Electricity will naturally flow to the earth if there is a path. Current also flows along the path of least resistance. If a human body provides the path of least resistance, the current will flow through it. When an electric appliance has a plug with three prongs, one of the three prongs serves as the ground, or zero volts. The ground provides a conducting path for the electrons to flow to the earth because the resistance traveling through the body would be greater than the resistance flowing directly to the ground.

Ground typically means the zero volts level, when making electrical measurements. Voltage is created by the separation of charges, which means that voltage measurements must be made between two points.

A water analogy helps to explain concepts of electricity. The higher the water and the greater the pressure, the more the water will flow. The water current also depends on the size of the space it must flow through. Similarly, the higher the voltage and the greater the electrical pressure, the more current will be produced. The electric current then encounters resistance that, like the water tap, reduces the flow. If the electric current is in an AC circuit, then the amount of current will depend on how much impedance is present. If the electric current is in a DC circuit, then the amount of current will depend on how much resistance is present. The pump is like a battery. It provides pressure to keep the flow moving. 

The relationship among voltage, resistance, and current is voltage (V) = current (I) multiplied by resistance (R). In other words, V=I*R. This is Ohm’s law, named after the scientist who explored these issues.

Two ways in which current flows are Alternating Current (AC) and Direct Current (DC). Alternating current (AC) and voltages vary over time by changing their polarity, or direction. AC flows in one direction, then reverses its direction and flows in the other direction, and then repeats the process. AC voltage is positive at one terminal, and negative at the other. Then the AC voltage reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive. This process repeats itself continuously.

DC always flows in the same direction, and DC voltages always have the same polarity. One terminal is always positive, and the other is always negative. They do not change or reverse.

An oscilloscope is an electronic device used to measure electrical signals relative to time. An oscilloscope graphs the electrical waves, pulses, and patterns. An oscilloscope has an x-axis that represents time, and a y-axis that represents voltage. There are usually two y-axis voltage inputs so that two waves can be observed and measured at the same time.

Power lines carry electricity in the form of AC because it can be delivered efficiently over large distances. DC can be found in flashlight batteries, car batteries, and as power for the microchips on the motherboard of a computer, where it only needs to go a short distance.

Electrons flow in closed circuits, or complete loops. Figure shows a simple circuit. The chemical processes in the battery cause charges to build up. This provides a voltage, or electrical pressure, that enables electrons to flow through various devices. The lines represent a conductor, which is usually copper wire. Think of a switch as two ends of a single wire that can be opened or broken to prevent electrons from flowing. When the two ends are closed, fixed, or shorted, electrons are allowed to flow. Finally, a light bulb provides resistance to the flow of electrons, causing the electrons to release energy in the form of light. The circuits involved in networking use a much more complex version of this very simple circuit.

For AC and DC electrical systems, the flow of electrons is always from a negatively charged source to a positively charged source. However, for the controlled flow of electrons to occur, a complete circuit is required. Remember, electrical current follows the path of least resistance. Figure shows part of the electrical circuit that brings power to a home or office.

Lab Activity Lab Exercise: Series Circuits In this lab, the student will build and explore the basic properties of series circuits.

     

Web Links Circuits http://www3.iptv.org/exploremore/energy/ Energy_In_Depth/sections/ circuits.htm