. There are two types of thermistors:

• Negative temperature coefficient (NTC) : The resistance decreases as the temperature rises.
• Positive temperature coefficient (PTC) : 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.

Capacitors.
A capacitor can store electrical energy from a battery and then use it to power a light bulb until the charge is totally depleted.
Capacitors are components that can store an electrical charge.
The value of a capacitor indicates the charge in volts that it can store. This is measured in farads (F).

Diodes.
A diode is an electronic component made from semiconductor materials. A diode has two electrodes: an anode (A) and a cathode (K).
A LED only gives off light when an electric current flows throught it.

Transistors.
They are made from semiconductor materials and have three electrodes called the base, the collector and the emitter.
There are two types of transistor: NPN and PNP.

• 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.
• 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.
• When the flow of electrons through the base is between the cut off and saturations levels, it will be proportional to th flow of electrons between the collector and the emitter. The transistor is in the active region.

8.2. Basic devices made with eletronic components.
Timers.
A timer is a device that operates for a cetain period of time and then shuuts itself off automatically.
Integrated circuits.
Integrated circuits consist of miniature electronic components, such as transistors, resistors and capacitors.