Q: What is Resistance? (Also referenced here are Voltage and Current)
A: Resistance (which is measured in Ohms) is the tendency of a material to resist (not prevent) the flow of an electrical current (which is measured in Amps). Electrical resistance is analogous to friction in a mechanical system. Friction, like resistance, converts energy to heat and dissipates it to the surrounding environment. Therefore, electrical resistance can sometimes be thought of as a braking or dampening mechanism for a circuit. And this is important because there is no true loss of energy. Instead the energy is converted and redirected. When enough current is impeded (i.e. resisted) this energy can even be converted directly into light which reduces the current flow. And this is precisely how a resistive wire in light bulb glows.
Several factors impact resistance. The nature of the material, its thickness and length, and the temperature all play a part. Resistance is naturally low in materials with high conductance, such as metals. Part of the reason for this is the strongly structured crystalline properties of metal and the ability for electrons to freely skip from the conduction band of one atom to the next, while encountering little resistance.
Conversely, it is high in materials that are insulators, such as plastic and rubber. However, not all highly structured materials have low resistance. If the structure is very uniform, but complex, then the electrons will still encounter numerous collisions making it more difficult for them to move through the material. Glass is similarly structured and as an insulator it has extremely high resistivity, but this is due primarily to the fact that there are virtually no free spaces to be occupied in their conduction bands that would allow electrons to move through them. They are too tightly packed.
So, in materials where the atomic structure is both well aligned to min. collisions and the electrons are loosely held, then there is less likelihood of the current flow electrons bumping into other atoms and creating resistance. In this case, we would say that the conductance is generally high, as is the case with metals and some liquids containing free ions. It is this “bumping” that creates disruption (aka resistance) and it is the “barricade-free multi-pathways” that encourage conductance.
Length therefore greatly affects resistance; the longer the length, the higher the resistance. Picture a material one atomic layer thick but incredibly long. There is only one pathway forward with lots of barrier bumps assuming this is a poor conductor. Now as the material is thicker, it frees up more pathways and it reduces the amount of barrier obstacles. So, thicker substances will generally have lower resistance and higher capacitance.
The so with this in mind, the longer a substance is, then there is more of a chance for increased “bumping” due to the obvious increase in overall atoms. The same is true of a substances thickness. The more of it there is, the more chance for resistance. But now imagine you are down South. It’s really hot and the heat makes you slow down and want nothing more than a tall glass of lemonade.
Well the same is true of our poor electrons. As the heat spikes from the barrier bumps and the material temperature increases so does the resistance. Those electrons just want a tall glass of lemonade. So materials with lower conductivity (i.e. high resistance) need higher voltage (or electromotive force) to get the job done and drive the electrons forward. And higher voltage means more power (aka a bigger battery or drive unit).
So this makes sense when looking at the definition of voltage. It is the amount of “potential energy” that electrons have relative to another point, usually called the "ground", which is defined as having a potential of 0 volts.
P = V x I = VI
Here, P is the power (measured in watts. Watts are equal to 1 joule per second of energy transfer), I is current (measured in Amps) and V is the voltage.
Thus Watts = Volts x Amps
A great analogy for this is by comparing it to a hose (i.e. the material (length/width) and a water tank that the hose is attached to. When the tank is full, there is more pressure on the column of water and the water comes out of the hose with greater force than when the tank is almost empty.
With electricity, if you increase the voltage (through a voltage stabilizer) (i.e. the water tank), you tend to send more current (i.e. water) through the resistive hose. And conversely, if the hose is bigger (i.e. the material is thicker and less resistive), then this has the same effect as delivering more water at a greater force than a thinner hose would. Everything is a tradeoff to find just the right mix of materials and properties that reduce cost and meet the performance needs.
The following formula sums it up nicely:
V = I x r = Ir
Here V is the voltage, ‘I’ is the flow of current in amps, and r is the resistance of the body.
So, voltage is the product of the amount of current flowing in a material and its resistance. In other words, current (amps) is the voltage divided by the resistance of the body. And Ohm’s Law states that R = V/I so if you know the voltage and current (which is already a factor of the resistance/conductance of a material), then you automatically know the resistance.
So is it current (amperage) or voltage that is dangerous? Well the answer is actually IT DEPENDS on the conditions. Why, well there is a couple of popular proverbs in electrical engineering worth repeating.
V will not kill you, but I will. And …
I will follow the path of least resistance.
Yet despite these truths Nikola Tesla, who invented AC current claimed that a few million volts of current could be sent through a body when the frequency is above 700Hz. Frequency is the speed at which the electrons are pumped in an AC circuit creating an alternating positive and negative waveform. DC (Direct current) has no frequency or wave form. It is a flat line constant and asynchronous signal, so the above proverbs are spot on. Nikola stated that anything over 700 Hz simply flowed over the body. Fascinating if true!
