Lesson 3: Electricity Basics

Understanding basic concepts of electricity is essential when working with photovoltaic systems. This section provides an elementary overview of electrical concepts and will introduce you to some common terminology.

What is meant by “electrical charge” ?

Electricity is made of electrons, which are tiny particles that have a negative charge. Electrical charge can either be positive (lacking electrons) or negative (having electrons). When two positively charged objects or two negatively charged objects come into contact, they repel one another. When a positive and a negative object come together, they attract one another. The charge difference between the two oppositely charged objects creates an electrical field. Check out Physics4Kids.com for a handy explanation of an electrical field in greater detail.

What is electricity?

Electricity is the flow of electrons. Those electrons exert a force and also have a flow rate. It may be helpful to think of it like a pipe full of water where the electrons are the water and the pipe is the wire. The force or pressure exerted by the electrons is measured in volts, and the flow rate, or current, is measured in amps. The wires through which the electrons travel also provide resistance, which affects current. There are two types of current: alternating current (AC) and direct current (DC). Together, current and force produce a certain amount of power, which is measured in watts.

What’s the difference between AC and DC current?

Alternating current (AC) flows back and forth in opposite directions in a regular pattern while direct current (DC) flows in only one direction. In AC current only, the frequency at which the flow changes direction is measured in Hertz (Hz), or the number of times per second the flow direction changes. The power company, or “the grid,” provides AC power to customers and that is the type of current that comes through your electrical outlet at home. DC current is stored in batteries and is also provided by PV arrays and windmills. Some electrical devices require DC current, such as those powered by batteries. Others require AC current, such as those plugged into the wall. Laptop computers, for example, are a special case. They require DC power because they have a battery. But when they are plugged in, the black box on your laptop cable converts AC power from the wall outlet into the DC power that is stored in the battery.

Describing direct current is easier to explain than alternating current because it can be thought of as water flowing through a pipe. We have everyday experience with such behavior, for example, when using a garden hose. We know that turning the flow rate up will cause more water to come out of the hose and turning up the water pressure will cause it to come out faster. The water can then drive a water wheel or other “load” that requires current and force to run.

For a circuit wherein the flow changes directions at regular time intervals, we have a harder time imagining the purpose of that. Therefore, the water analogy is not appropriate for describing alternating current.

To deeply understand how alternating current works, it requires proficiency with vector mathematics, which is beyond the scope of this website. Nevertheless, in solar systems, the DC power provided by the array must be converted into AC power in order to be useful in homes and buildings. This conversion is performed by a component of the electrical system called an inverter. To learn more about the benefits of AC power, follow this link to read more about how power inverters work.

Bringing it all together with Ohm’s Law

Ohm’s Law states that the current is directly proportional to the voltage and inversely proportional to resistance if temperature remains the same. Resistance, however, is constant and is related only to the type of wire, or conductor, used.

Figure 3. (Left) V=I*R and (Right) Watts = Amps*Volts or Power = Current * Pressure

What are the types of electrical circuits?

There are closed circuits, open circuits, and short circuits. A circuit is the path that electrons follow to produce power and do work (i.e. light a light bulb, run a hair dryer, etc). Below are some visual representations to make this explanation easier.

Closed and open circut

Figure 1. A closed circuit is a functioning circuit. An open circuit is when the path is broken somehow.

Short Circuit

Figure 2. Electrons will travel the path of least resistance. If an easier, alternate path is available the electrons will take it, resulting in a “short” circuit.

What are some common electrical system components?

There are many terms used to describe the parts of electrical systems. It is important to be familiar with them because you will see them in the field. A helpful activity to familiarize yourself with these terms is look these words up online and make flashcards! Each term is linked to a more detailed explanation.

  • AC Power: Current that changes directions
  • DC Power: Current that goes in one direction
  • Conductor: A material that electrons flow through
  • Insulator: A material that electrons do not flow through well
  • Conduit, Raceway, Enclosure: Coverings for piping or wires
  • Over-current device: Circuit breakers, fuses, or temperature sensors are examples of over-current devices.
  • Diode: An electrical component containing an anode and a cathode
  • Power Inverter: Changes DC power to AC power
  • Switchgear: Controls the flow of electrons through a circuit, allows isolation of some part of the circuit by switching it on or off
  • Transformer: Changes electricity from high to low voltage
  • Terminal, Connector: The point where wires come to an end or join
  • Grounding Equipment: Grounding directs electrical energy into the earth by providing a conductor that is less resistant than you are.
  • Resistor: Restricts the flow of current
  • Thermistor: Temperature-sensitive resistors
  • Inductor: Can slow and reshape alternating currents  in systems where current and voltage change a lot.
  • Capacitor: Two conductors separated by an insulator, stores energy, blocks DC but allows AC to pass
  • Test meters: Voltmeter (measures volts), ammeter (measures current), ohmmeter (measures resistance), watt-hour meter (measures the amount of electricity being used)

References

  1. VR-Zone: AC and DC current: Fundamental differences and a simple explanation. http://vr-zone.com/articles/ac-and-dc-current-fundamental-differences-and-a-simple-explanation/11194.html
  2. John Balfour, Michael Shaw, and Nicole Bremer Nash. The Art and Science of Photovoltaics: Review Guide for the NABCEP Entry-Level Exam
  3. Figure 1 Image Credit: http://www.cdn.sciencebuddies.org/Files/4911/7/closed-open-circuit-diagram_img.jpg
  4. Figure 2 Image Credit: http://www.cdn.sciencebuddies.org/Files/4912/6/short-circuit-diagram_img.jpg
  5. Figure 3 Image Credit: http://i132.photobucket.com/albums/q27/stesul411/ohmslaw3.gif

Further Reading:
For even more quality information on the basics of electrical systems, the Siemens corporation’s quickSTEP Training Program offers very easy-to-read instructions for numerous subtopics of electricity. (Note: this training material focuses on products made specifically by the company, but it is still a very useful primer for beginners.) Click here to Browse their PDF Downloads.


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