Myths About Electricity

[Updated 2/19/2017]

Electricity is always hard to understand. It can't be easily measured or observed, and knowledge of math and physics is usually required to have a decent understanding of its behavior. Because of that, lots of assumptions are made in an attempt to explain its behavior. Some of them are too oversimplified, while others are simply wrong.

Below is a list of the most common myths and misconceptions I usually hear or read about. 


1. Current takes the path of least resistance

We have all heard it. This oversimplification is commonly used to explain the behavior of electric current and oftentimes, electric shorts. In reality, current takes all paths in a circuit. Most current will take the path of least resistance, but some will still flow through the paths of lower resistance.

A good analogy to this would be a bucket full of water. If you drill small hole and a large hole at the bottom, water will leak from both holes. While most water will be coming out of the large hole, some water will still pour from the small hole.

This statement can be easily proven. If we build a circuit with a 9V battery and two resistors in parallel, R1=10kΩ and R2= 500Ω, R2 would be the path of least resistance. Using Ohm’s Law and some basic circuit analysis, we will find that 18mA will flow through R1 and 0.9mA through R2. Therefore, the path of higher resistance will still have a small current flow.

Fig 1. Parallel resistor circuit

2. Digital signals aren't always square

Computers, phones and most devices around us use some sort of digital interface. A digital system uses discrete values to represent information. Most digital systems are based on the binary system which uses electric signals at two different voltages (e.g. 0V and 3.3V) to represent the numbers 0 and 1. Because of that, a lot of people believe that digital electric signals are shaped like perfect square waves (figure 2).

For real circuits, we have to take into consideration the effects that the physical components (cables, connectors, electric noise, etc.) can cause on the signal since they can add small resistance, capacitance and inductance to the circuit. These variables cause signal degradation due to crosstalk, coupling, ringing, noise, attenuation, etc. depending on how the circuit is build. For low frequency signals, the waves can look perfectly square (figure 3). However, the degradation becomes more noticeable with higher frequency signals because the effects on inductance or capacitance are proportional to the frequency (figure 4).

So, how does this affect us? An electronics designer has to take signal integrity into consideration when creating a new device. Signal integrity can be compromised if the frequency of the signals is too high, if the impedance of the transmission lines is mismatched, if the cables used are too long, or if the PCB packs too many traces, has a poor ground layout or places data lines too close to sources of noise, among other reasons. These effects can ultimately degrade the signal to a point where the 1's and 0's can't be recognized anymore.

That's the reason why engineers have come up with different ways for preserving signal integrity, such as using circuits (filters, equalizers, twisted pair wiring, etc), communication standards (ethernet, bit encoding, SDI, etc) or even special cables (twisted pair, coaxial cable, RF conectors, etc.). This doesn't mean you need to buy overpriced cables for your home theater system though (figure 6).

Fig 2. 8-bit digital signal representing number 77

Fig 3. Low-speed digital signal shown in the oscilloscope

Fig 4. High-speed digital signals showing attenuation

Fig 5. Degraded digital signal

Fig 6. Impedance mismatches can attenuate digital signals. You still DON'T need one of these cables.

3. Batteries supply charge

After using a cell phone for an entire day, we “recharge” the battery. This term tends to confuse many people by giving the wrong idea that batteries store electric charge.

First of all, electric charge is present everywhere. Atoms have charged particles (protons, electrons) and therefore, copper wires and other circuit components have charge. It's a part of their atoms. This means that unplugging a battery won’t take the charge away.

Batteries stores chemical potential energy, which is then converted to electrical energy. As long as this chemical energy is available, the battery will maintain an electric potential (aka voltage). The electric potential simply “pushes” the charge already existing in the circuit just as a water pump pushes the water already present in the pipes (see figure 7).

In other words, batteries store energy, not charge. I know this is just semantics, but now you have something new to argue with people in the internet.

Fig 7. Water pump system compared to electric circuit. A battery works like a water pump by using energy to push the electric charge in the circuit.