In fact, almost any voltage can be withstood as long as the person never makes a connection to ground so this is generally why V will not kill you. It is merely a potential. This is how birds sit on electrical wires. It is the ground (or connection to ground) that releases the voltage potential and allows the flow of electrical current.
In addition, the amount of time that a current is passing through your system is critical. A brief moment can sometimes be tolerated but too long and it is catastrophic. People have been struck by lightning, which can pass anything from 100 to 1000 amperes of current through the body, and they have survived. Remember, voltage is the “potential” that is standing still, while current (amperage) is the voltage now moving to ground and it is the voltage that determines the force of the amperage.
A good way to look at it is, voltage is the caliber of the bullet and the amperage is the bullet being fired. So, much like a bullet when the amperage passes through something, that object or substance creates resistance. And the more resistance it encounters the more damage it can do. If the force and heat generated are too strong and too sudden, then object could quite literally blow up. This is often seen when a tree is struck by lightning. Trees are poor conductors and great insulators, hence they have high resistance. A sudden large current flow due to the massive voltage drop to ground leaves the force of the energy wanting to go in all directions, so the tree explodes. A person on the other hand is a relatively good conductor of electricity and therefore they have less general resistance so surviving a lightning strike is possible.
But in general, when not floating in air, being struck by lightning, or being hit with a bullet, it is safe to assume that as the flow of the electrons increases, so does the risk of injury. If the voltage passing through our bodies is at a low rate of speed (i.e. low current/flow) then we barely feel a thing. But when they flow is at very high rate of speed due to an increased potential and sudden release, and then bad things happen. As long as the electric charges pass through your body at a rate that is less than 1/1000 of an ampere (or one milliamp), they're not dangerous. But any voltages over 45 volts can be deadly.
Generally it is believed that larger voltages will do more harm than smaller ones. But in reality it is the current that flows between the two points that is ultimately responsible for the Electric Shock. It is not the voltage per se but the actual flow itself leading to ground. As the voltage potential increases the outcome for a larger current flow also increases, especially if the resistance of the object remains constant. This is because of the above formula: V = IR, higher the voltage; stronger the punch.
So if the resistance of the material is sufficiently low (for example a human with wet feet or hands) then even a small amount of voltage can produce enough current to create a decent shock. There's no widely accepted standard for safety on this matter but it is safe to assume that anything beyond 40mA is dangerous.
The minimum current a human can feel depends on the current type (AC or DC) and the frequency (assuming AC).
In general for a normal 68 kg (150 lb.) human applying (AC at 60 hz):
1mA (mill Ampere) or (.001A) of current can be felt. This is perception level creating a slight tingling sensation. It can still be dangerous under certain conditions.
5mA of current can be disturbing or painful given ones tolerance level. An average individual can let go. However, strong involuntary reactions can lead to other injuries.
6mA - 16mA will cause a painful shock and a person will begin to lose muscle control. This is commonly referred to as the freezing current or “let-go” range.
17–99mA, will cause extreme pain, respiratory arrest, severe contractions and individuals cannot let go. Death is possible.
100mA – 2,000mA will trigger ventricular fibrillation (uneven, uncoordinated pumping of the heart.) Muscular contraction and nerve damage begin to occur. Death is likely.
> 2,000mA will cause cardiac arrest, internal organ damage, and severe burns. Death is almost certain.
Alternating current (AC) and Direct current (DC) have slightly different effects on the human body, but both are dangerous above a certain voltage. As the voltage increases our body resistance drops. Essentially the body is less immune to the force and time that it can withstand the force. In a sufficient enough amount of time, the current can quickly tear down the skin and create a more favorable low resistance pathway which allows the high current to pass unimpeded and increases the risk of death.
An average normal human (under * normal conditions) above the ground level can withstand a voltage of about 26V DC & 60 V AC since 1 DCV ≈ 2.23 ACV. * Normal conditions refers to resisted, dry skin, no direct earth contact, low amperage (around 10 mA) & normal functioning of body. Note that since the magnitude of AC is roughly 2 times that of DC, it is generally viewed that AC is more dangerous since more DC current (roughly two to four times greater) is required to induce the same harmful effects as AC. With DC it is comparatively easier to “let-go” of ‘live’ parts that have been gripped, which is contrary to popular belief and experimental evidence. And DC does not affect the heart as badly as AC due to the AC waveforms that trigger the disruption.
Think of this as well, a person can discharge on the order of 50,000V when they get zapped from a carpet or from touching furry clothes. But they don't feel much because of the very small current (flow) that occurred in a very short-burst of time as it discharged.
On the other hand, a person will get on heck of a painful shock if they touch 220/110V mains, because of the larger current. Either of these uninterrupted will almost certainly cause death. In general, voltages above 50V are capable of delivering enough current to kill providing the skin resistance at the time is low enough and there is a suitable return path.