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Voltage versus current

by Jimmy87
Tags: current, versus, voltage
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Jimmy87
#1
Apr1-14, 02:50 PM
P: 131
Hi, please could someone help me clear up an issue I am having of whether or not voltage can kill you. I know there are numerous threads on this but they don't quite answer my question. This crops up all the time in lots of forums but I was wondering if I am missing something because as far as I am concerned voltage can NEVER kill you because it isn't anything physical.

If I understand voltage correctly then voltage is the difference in energy per charge between two points. Therefore voltage is simply a numerical description of the difference in energy per charge. It therefore surely cannot physically exist. Energy is real and charge is real but voltage is not a physical thing. How can a description of something kill you. Surely its the ENERGY that kills you?

Looking at an analogous situation, if you divide gravitational gravitational PE by mass then you get gravitational 'voltage' which is the energy per mass (instead of energy per charge). If you dropped a rock from a height it could kill you if it landed on your head. This is because the rock has mass and energy which are real things. To say that the energy per mass killed you would be ridiculous because it isn't anything real. Likewise, the energy per charge (voltage) surely cannot kill you?

Is what I have said correct or not?

Thanks for anyone who can take the time to respond.
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stevendaryl
#2
Apr1-14, 03:24 PM
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I don't agree that "voltage" isn't real, but you are right that voltage won't harm you if only a tiny amount of current flows across that voltage. A Van de Graaff generator produces really huge votages--over 100,000 volts--but isn't very dangerous, because only a tiny amount of current flows across that voltage when you touch it.
Jimmy87
#3
Apr1-14, 03:41 PM
P: 131
Quote Quote by stevendaryl View Post
I don't agree that "voltage" isn't real, but you are right that voltage won't harm you if only a tiny amount of current flows across that voltage. A Van de Graaff generator produces really huge votages--over 100,000 volts--but isn't very dangerous, because only a tiny amount of current flows across that voltage when you touch it.
Thanks for the reply. I understand that a potential difference is needed but the p.d. itself is just a mathematical model telling you the difference in energy per charge. If you developed a microscope powerful enough you could draw a picture of an electron because its a physical thing. You could never draw a picture of voltage because it isn't physical its just a comparison of energy between two points.

BiGyElLoWhAt
#4
Apr1-14, 03:43 PM
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Voltage versus current

Voltage itself can't kill you. It's the current that kills you. V=IR and the average resistance of the human body is (depending on the path the current takes) between 2k ohms and about 120k ohms, and 15 mA through your heart can kill you. I'll let you work out the math. Again, voltage in itself cannot hurt you, only current (which can and most certainly does result from a voltage)
Drakkith
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Apr1-14, 07:57 PM
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Quote Quote by Jimmy87 View Post
Thanks for the reply. I understand that a potential difference is needed but the p.d. itself is just a mathematical model telling you the difference in energy per charge. If you developed a microscope powerful enough you could draw a picture of an electron because its a physical thing. You could never draw a picture of voltage because it isn't physical its just a comparison of energy between two points.
In the end, it is the electromagnetic force that causes charges to move. Voltage is merely a way of describing the properties of the EM field of the circuit that makes it much, MUCH easier to work with than trying to do something like field equations. (Or whatever equations would be needed)
UltrafastPED
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Apr2-14, 12:35 AM
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I've had 28,000 volts discharge from a defective cable through my ear. It did make my hair stand on end, but other than that there was only a small red spot.

This is because there was less than 1 joule of total energy.

It is the _energy_ transferred through your body which causes damage. For AC the power is V x I, energy per second. So if the produce V x I is greater than 10, and the contact is more than 1 second - then you will accumulate damage. For example, if the discharge is from your hand, and out through your feet, or through your other hand - then it passes through your heart.

But their are many physiological effects, so best to keep your hands off of the live connections.

Some references:
http://www.physics.ohio-state.edu/~p...l_current.html

http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

https://www.youtube.com/watch?v=XDf2nhfxVzg

http://en.wikipedia.org/wiki/Electric_shock
Jimmy87
#7
Apr2-14, 07:26 AM
P: 131
Thanks for the answers people. I just thing it's fundamentally wrong to say voltage is something physical. It all boils down to energy in my view and this is DIRECTLY the cause of what kills you. Imagine my friend told me to go to the shops and get him a newspaper but I got run over by a truck and killed. My friends instruction would be analogous to the voltage. It is required for me to have got hit by the truck but it would seem ridiculous to say I got killed by his instruction. Similarly, voltage is simply a mathematical model that humans have used to explain electricity. You need a voltage for charges to do work but at the end of the day voltage is just a number whereas charge and energy are REAL.
stevendaryl
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Apr2-14, 08:02 AM
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Quote Quote by Jimmy87 View Post
Thanks for the answers people. I just thing it's fundamentally wrong to say voltage is something physical. It all boils down to energy in my view and this is DIRECTLY the cause of what kills you. Imagine my friend told me to go to the shops and get him a newspaper but I got run over by a truck and killed. My friends instruction would be analogous to the voltage. It is required for me to have got hit by the truck but it would seem ridiculous to say I got killed by his instruction. Similarly, voltage is simply a mathematical model that humans have used to explain electricity. You need a voltage for charges to do work but at the end of the day voltage is just a number whereas charge and energy are REAL.
In electromagnetism, the electric and magnetic fields are definitely something real. Light is a fluctuating electromagnetic field. The electric field has an energy density, even if there are no charges around to "feel" the electric field. It takes energy to create an electric field.

