July 24, 2015    Newsroom

Arriving home after a long day of work, you might turn on the lights, put some dinner in the microwave, and put on the TV to watch that episode of your favorite show to help you unwind – pretty typical. Many people never come to the realization that all of these scenarios involve electricity. In fact, almost everything we do and take for granted requires electricity to function (even your car requires a battery). For many people, the mention of this word, or the words “voltage” and “current” are enough to send the brain into a state of either panic or indifference.

Electricity is that idea that your teacher tried to explain to your science class in high school for maybe a week or two, but somehow made everyone even more confused than they were before. The two things you may have gotten out of it: electricity is dangerous, and it’s magical (one of those things may be true) – it’s a concept that seems to betray our common sense at every turn. And that’s precisely why I made the decision to pursue Electrical Engineering in college – partially because of the challenge, and partially due to simple curiosity. I wanted to know more about this elusive force that controls literally every facet of our lives. What I learned is that when it’s broken down, the very basics of electricity can be surprisingly simple. I’d like to share some of this basic knowledge in the hopes that many of you will learn something new, and also that I might help spark some of your own curiosity into this magical subject.

First, the basics: Current (measured in amperes) 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 function at a 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 (potential energy) difference in the circuit. This can be easily seen in a drawing of a typical simple circuit:

There are several things to observe from the diagram: the battery creates a potential difference (or voltage) in the circuit. Note the 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 light bulb 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 of these 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 largely 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 (0.03 A) 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. light bulb, refrigerator, or any appliance which draws electrical power) is equal to Voltage (V) * Current (I). For example, a 60 Watt light bulb rated for 120 Volts will draw 0.5 Amps of current. The light bulb not only requires approximately 60 Watts to function properly, but also requires the voltage to be relatively constant at 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.

In reality, a circuit can feed multiple lights and appliances. To determine the total power of any circuit, it’s as simple as adding up the power requirements of each load. For example, if your circuit contains a 60W light, 120W Computer, and a 180W fan, the total power draw on this circuit is (240 + 120 + 60) = 360W. If the nominal voltage is again 120 Volts, you can easily find the typical maximum current you’d find on this particular circuit. (360/120) = 3 Amps.

A circuit with 3 loads (or resistances). In a typical power circuit, theInput voltage would be higher, resistance lower, and current higher than seen in the figure. Note that larger loads = more current = lower resistances.

However, it is important to differentiate between the power absorbed by a load (which is useful) and incidental power lost before it reaches its destination. No conductor is perfect (even copper has resistance) and this resistance can add up and cause electrical power to be lost before its target destination, which is unwanted and something engineers try to limit as much as realistically possible. This power loss is calculated seemingly a little bit differently – P(Loss) = I^2*R. Although it may seem different, a quick look at Ohm’s law will tell you it’s exactly the same as above.

P = I*V

V = I*R

P = I*(IR)

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 I’ve encouraged some of you to learn even more about this unique and interesting subject, and I’d love to answer any questions you might have. 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.

– Daniel Camporese, Electrical Engineer