4. It’s not the volts that kills, it’s the amps
[Updated - thanks to Mustang Irving]

Lots of people love to use this statement to show-off their knowledge of electricity. The main explanation I usually hear is that it only takes 10mA to kill a person, and that an electrostatic discharge can have thousands of volts but can’t kill people.

In reality, we need both and you can't have one without the other. If we take a look at Ohm’s Law (V=iR), we can see that voltage and current are proportional to each other. Therefore, assuming we have a constant resistance, we need to increase the voltage to increase the current flow. Also, for some materials, there's something called the breakdown voltage, where a certain voltage needs to be reached in order to conduct electricity at all (this is the main concept of semiconductors). We also need to take into account the amount of time electricity is being applied. Regarding the 10mA needed to kill a person, this occurs when that current flows through the heart. However, we need to keep in mind that the human skin has several thousand ohms of resistance and because of that, a very high voltage needs to be applied. We also need to add the fact that current can take different paths depending on the shape and composition of the conductor (i.e. our organs), as well as where the current is being applied. This explains why we can touch the terminals of a 12V car battery, which can put out more than 75A, without being electrocuted. Now, let’s explain why electrostatic discharges (ESD) don’t kill. An ESD is caused by the buildup of electrostatic charge on our skin and can have a potential of 7kV to 10kV, as well as a few Amps of current. However, because of how quickly it discharges, the total amount of energy going through you is negligible.
Physics time: The equation for energy is voltage x charge (E=V*q). The equation for charge is current x time (q=i*t). Thus, energy is the product of voltage, current and time (E=V*i*t). The length of an ESD is usually measured in the nanoseconds which results in tiny amounts of energy. Just as a reference, the IEC61000-4-2 ESD Safety Standard defines a human body discharge as a pulse with a 25ns rise time and usually completely dissipates within the 500ns. That's 1 second divided by 2 millions.

So, what does all this mean? While it is the amps that kills you, we won’t see any current without a voltage high enough to push all those electrons. Remember, while current is the charge flowing through a circuit, voltage is the force that pushes the charge. Also this needs to happen for long enough to actually cause any damage.

5. Free energy. Perpetual motion machines.

Anyone who has ever sat in a physics class knows that energy can’t be created or destroyed, only transformed. This is what we know as the 1st Law of Thermodynamics or the Law of Conservation of Energy. In other words, you can’t expect more energy coming out of a system than what you put in. Nevertheless, we keep seeing all these perpetual motion machines or devices capable of generating energy out of magnets or thin air. Since the appearance of crowdfunding websites like Indiegogo and Kickstarter, a lot of inventors have appeared asking for money to fund their perpetual motion machines. Sadly, all it takes is some pseudo-science, a good graphic designer and a good social media campaign to take money from the unsuspecting.

Fig 8. Examples of perpetual motion machines that won't work

Another crowdfunding campaign worth mentioning is the solar roadways project which thanks to an impressive video and social media marketing went viral and even got mentioned in several technology websites. The idea of the project is to build roads out of PV panels to power up street lights, signs, melt snow, etc. The PV panels would be protected by hard glass panels.

Why am I mentioning this project? Because the creator of the project is using solar panels under the worst possible conditions, both in terms of price (PV panels are expensive, hard glass panels are expensive, replacing or fixing a PV sealed under a glass panel on the floor is expensive) and efficiency (the glass panels can get dirty and block sunlight). Yet, it was able to collect more than $2.2M in donations. Solar panels do produce electric energy, but you can’t promise to produce more energy than standard rooftop PV panels, which happen to be placed in much better sunlight conditions, cost multiple times less and are easier to fix and maintain. One of the main goals in engineering is to be efficient, and this project is the total opposite to that.