Voltage is simply the integral of the electric field. When you connect a wire to a power source, it creates an electric field inside the wire, and the magnitude of the field is related to the voltage by: [itex]V = EL[/itex] where [itex]V[/itex] is the voltage drop from one end of the wire to the other, [itex]E[/itex] is the average electric field in the wire, and [itex]L[/itex] is the length of the wire.
nasu
#9
Apr2-14, 08:45 AM
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Quote Quote by Jimmy87 View Post
Thanks for the answers people. I just thing it's fundamentally wrong to say voltage is something physical. It all boils down to energy in my view and this is DIRECTLY the cause of what kills you. Imagine my friend told me to go to the shops and get him a newspaper but I got run over by a truck and killed. My friends instruction would be analogous to the voltage. It is required for me to have got hit by the truck but it would seem ridiculous to say I got killed by his instruction. Similarly, voltage is simply a mathematical model that humans have used to explain electricity. You need a voltage for charges to do work but at the end of the day voltage is just a number whereas charge and energy are REAL.
Energy is also a physical quantity and a mathematical model. Just like the voltage.
What you call "physical" may be a matter of your choice in some sense.

You can think in terms of phenomena rather than quantities.
But is still not straightforward. You could say that it is the electric current (the phenomenon, the charge displacement, not the intensity of the current) that "kills". But if the current would pass without disrupting some biological processes it won't kill. It is the muscle contraction that kills? Or the heat developed? Or the interference with the nerve signals to the heart muscle?
It's not so meaningful to look for a simplistic answer of the type "voltage kills" or "current kills".
sophiecentaur
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Apr2-14, 11:00 AM
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Quote Quote by Jimmy87 View Post
Thanks for the answers people. I just thing it's fundamentally wrong to say voltage is something physical. It all boils down to energy in my view and this is DIRECTLY the cause of what kills you. Imagine my friend told me to go to the shops and get him a newspaper but I got run over by a truck and killed. My friends instruction would be analogous to the voltage. It is required for me to have got hit by the truck but it would seem ridiculous to say I got killed by his instruction. Similarly, voltage is simply a mathematical model that humans have used to explain electricity. You need a voltage for charges to do work but at the end of the day voltage is just a number whereas charge and energy are REAL.
Field and Potential are both equally valid ways to approach 'Electricity'. It depends upon what situation you are trying to deal with. With a 6V battery and two wires, you could produce a field of 1V/m, .001V/m or 1000V/m, merely by altering the separation of the ends of the wires. What use would the notion of Field be in that case?
The Voltage is not 'just an instruction'. It is the energy the battery or generator is producing for each coulomb of charge. When you tell your friend to do something, you are not supplying any energy (unless you kick him out of the door) It is the truck that supplies the energy to do the damage. The electrical analogy of your instruction is the instructions about how to build the circuit in the first place.

I don't think you will get anywhere if you insist that Energy is any more real than Voltage. You should read about it more with an open mind. Maths is only a model of anything but it is an excellent way of describing the 'how' but not the 'why' of life. The 'why's of life are not explainable, ultimately.
analogdesign
#11
Apr2-14, 11:16 AM
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One thing to ask yourself is whether you think gravity is "real". You can't get a microscope and see gravity either.
dauto
#12
Apr2-14, 11:17 AM
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Quote Quote by Jimmy87 View Post
Thanks for the answers people. I just thing it's fundamentally wrong to say voltage is something physical. It all boils down to energy in my view and this is DIRECTLY the cause of what kills you. Imagine my friend told me to go to the shops and get him a newspaper but I got run over by a truck and killed. My friends instruction would be analogous to the voltage. It is required for me to have got hit by the truck but it would seem ridiculous to say I got killed by his instruction. Similarly, voltage is simply a mathematical model that humans have used to explain electricity. You need a voltage for charges to do work but at the end of the day voltage is just a number whereas charge and energy are REAL.
All physical quantities are based on some mathematical model and equations. Voltage isn't more real or more physical than energy in any meaningful way.
Drakkith
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Apr2-14, 11:38 AM
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Quote Quote by dauto View Post
Voltage isn't more real or more physical than energy in any meaningful way.
I'm not so sure. Voltage is a difference in electric potential, which is analogous to a difference in gravitational potential between two points at different heights. Both are "real" in the sense that a difference in potential exists, but I'm not sure i'd go further than that.
phinds
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Apr2-14, 12:35 PM
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Quote Quote by dauto View Post
All physical quantities are based on some mathematical model and equations. Voltage isn't more real or more physical than energy in any meaningful way.
I'd say that you have that exactly backwards. The math models arise because of the fact that they describe reality. The reality is not in any way based on the existence of a math model to describe it.
sophiecentaur
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Apr2-14, 01:04 PM
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Quote Quote by phinds View Post
I'd say that you have that exactly backwards. The math models arise because of the fact that they describe reality. The reality is not in any way based on the existence of a math model to describe it.
Maths models describe / predict the relationship between measurable quantities. Whether or not that is 'reality' is another matter. But this is very philosophical; when you drop a brick on your foot, the sensation is 'real' enough but at what level (macroscopic / molecular / quantum) would you say the situation was real? You can simulate 'weight' with a magnetic force so that an observer could not tell the difference. What is reality then?
People tend to use the word 'real' to mean 'familiar' and I don't, personally, go along with that.


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