Getting home after a long day of work, you might turn on the lights, get some food out of the fridge, and put on the TV to watch an episode of your favorite TV show to help you unwind. Many people never take the time to realize that all of these ordinary scenarios involve electricity. In fact, almost everything we do and take for granted requires electricity to function—even your car, in its most basic function, requires a battery. For many people, the mere mention of the word “electricity”, or the words “voltage” and “current”, are enough to send the brain into a combination of panic and indifference. Electricity is that concept that your teacher tried to explain to your class in high school for maybe a week or two, but somehow it made everyone even more confused about it than they were before. The two things you may have gotten out of that brief lesson: electricity is dangerous, and it’s magical—it’s a concept that seems to elude our common sense at every turn. And that’s precisely why I made the decision to switch from Chemistry to Electrical Engineering in college – partially out of the challenge, and partially due to simple curiosity. I wanted to know more about this strange force that controls virtually every facet of our lives. What I learned is that when it’s broken down, the basics of electricity are surprisingly simple. I’d like to share some of this basic knowledge in the hopes that many of you will learn something new, and that I might also help spark some of your own curiosity on the subject of all things electrical.
First, the very basics: Current (measured in amperes, or amps) measures movement of electric charge, or simply the flow of electrons in a wire or any conductor. Voltage (measured in volts) is a measurement of the difference in potential electrical energy between two points in a circuit—generally it is between one point and a “ground,” or a reference point that is perpetually at 0 volts. However, we sometimes measure the difference between two separate “hot” wires. The majority of household appliances and receptacles are held at a nominal voltage of 120 volts (that is, the difference between the “hot” wire and the ground is 120 volts). In order to create a current, you need two things: a continuous path for electrons to flow (a circuit) and a voltage difference in the circuit. This can be easily seen in a drawing of a typical simple circuit: There are several things to note here: the battery creates the potential difference (or voltage) in the circuit. Note the circuit, or continuous path for current to flow from the positive to negative terminal of the battery. This creates a flow of electrons in the circuit (strangely enough, the measure of current is always opposite the flow of electrons, from positive to negative - technically current measures the flow of POSITIVE charge). In this case, the lamp can be seen as a resistance – it opposes the flow of current.
There is a very simple linear relationship between current, voltage, and resistance in DC circuits. This is known as Ohm’s law: V = IR. In a DC circuit where any two quantities are known, the other can easily be calculated. The lower the resistance in a circuit, the more current will flow for any given voltage V. Copper wires are used in many circuits partially for this reason – it has an incredibly small resistance (1.68×10−8 ohms per meter of wire). On the other hand, the average resistance of the human body under dry conditions is roughly 100,000 ohms (quite large). If currents as low as 30mA can be lethal, what is the lowest lethal voltage for a person with resistance of 100,000 ohms? Disclaimer: human resistance can be as low as a few hundred ohms under certain conditions. DO NOT try this at home.The last thing I’d like to mention is the concept of electrical power. In its most basic form, the power (in Watts) absorbed by an electrical load (i.e. a light bulb, refrigerator, or any appliance which draws electrical power) is equal to Voltage * Current. For example, a 60 Watt light bulb rated for 120 Volts (typical) will draw 0.5 Amps of current. The light bulb not only requires approximately 60 Watts to function properly, but also requires a voltage close to 120 volts. Input voltages which are too high or too low will cause the light bulb to function erratically or not at all, and in some cases can also damage or destroy the circuit.When looking at a line with multiple loads (in reality, a circuit can feed multiple lights and appliances), it’s as simple as adding up the power requirements of each load. For example, if your circuit contains a 60W light, 120W light, and a 240W light, the total power draw on this circuit is (240 + 120 + 60) = 420W. If the nominal voltage is again 120 Volts, you can easily find the typical current you’d find on this particular circuit.
A circuit with 3 loads (or resistances). In a typical power circuit, the Input voltage would be higher, resistance lower, and current higher than in the figure. Note that larger loads = more current = lower resistances. However, it is important to differentiate the power absorbed by a load (which is useful) and incidental power lost or dissipated on the line before it reaches its destination. No conductor is perfect (even copper has resistance) and this resistance causes power to be lost on the line, which is unwanted (and something engineers try to limit as much as realistically possible). This power loss is calculated a little bit differently - . Although it may seem different, a quick look at Ohm’s law will tell you it’s exactly the same as above. This number should be as low as possible, and there are two simple ways to accomplish this – you can lower the resistance of the circuit (better conductors, decrease lengths of wire) and more importantly, lower the amount of current on the circuit (while keeping the same amount of power).
This is precisely why large transmission lines have such high voltages (some are over 500 kV), and also one of the reasons why the grid uses Alternating Current as opposed to Direct Current – but perhaps I should leave that for another discussion. It is my sincere hope that you understood most of what I said (I’d be happy to answer any questions you might have) and that I've encouraged some of you to learn even more about this unique and interesting subject. If you want to learn more, I recommend checking out http://www.allaboutcircuits.com/. With a working knowledge, electricity can be a safe and powerful tool – even if some of it can still only be best described as magic. Dan Camporese is a Staff Electrical Engineer in H2M's MEP group. He can be reached at firstname.lastname@example.org.