Fig 9. Examples of crowdfunding projects that promised the impossible

6. Cell phones can give you cancer

First of all, we live surrounded by electromagnetic waves. These waves are emitted by TV and radio stations, cell phone towers, radars, wi-fi routers, satellites and the sun produce radiation. Even people produce EM wave (brain waves and infrared radiation). All these EM waves are what we’d consider non-ionizing radiation. Unlike ionizing radiation (x-rays, gamma rays, cosmic rays, higher UV, etc), non-ionizing waves don’t have enough energy to remove the ions from an atom to harm us.

The only effect that non-ionizing waves could have is to create heat. However, this only occurs if the waves have enough energy (such as the waves in a microwave oven), which an average cell phone or TV station transmitter is unable to create.

Additionally, according to the American Cancer Society, multiple research studies have found no evidence that cell phones can cause cancer.

What about microwave ovens? Just like as other non-ionizing EM waves, microwave ovens produce radiation that is absorbed by the water molecules in food. These water molecules vibrate and produce heat without altering their molecular configuration or becoming radioactive, as many believe. In other words, food is heated just as in any other cooking method. Also, a well designed microwave oven won’t let any waves leave its enclosure. The waves are blocked by the mesh in the oven’s door (like a Faraday cage), so staring at your food won’t give you brain cancer either.

Fig 10. If you are still concerned about cell phone radiation,you can always protect yourself.

7. Transformers can convert any voltage source

This isn't a common one, but every now and then someone will ask what transformer to use to increase or reduce the voltage of a DC power source.

Transformers only work with alternating current (AC). A transformer is made of a ferromagnetic core and two wires coiled around it. When a current passes through the input wire (primary), a magnetic flux is generated and absorbed by the core. At the same time, when a magnetic flux passes around a wire (secondary), the energy is absorbed by it as electric current. This wire can then be used as an output. Since the voltage is proportional to the number of turns coiled around the core, we can use this proportionality to increase or decrease the voltage of our AC signal. However, in order to create a magnetic flux, the electric current needs to fluctuate (alternate). This doesn't occur with DC current, unless you count the instants when you turn the power on and off.

To change the voltage level of DC current, we can use buck, boost or buck-boost converters. These circuits use a combination of inductors, capacitors, resistors and diodes in a circuit that is switched on and off at high speeds to produce different levels of DC voltages.

Fig 11. A basic transformer

8. Rubber tires protect car occupants from lightning

Rubber is a well known insulator. It is also known that your car is the safest place to be if you are outdoors during an electric storm. However, unlike popular belief, the tires are not the reason why people don’t get electrocuted in their cars. All insulators can conduct electricity if the voltage is high enough. This voltage is called breakdown voltage and once this threshold is reached, there is a rush of current and electricity simply flows.

Also, the electrical resistivity of rubber (what makes it a good insulator) is around 1000 times lower than air (according to Wikipedia) and in case you didn’t notice, we are talking about lighting bolts with enough force to travel through hundreds of feet of air (already a good insulator) before hitting the car.

The reason car occupants aren’t electrocuted is because the car acts as a Faraday cage, an enclosure made of a conductive material which channels electricity around it, but not through it. This can be explained by Gauss’ Law, which states that the electric charge inside a hollow conductor is zero, and therefore, the electric field inside is also zero. This is the same reason airplane passengers aren't electrocuted when the plane is hit by lightning.

Fig 12. A Faraday cage blocking electric bolts from Tesla coil

9. Ground

Understanding ground is often confusing. When we are taught circuits for the first time, a ground symbol is always connected to the lowest voltage rail; when we create a circuit in SPICE, the tool won’t run simulations unless you define a ground; modern appliances have a ground pin, however it uses the neutral line as a the return path for current.

So, what is ground? Whenever we analyze a circuit, we look at the voltages across the different components. The ground is simply a reference point used to measure the voltage across the different node in an electric circuit.

The term ground started being used after power companies started using the earth as a return path for their power distribution circuit. After all, everyone has access to it. Because of this, the term ground was coined as the reference point for electric circuits. By the time battery operated devices made an appearance, the industry was already using the term ground for the voltage reference. By the way, utility companies still use a neutral wire in transmission lines since the impedance is much lower than using the ground.

