Don Cass

January 9, 2018

January 9th = Static Electricity Day!

Filed under: Uncategorized — doncass @ 11:30 am

Who knew such a holiday existed? But it makes sense. Not because of how it makes hair unruly and gives people uncomfortable little zaps – but because such an “innocent” little thing powers so much of out modern world.  So…I thought I’d take the opportunity to share some of my thoughts on the subject…

Most people know that while an object’s mass is conserved, its weight depends on the mass of the object to which it’s attracted. For example, a 1kg object is attracted to the earth with a force (weight) of 9.8 Newtons, but it’s attracted to the less massive moon with a force (weight) of only 1.6 Newtons. But the cool thing is that not all 1kg objects are equally attracted to an object like the earth – because there is force besides gravity that influences how strongly two objects are attracted to each other: electromagnetism. For example, if your 1kg object is “electrically charged,” it may be pulled away from the earth by an oppositely charged object – making its net attraction to the earth less than 9.8 Newtons. Or it may be repelled toward the earth by another object with the same charge, making it’s net attraction to the earth greater than 9.8 Newtons. What I find even more amazing is that, unless you’re dealing with objects as massive as “the earth,” such “electromagnetic” forces are usually stronger than gravitational forces. On the atomic scale, such electromagnetic forces are much more important than gravitational forces. Indeed, some people feel that chemistry is nothing but “applied electromagnetism.”

Historically, it’s believed that the first observations of electromagnetic phenomena occurred in ancient Greece. “The story” is that around 900 BC, a Greek shepherd in the region known as Magnesia (red on the map to the right) observed that some black minerals that he walked on pulled iron nails out of his shoes and the iron tip off of his staff. magnesiaIt took another 300 years for Mr. “earth floats on water” Thales to record the fact that if you rub some materials (like amber – elektron, in Greek) with animal fur, they gain the ability to mysteriously attract small objects like pieces of straw or feathers. Such attractions were certainly not gravitational or they would be independent of the type of rock you walked on or whether or not you had rubbed something. These attractions were something quite different…

Around 1600, Gilbert tested a wide array of materials and called those which shared amber’s ability to attract other things when rubbed “electric.” I highly recommend that, regardless of how people around you may stare, you try rubbing a bunch of different materials with fur (aka “your hair”) & see which become “electric.” Here’s Gilbert’s list:

Electric: diamond, sapphire, carbuncle, iris stone, opal, amethyst, vincentina, English gem (Bristol stone, bristola), beryl, rock crystal, glass, artificial gems made of (paste) many fluorspars, belemnites, sulphur, mastich, sealing-wax [or lac],

Weakly electric: Salt, mica, rock alum. and orpiment

Non-electric: emerald, agate, carnelian, pearls, jasper, chalcedony, alabaster, porphyry, coral, marble, flint, bloodstone, emery or corundum, bone, ivory, hard woods (ebony); some other woods (cedar, juniper, cypress), metals (silver, gold, copper, iron), loadstones

In 1629, Niccolo Cabeo noticed that if two electric bodies which attracted each other were allowed to touch, they repelled each other – so “electric” forces could be repulsive as well as attractive.

generatorBy the late 1600s, people were rubbing different materials together to generate large amounts of ‘electricity’ which could produce impressive sparks. Some of these (as when a glass globe was rotated touching a hand or sulfur) produced light that was bright enough to read by – sort of the 1st “electric light.”

In 1729, Stephen Gray Stephen Gray showed that electricity  could also be transferred from one place to another with some materials (metals) but not others (silk thread).

In 1733, the French chemist Charles-François de Cisternay du Fay proposed that rubbing could create either of 2 types of ‘electric fluid.’ He said that when amber was rubbed with fur it acquired “resinous” electricity and that when glass was rubbed with silk, it acquired “vitreous” electricity. Each of these types repelled a suspended piece of the same type of rubbed stuff and attracted a suspended piece of the opposite type:static forces

When something with resinous electricity touched something with vitreous electricity, their electric strengths decreased.

What was really cool was that anything that du Fay rubbed with anything either had no effect on his suspended rods or behaved in the same way as either the rubbed glass or the rubbed amber. No rubbing of anything by anything ever produced a material that was attracted to or repelled by both the amber and the glass rods (The strength of the attraction or repulsion may vary, but its basic attractive or repulsive character does not.)

This means that any rubbed object was always in one of three states:

i) resinous = attracted to rubbed glass and repelled by rubbed plastic
ii) vitreous = attracted to rubbed plastic and repelled by rubbed glass
iii) neither attracted nor repelled by rubbed glass or rubbed plastic.

