What is Electricity?
Electricity is the continuous movement of a sea of electrons (stream of electrons) from one point to another, through a conducting material such as a copper wire. An electric circuit is like a pathway through which an electric current (stream of electrons) flows. A circuit consists of a source, a conductor and a load (Resistance). See Figure.1.1 for an illustration of a DC circuit.
Figure 1.1: A DC circuit
DC Quantity
A DC Quantity is that which has a constant magnitude or size and flows in one direction, hence the term Direct Quantity.
A DC circuit consists of:
- DC Voltage source,
- Direct Current and
- Conductor (Resistance).
Before we look at each in detail, let’s use the analogy of a water tank to describe the three parts of a DC current, namely: voltage, current and resistance. Using this analogy,
- the charge is represented by the amount of water in the tank,
- the voltage is represented by the water pressure,
- the current is represented by water flow, and
- the resistance is represented by the friction of the hosepipe against water moving through it.
Let’s now look at each of the three parts of a DC circuit in detail.
DC Voltage Source
A DC voltage source is that potential or pressure that pushes electricity in a circuit in one direction. The direction of this electricity moves from a region of high pressure (potential) to a region of low pressure (potential), similar to water pressure building up in a hosepipe. The pressure causes the electrons to move through the wire to the area of positive charge. This potential energy is called DC Voltage. Its unit of measurement is the Volt usually abbreviated as “V”.
Direct Current
A current is defined as the flow of electrons through a conducting medium, such as a copper wire. There are two types of currents in use today. The Direct Current abbreviated DC, and the Alternating Current abbreviated AC. In a Direct Current, electricity flows in one direction only and the voltage remains constant. For example, batteries create a direct current because the electrons always flow from the negative side to the positive side. In Alternating Current, the flow of electrons constantly changes direction, sometimes going forward and then going back. The voltage for AC also changes all the time. In this book, we will concentrate more on the DC because, as we said in the introduction, the internal circuit of a solar PV system functions as a DC circuit.
A current is denoted by the Symbol “I” and its unit of measurement is the ampere (A). An electrical current can be compared to the flow of water through a hosepipe.
Conductor (Resistance)
A conductor is a material that allows electrons to readily pass through with little opposition or Resistance. In a non-conductor, atoms are arranged in a way that causes them to resist the flow of electrons. The tendency to resist the flow of electrons (Current) is called Resistance. It is usually denoted by the symbol “R”. An ohm is the unit of measurement of resistance and it is denoted by the symbol “Ω”
Power
Power is the rate of doing Work. Mathematically, it can be determined by multiplying VOLTAGE (V) by the CURRENT (I). A unit of power is expressed in Watts (W).
The mathematical formula of power is as follows:
Power = Voltage x Current (in watts)
P = V × I (W)
The Power triangle in Figure 1.2 can be used to determine one quantity given any two as follows:
- Power (P) = V x I (Voltage x Current in watts)
- Voltage (V) = P/I (Power divided by Current)
- Current (I) = P/V (Power divided by Voltage)
Energy
Energy is defined as the ability to do work. Electrical energy is the product of electrical Power (P) and Time (T) and it is measured in Joules. The formula for energy is as follows:
E = P × T
Where P is Power and T is time in seconds, then the units become Watt-Seconds, also known as JOULES.
Intermediate switch connection and wiring diagram