When designing wireless devices, we need to keep in mind that their grounds pins aren’t really connected and therefore, they don’t share the same reference. This can be easily fixed by connecting the grounds in the devices.

Fig 13. Besides handheld or portable devices, all systems are somehow connected to the earth.

10. Benjamin Franklin's kite was never hit by lighting

Ben Franklin was one of the Founding Fathers of the United States. However, he is better known by many as the scientist who flew a kite in the middle of a storm to prove the electric nature of lightning. The common belief is that the kite was struck by lighting, electricity went down the string and was absorbed by a metal key tied to the string... and that's as far as people know the story.

Many scientists still believe the experiment didn't happen at all. But for those who do believe, this was the real story. According to the legend, Ben Franklin flew a kite with a key tied to the string. The key was connected to a Leyden Jar, an early form of capacitor meant to absorb the electric discharge. For safety reasons, his end of the string kept dry to insulate him from the electric discharge.

Lightning never hit the kite, since the dry string section would have failed to protect Franklin from the tens of thousands of volts in lightning. People are electrocuted when lighting hits the ground several feet away from them. In fact, he though that his experiment had failed. However, Franklin observed that the end of the string was repelling another piece of string he had around and when he went to grab the key, he was shocked by a small electric discharge. This proved the existence of electric charge in the atmosphere during an electric storm, which at the time was a huge discovery.

Fig 14. Ben Franklin being shocked by the key at the end the kite string

11. Overhead power lines are not insulated

A lot of people believe that the overhead lines used to deliver electric power to our homes are insulated. After all, birds stand on them without being electrocuted.  The truth is that the great majority of lines aren't insulated and birds aren't electrocuted because they do not complete a circuit. Birds are only touching one line in a 3-phase circuit. If they touched two, the circuit would be closed and they would be toasted. The biggest power lines can have as much as 700,000kV and are usually placed on very tall towers. However, the lines that usually kill people are the ones with just a few hundred volts. The most common accidents occur when people do construction or backyard work and a ladder, a crane or some some other piece of equipment touches the power lines.

So, why aren't power lines insulated with rubber? The reason is very simple: cost.  

Transmission lines are designed to last for decades. However, the rubber insulation is only good for some years before it starts peeling off and needs to be replaced. Rubber is an expensive material and higher voltages require more insulation (around 5mm for every thousand volts). To that we have to add the cost of replacing the wires since the insulation can't be replaced on-site.

If we take into consideration the number of power lines going through an entire city, county or region, not using rubber insulation is still cheaper than using towers and glass or ceramic insulators.

So, make sure you keep an eye on those lines next time you decide to trim your trees.

Fig 15. Overhead power lines outside a house

12. Rubber gloves and shoes can protect you from electrocution

This is another common belief that could end up hurting people. If you read item 8 and 11 in this article, you probably already know what I am talking about. Insulators such as rubber have a property called breakdown voltage in which electricity is able to flow if the voltage is high enough. According to AllAboutCircuits the breakdown voltage of rubber is 450 to 700 kV per inch. What does this mean? Well, despite being less than 1mm thick, your kitchen rubber gloves may be good enough to handle the voltage in the mains power (or maybe not, depending on the materials used in the gloves). But for any high voltage application, that simply wouldn't work. The same can be said about rubber shoes. The main problem with shoes is that there's no guarantee that they are made of pure rubber and therefore the insulation properties may vary.

Fig 16. Do I really need to label this image?

13. Surge protectors can protect my electronics from lightning

This explanation just derives from the previous item so I'll be brief. A lighting bolt is an electrostatic discharge (ESD) where the voltage was so high that it surpassed the breakdown voltage of hundreds of feet of air. The few millimeter gap created when your surge protector is turned off is just a grain of sand compared to the distance already traveled by the discharge. The best solution during an electric storm is to unplug your appliances.

Fig 17. Standard surge protector