Thursday, July 3, 2014

Basic DC Electricity

If you're reading this, you're using electricity (unless you printed it out and saved it in hard-copy). Most of our toys and tools use electricity as well - it's hard to find things that don't come with lights and sound in today's world.
In the event of a "grid-down" crisis (tornado, hurricane, terrorist attack, EMP, solar flare, etc.) knowing how to work with basic electricity could make life a lot easier. Making light after the sun goes down with a reduced risk of fire, communicating (or at least being able to receive broadcast news), and being able to run small things like a fan or a laptop computer could be very handy.

I'm going to cover DC (Direct Current) electricity. Low voltage DC is safer to work with than AC (Alternating Current) unless you're trained to play with AC. I believe another of our authors is going to cover AC power in a future article, and since he has more training in that area I will leave it to him. Low voltage is defined as anything under 50 Volts, which is about the minimum it takes to shock you through dry, unbroken skin.

What is electricity and how does it work?

There are a few theories of electricity out there. I was taught, and prefer, the electron-flow theory. It states that electricity is the flow of individual electrons from one place to another through a conductor. As they move through the conductor we can make use of the effects that they have on the conductor and the fields that are generated by the movement of electrons. The easiest way to visualize this is to imagine that the electrons are water, the conductors are pipes and batteries/ generators are pumps. I'll use the water analogy as best I can - it doesn't really work too well with the field effects, but is sufficient for direct effects.

Some definitions and common practices.

Units of measurement

• Amp or Ampere - the base measure of electricity, 1 Amp = 6.241×1018   (1 Coulomb) electrons per second moving past a point.
• Volt - electromotive force, or "pressure". It takes 1 Volt to move 1 Amp of electricity through 1 Ohm of resistance.
• Ohm - resistance to the flow of electricity.
• Watt - a measure of work. 1 Amp of electricity at 1 Volt = 1 Watt of energy. Watts= Volts x Amps

Common wire colors

• Green -  More commonly seen in AC use, green is always the ground wire. "Green is ground the world around."
• Red - In DC use, red is the positive wire.
• Black - In DC use, black is the negative wire.

Circuit types

• Series - when you put batteries in your flashlight, they go in with the negative and positive  ends pointing in the same directions. This means they connect positive-to-negative and create a series circuit. In a series circuit, the voltage of each battery is added to the others in the circuit. In the water analogy, it would be the same as connecting the discharge of one pump to the intake of the next one and running them at the same time. You would have the same flow, but at a higher pressure.
• Parallel - when you hook up car for a jump-start, you connect the positive to positive and negative to negative. This creates a parallel circuit with an unchanged voltage, but the amperage available is the sum of the batteries. Using the pumps again, it would be the same as hooking two pumps to the same source and discharge points giving the same pressure but more flow.

Hardware

• Switches - make and break a circuit. Generally used to turn things on and off.
• Fuses - Unlike circuit breakers, fuses don't care if they're used on AC or DC circuits as long as their voltage rating is kept in mind. Fuses are a safety device and should be included in every circuit, if possible.
• Tape - Common black electrical tape is rated for 600 Volts. One layer of electrical tape is plenty for electrical safety when working with low voltage DC, but you may need more for mechanical safety (abrasion, weather, pulling, etc.)
• Wire - Wire sizing is best done by using a chart. Be aware that there are separate charts for AC use and DC use. DC power loses voltage in long wires faster than AC does. This is one of the reasons AC is the standard for commercial and residential use - the power plant can be further away from the point of use. The whole Tesla vs. Edison debate is grist for another blog.

Where does electricity come from?

Electricity is produced through 4 main methods.
• Field effect - When a conductor is moved through a magnetic field (or a magnetic field is moved past a conductor- same thing), electrons will flow through the conductor. This is how generators (DC) and alternators (AC) work.
• Chemical effect - Basically, electrons will flow from a material with a higher electron charge to one with a lower charge under the right circumstances. Your average car battery stores electricity in chemical plates that change as they trade electrons.
• Semiconductors - When excited, some semiconductors will produce electricity. Photovoltaic cells (solar cells) are a good example: when exposed to a source of light with the proper "colors" or frequencies of light, they will produce electricity.
• Physical effects - The most common physical effect is ionization, which produces a static (non-moving) electrical charge in an object or area. When the "pressure" (voltage) gets to a sufficiently high level, the electricity jumps to an object or area of lower "pressure". Lightning is the best example of static electricity in motion.
Next week I'll tie some of this together and give some examples of how this knowledge can be useful in times of need.