Batteries and Direct Current
Batteries have two terminals for attaching to a circuit. One terminal is positively charged, and the other is negatively charged. When the two terminals are connected together, current flows from the negative terminal to the positive terminal (this is because electrons are negatively charged). This current flows in only one direction and it is known as Direct Current (DC). A NiCd (Nickel-Cadmium) battery produces DC.

Alternating Current
Electricity found in common house outlets cycles back and forth. For a moment, it flows from one terminal, and then the next moment it switches and flows from the other terminal. This is known as Alternating Current (AC). A commercial generating station is an example of a source of AC.

Frequency
The rate that alternating current flows back and forth is known as frequency. Frequency is measured in Hertz (Hz). If the current from a generator was alternating at 60 cycles per second, then the frequency of that AC would be 60 Hz. Likewise, if a current was flowing back and forth 1000 times per second, the frequency would be 1000 Hz.

Sound and Audio Frequency
The sound we hear is made up of vibrations carried through the air. These vibrations also go back and forth at various rates. For example, when a violen player makes a single note, the string of that violin vibrates back and forth at a certain frequency. The air molecules around that string also go back and forth, and these air molecules bounce into other air molecules. The sound we hear are those last few vibrating molecules of air impacting our ear which then senses that sound.

The human ear can hear both high and low frequencies. The average range of human hearing is 20 Hz to 20,000 Hz. This range of frequency is also called the audio frequencies because the human ear can sense sounds in this range.

Since 20,000 Hz is the highest frequency considered to be an audio frequency, it is also the lowest frequency considered to be a radio frequency. But an audio signal (that travels via air) is different from a radio signal. Radio signals can travel through the vacuum in outer space. Sound cannot travel in a vacuum (where there is no air or other matter present).

An antenna radiates a signal in a similar fashion that the violin string did. As a radio signal is applied to an antenna, that signal is radiated into the space around it. The signal travels out into space as it continues to alternate back and forth as it did when it left the antenna.

All radio signals travel at the same speed (unless slowed down by matter). This speed is the speed of light. This is 186,000 miles per second. That's fast enough to go around the earth 7 times each second. An equivalent value commonly used in radio is 300,000,000 meters per second.

As a radio signal travels, it is alternating back and forth. If a signal was at a frequency of 300,000,000 Hz (i.e. alternating back and forth 300 million times per second), then it would complete a single cycle in one meter as it traveled at the speed of 300,000,000 meters per second.The distance an AC signal travels during one complete cycle is known as wavelength. Therefore, the wavelength of a signal at 300,000,000 Hz would be 1 meter.

As the frequency of a signal increases, it's wavelength becomes shorter because there are more cycles made as the signal travels (and the speed of the signal stays the same). Likewise, as the frequency of a signal decreases, it's wavelength becomes longer.

One easy way to remember this is to picture a piano or a harp. The short strings make the higher frequencies, and the long strings make the lower frequencies. Likewise, the higher radio frequencies have shorter wavelengths, and the lower radio frequencies have longer wavelengths.

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