Insulating Materials

[soccersparks]

Ok thank you i think i got it. One other question though. What is in the palstic or any of the good insulating materials that makes the spark not go through it?
Thank you,
Soccersparks

[bradleyshanrock-solberg]

There are whole college courses on that topic, but lets see if I can get a few key concepts across.

There are three rough categories of materials. How they behave with electricity depends on the atomic structures. Atoms are made of protons and neutrons in the core or nucleus, plus electrons which kind of orbit around the nucleus. It is sort of like the solar system, with planets rotating around the sun...electrons would be the planets. (it's not a perfect match in concepts, electrons are more like a cloud around the center than solid objects going around in predictable orbit, but the basic idea is useful for thinking about it.

Conductors - most metals, for example. These materials have electrons that are weakly bonded to the atom as a whole....they can be "stolen" by another atom (or used kind of as glue to hold two atoms together, shared by both. They can also just move away completely when there are charge differences attracting elecrons in a direction. The concept is called "mobile charges". The atomic structure doesn't resist motion of electrons, at least not much.

Insulators - plastic, wood, air and most porous substances like styrofoam that have air in them. These materials have electrons that are more tightly bonded ("no mobile charges"). It's tough to get the electrons to move away from the basic crystal structure. Attempts to get them to move will just result in waste energy - heat, rather than electric current.

There is also a category of materials called semiconductors - at useful amounts of charge for the kinds of things built at human scale, these will sometimes be conductors, sometimes insulators. This switching behavior is very valuable in computers, which at the core are based on simple "on" and "off" operations, where "on" might be "current can flow" and "off" might be "current can't flow", switchable by small changes in electric current.

The distinction is arbitrary, as "Strong" or "weak" attraction depends on how big the charge difference is. Put a big enough charge on and any material will conduct electricity. On the flipside, even the best conductors have a bit of resistance and lose some energy as heat (and won't transmit very small charge differences).

In the end though, the weaker the electrons are attached to the atomic structure of the material (many materials have multiple components in them, so it's the structure as a whole that matters, not just the individual atoms in isolation), the better a conductor it is. Your insulators "hold onto" their electrons better than the air does. That's the best I can explain it without you taking courses in chemistry and physics that are at minimum high school level and likely college level. The math behind it is especially awful, especially down at the atomic level but the basic concepts that result from all that science aren't that hard to understand.

People figured this out by experiment, just like you're doing. They learned about the atomic attractions indirectly, long before they had tools to see what's going on at that level. Even now, the structures are so small that you can't really look at an individual electron. You can see atomic level structures, but not the components of atoms. (at those sizes, "looking" means either bouncing photons off things (reflection) or having them pass through things (like light through a window, or an X-ray)....and the photons have enough energy to knock the subatomic particles around...looking changes what you are looking at)

[Craig_Bridge]

The equation you started with is an approximation used by engineers and physicists that maybe dealing with pressure vessels or high altituded so it provides a crude adjustment for special pressure conditions that you probably don't require.

At sea level, the median atmospheric pressure of 30 millibars is defined to be 1 atmosphere. Anyplace where you measure the pressure at 30 millibars, you have 1 standard atmosphere of pressure!

The humidity will have far more effect on the validity of this approximation than any atmospheric pressure differences for most places on the surface of the earth so just use "1".

[soccersparks]

Hi,
I have a question. Are hollow objects good insulating materials? If so, why?
Thanks,
Laine

[bradleyshanrock-solberg]

A hollow object is a good insulator if it is made of something not very conductive but less conductive than air (or whatever is inside the object). If the material inside the object is nonconductive, the charge has to travel around the outside of the object instead of through the middle, so it has farther to go. Air is pretty good at insulating and a vacuum is excellent (no atoms means no free electrons to carry a charge)

The overall resistance of a circuit depends not only on the materials, but the distance involved. Hollow objects introduce greater distances. But if it is a hollow copper jar, it's still going to conduct well. A hollow plastic one will take more charge than a solid one of the same geometry to conduct electricity (and given the low melting points of plastic, it might melt from resistance heat before it conducts charge)

[Craig_Bridge]

(and given the low melting points of plastic, it might melt from resistance heat before it conducts charge)

Excuse me but resistance heating affects require current flow (The power producing heat is proportional to the current squared times the resistance).

Hollow objects in an Electric Field are very complex things to analyze and the geometry of the field and the object and how they interact can be extremely challenging. If you are really interested in getting an appreciation for the analysis involved, look up "dielectric" properties. The simple explainations start with a parallel plate capacitor where the charge density is uniform. If you alter the shape so there are sharp points like lightning rods, the charge density is concentrated which makes the dielectric field to be concentrated and more easily broken down. The geometry of each boundary between different materials has a dielectric field and charge density distribution associated with it. This makes things like thin non-conductive tubing perpendicular to an electric field a really complex problem to figure out if the breakdown or leakage will occur on the surface or by penetrating the surfaces or if the breakdown will occur independent of the object entirely by bypassing it.

[bradleyshanrock-solberg]

Right, Craig. You need current for heat. But you also need a medium to conduct the electricity for more than a brief period of time. (same principle as a fuse...if you overload it, the conductive surface is destroyed, usually by melting, which breaks the circuit).

If the plastic melts when the current finally overcomes the insulating properties of the plastic, the circuit won't exist very long. However it's a good point. If voltage is very high but current is low, plastic could conduct without melting, at least for a while. Voltage and current aren't the same - one causes the circuit to happen at all, the other is what generates power (or causes injury/damage). To use a hydraulic analogy, voltage might be the water pressure, but current is how much water moves through the pipes. A person can be exposed to enough voltage to create a large spark, but if current is low you will barely feel it. Relatively low voltages, that won't spark across much of a gap, can kill if the current is large. (when electricity grounds through a person, your tissues are the conductive medium. It has a resistance, which will generate heat. If heat is high enough, your tissues will burn, internal or external. Plus there is also the fact that electricity can interfere with things like the signals from your brain to other parts of the body, including the heart. It takes a lot less electricity to cause a heart attack than to cause serious injury by the burns)

And yes, current in hollow is very challenging to predict with any accuracy.

[soccersparks]

Hello Science Buddies,
You guys have been such a big help on my last project and now i have another one.
The topic is Asteroids Comets and Meteors. I am missing information and having trouble finding it. I need to know what the differences and Similarities are between them?
Thank you for all of you help!
Soccersparks

[agm]

Soccersparks,

There's quite a bit of info if you google "asteroids comets meteors" -- these looked like some of the best results:

http://www.ioncmaste.ca/homepage/resour ... f_CMA.html# (click the link to "Intro to Comets, Meteors and Asteroids Movie" -- it won't let me link directly)
http://discovermagazine.com/topics/spac ... ids-comets (recent research related to them)
http://curious.astro.cornell.edu/comets.php (looks like it describes the differences)
http://www.ioncmaste.ca/homepage/resour ... dule5.html
http://www.adlerplanetarium.org/cyberspace/cma/

What kind of project are you thinking of doing? Are you interested in looking at photos taken by telescopes, analyzing data about when they are observed, examining objects that have fallen to the ground...?

Good luck and hope that helps,
Amanda