An electromagnetic wave can have an electric field magnitude of more than 1000 V/m, which is a very high potential difference. When the light shines on you, why don't you feel an electric shock?

  • $\begingroup$ there’s something inconsistent, as you speak of potential difference but correctly quote the units of $\vec E$ as V/m. What of $1V$ over $1/1000$m? that’s still $1000 V/m$ but not much of a voltage difference. $\endgroup$ – ZeroTheHero May 3 at 3:03
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    $\begingroup$ FWIW, the breakdown voltage of air is about $3\times 10^6\ {\rm V/m}$, so 1000 V/m is well short of what's needed to cause a shock. $\endgroup$ – The Photon May 3 at 5:01
  • $\begingroup$ The peak electric field of sunlight is about 300 $V/m$, so yes 1000 $V/m$ is pitifully small and not a useful number to think about. I agree with @ThePhoton that the breakdown voltage of air is a more useful number to think about $\endgroup$ – KF Gauss May 4 at 3:59

When the light shines on you, why don't you feel an electric shock?

I am posting a new answer to your question because, in my opinion, there appears to be a lot of confusion about what “electric shock” means.

The short answer to your question is if it doesn’t cause electric shock, it is because the field from the light does not produce enough current in the body to cause “electric shock”.

But we need to be clear on what we actually mean by “electric shock”. In the safety community (in which I had the privilege of working in for 40 years) the term is normally associated with what the potential harmful physiological effect of current through the body is.

First of all, in spite of all the discussions of “high voltage,” it is not voltage that harms you. It’s the current through the body that the voltage is capable of producing that can harm you.

Typically, the physiological effects of current in the body can vary depending on the magnitude, frequency, duration and other factors. The following is only a brief overview.

1. Perception- For example, a tingling sensation, feeling of pins and needles, etc. For 60 Hz sinusoidal current, the levels can range from hundredths to tenths of a milliampere. These levels are generally not considered harmful. People have reported these sensations underneath high voltage utility lines where the voltage per meter is very high.

2. Startle Reaction- This means involuntary muscular contraction. For sinusoidal ac current the threshold is generally considered to be 0.5mA AC rms (0.7mA –peak). This level does not directly cause injury to the body. The potential concern is for the consequences of the involuntary contraction being injurious. An example is operating a hand held power saw and loosing control of it because of the reaction. Loss of control could result in a mechanical injury.

3. Muscle Tetanization- When the current gets higher, it can result in what is sometimes called “muscle freezing” or tetanus. Popular depictions are a person unable to let go of something he/she is gripping at the time the current flows. The range for 60Hz rms current for this effect may be as low as 5 mA for children and up to 10 mA for adult males. It varies with individuals. The consequences can also lead indirectly to injury like startle reaction.

4. Severe Electric Shock- At greater currents the body may suffer from life threating effects, such as ventricular fibrillation and cardiac arrest. The magnitude of 60 Hz sinusoidal current that can cause these effects will depend on the current path through the body. Current density through the heart is the critical factor.

I apologize if this is a bit long winded.

In closing, I want to make it clear that I am not saying the other answers are necessarily wrong. All I am saying is that whatever scenario is presented to you, you should ask what is the current in the body and what are the physiological effects of that current that they consider to constitute "electric shock".

Thanks for your consideration and hope this helps.

  • $\begingroup$ "...the field from the light does not produce enough current in the body to cause “electric shock”." So you are just saying that ordinary light (sunlight let's say) just isn't strong enough to shock anyone, but could if it was stronger? $\endgroup$ – KF Gauss May 4 at 4:02

It depends heavily on the frequency of the light!

If this light is on the scale of kHz, then you will definitely get shocked. But when you talk about optical light in the petahertz range, then the electric field changes direction every few hundred nanometers spatially and every few femtoseconds in time. This means that arcing that happens when you get shocked can't occur! The electric field would switch direction billions of times along the arc, so no shock can be sustained.

The fundamental equation here is $$\lambda = \frac{c}{f}$$

So as frequency increases, the wavelength gets shorter and you no longer can have shocks which occur on the centimeter/millimeter scale.

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    $\begingroup$ You will NOT definitely get an electric shock. It depends on the current available from the voltage source. $\endgroup$ – Bob D May 3 at 20:38
  • $\begingroup$ Do you have anything to back up your statement? $\endgroup$ – Bob D May 3 at 20:53
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    $\begingroup$ @Bob D you can put a fork into a microwave and get arcing! It doesn't make sense to talk about "current available" from the source in this case, just the amount of power in the radiation. $\endgroup$ – KF Gauss May 3 at 21:29
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    $\begingroup$ What does that have to do with it. You can get a substantial arc (several mm arc through air) from an electrostatic discharge from your body that involves thousands of volts, without getting injured. That's because the current is too low. You can't use arcing alone as a criteria for the potential for harmful electric shock. $\endgroup$ – Bob D May 3 at 21:53
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    $\begingroup$ @Bob D no one said anything about "harmful" or "unharmful". The question is simply asking when do electric shocks occur. My point is that frequency of light that determines it. You can see shocks happen with low frequency light, but not high frequency $\endgroup$ – KF Gauss May 3 at 23:25

You are talking about two different things:

  1. electricity, an electric shock, that is caused by an electric current, electrons traveling in space between two points in space (inside a medium or between two mediums)

  2. when light shines on you, you are asking why you do not feel an electric shock. In the case when light shines on you, it is photons that travel in space, between two points in space (inside a medium or between two mediums)

So basically an electric shock is caused by electrons traveling, but when light shines on you, that is caused by photons traveling.

Now when light shines on you, photons interact with your body's atoms. These photons cannot cause an electric shock, because to have an electric shock you need electrons to travel.

When light shines on you, these photons might be absorbed by atoms and electrons in you body, but these photons rarely cause electrons to move off (get knocked off) the atom. Usually the electrons just move to a higher energy level around the nucleus as per QM. To have an electric shock, you need electrons that are loosely bound (delocalized) and can travel freely from atom to atom. Photons cannot cause that. Photons might cause some of the electrons to be knocked off the atoms but not to travel freely and create electricity.


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