Complete Electronics Self-Teaching Guide with Projects 4th Edition
Genres: Engineering & Transportation
Book Preface
Electronics cannot be studied without first understanding the basics of electricity. This chapter is a review and pre-test on those aspects of direct current (DC) that apply to electronics. By no means does it cover the whole DC theory, but merely those topics that are essential to simple electronics.
This chapter reviews the following:
- Current flow
- Potential or voltage difference
- Ohm’s law
- Resistors in series and parallel
- Power
- Small currents
- Resistance graphs
- Kirchhoff’s Voltage Law
- Kirchhoff’s Current Law
- Voltage and current dividers
- Switches
- Capacitor charging and discharging
- Capacitors in series and parallel
Current Flow
1 Electrical and electronic devices work because of an electric current.
What is an electric current?
An electric current is a flow of electric charge. The electric charge usually consists of negatively charged electrons. However, in semiconductors, there are also positive charge carriers called holes.
2 There are several methods that can be used to generate an electric current.
Write at least three ways an electron flow (or current) can be generated.
The following is a list of the most common ways to generate current:
- Magnetically—This includes the induction of electrons in a wire rotating within a magnetic field. An example of this would be generators turned by water, wind, or steam, or the fan belt in a car.
- Chemically—This involves the electrochemical generation of electrons by reactions between chemicals and electrodes (as in batteries).
- Photovoltaic generation of electrons—This occurs when light strikes semiconductor crystals (as in solar cells).
Less common methods to generate an electric current include the following:
- Thermal generation—This uses temperature differences between thermocouple junctions. Thermal generation is used in generators on spacecrafts that are fueled by radioactive material.
- Electrochemical reaction—This occurs between hydrogen, oxygen, and electrodes (fuel cells).
- Piezoelectrical—This involves mechanical deformation of piezoelectric substances. For example, piezoelectric material in the heels of shoes power LEDs that light up when you walk.
3 Most of the simple examples in this book contain a battery as the voltage source. As such, the source provides a potential difference to a circuit that enables a current to flow. An electric current is a flow of electric charge. In the case of a battery, electrons are the electric charge, and they flow from the terminal that has an excess number of electrons to the terminal that has a deficiency of electrons. This flow takes place in any complete circuit that is connected to battery terminals. It is this difference in the charge that creates the potential difference in the battery. The electrons try to balance the difference.
Because electrons have a negative charge, they actually flow from the negative terminal and return to the positive terminal. This direction of flow is called electron flow. Most books, however, use current flow, which is in the opposite direction. It is referred to as conventional current flow, or simply current flow. In this book, the term conventional current flow is used in all circuits.
Later in this book, you see that many semiconductor devices have a symbol that contains an arrowhead pointing in the direction of conventional current flow.
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