But were “resinous” and “vitreous” really the best names for two such states of varying strengths that when touched become smaller? In 1751, Ben Franklin realized that these properties were just like those of the numbers:

• numbers come in 3 forms (positive, zero and negative) two of which (+ and -) can have varying sizes
• if the opposite types (positive & negative numbers) combine, they become smaller (7 + (-3) = 4)

Because of this, Franklin suggested replacing “vitreous” and “resinous” with “+” and “-.”

WHAT’S GOING ON: Early scientists thought that friction actually “created” the “electricity” (their word for charge). They did not realize that an equal amount of opposite electricity remained on the fur or silk used to rub an object. Proteins like hair and fur are composed of molecules with both + and – places. (This is why it’s so easy to dye protein fibers like wool & silk: + dyes can stick to – places and – dyes can stick to + places.) Plastic is often made of chains of carbon atoms surrounded by hydrogen atoms. Hydrogen atoms only contain 1 negatively charged electron which, in plastics, is busy being attracted to the carbon atoms, leaving the surface of the plastic a bit positive. This means that when plastic is rubbed by hair, the negative places on the hair transfer some charge to the slightly positive places on the ruler, making the ruler become negative. Glass, on the other hand, is mostly composed of + silicon atoms surrounded by – oxygen atoms on the surface. When glass is rubbed on hair, the positive places in the hair rip negative charges off of the negative glass surface, leaving it slightly positive.

triboelectricIn 1757, Wilcke began to quantify static electricity by publishing the first “triboelectric sequence” (to the right, from the Greek for “rubbing”) which indicated the relative tendencies of materials to become positively or negatively charged:

From this chart you can see that if you rub a rubber balloon on your hair, the balloon becomes slightly negative and your hair becomes slightly positive. If you then bring the negative rubber balloon up to an uncharged wall, the balloon’s negative charge repels negative charge from the surface of the wall (into the wall’s interior), leaving the surface a bit positive – and the balloon and wall will attract each other. Ta da!

For a long time, only small electric charges could be created – because as an object became charged, it repelled addition charges of the same type. This limitation was overcome in 1745 with the invention of the “Leyden jar:”

leyden

Leyden jars were glass jars (red) with metal (gray) on both the inside & outside, and with the inner metal connected to a ball on top through a chain & rod. When a charged object was touched to the top ball, its charge spread onto the inner metal. (In this picture, it’s assumed this is a – charge.) This charge then pulls the opposite charge from ground onto the outer metal. This opposite charge helps attract more charge to the inner metal. Repeatedly touching charged objects to the top ball adds more and more charge onto the inner & outer metals. Large sparks could then be generated by cnnecting a wire between the outer and inner metals. By 1746, Leyden jars were so good that one was discharged one through a 1 kilometer line of monks! In 1747, Henry Cavendish began measuring the electrical conductivity of various substances by comparing the shocks he received when he discharged Leyden jars through them + himself.

In time, much larger static charges were generated by Wimhurst machines (~1880, see (http://techtv.mit.edu/videos/5303) and even larger charges by van de Graff’s 1929 ‘generator.’

electroscopeIn 1748, Jean-Antoine Nollet built the 1st “electroscope” to measure charge without having to be shocked by it. (He used a suspended pith ball that moved in response to the electrostatic attraction and repulsion of nearby charged objects.) In 1787, the “gold leaf electroscope” (to the right) was developed that measured charge by how much it could separate two pieces of thin gold foil:

 

 

coulombIn 1785, Coulomb used a “torsion balance” to measure how the repulsive force between electric charges varied with their separation. He first found that the force needed to twist a thread was proportional to the angle through which it was twisted. He then suspended a horizontal rod with a ball on the end onto a vertical wire in the middle of a draft-free enclosure – so that it touched a second ball on a vertical rod that extended beyond the enclosure to a 3rd ball:

Coulomb then electrically charged the 3rd ball by touching it with a rubbed object. The charge was conducted down to the 2nd ball and then onto the moveable ball. Since the 2nd and movable balls had the same type of charge they repelled each other. At some point, the electrical repulsion was balanced by the torsion in the thread and it stopped moving – say at an angle of 36 degrees.

Coulomb then twisted the wire (from the top) until the two charged balls were half as far apart (say separated by 18 degrees). To do this, he had to twist the top of the wire by 126 degrees, for a total twist of 126 + 18 = 144 degrees. But 144 degrees = 4 x the initial separation of 36 degrees, and since twist was proportional to force, the force between the balls when the distance between them was cut to ½ was 4x the initial force. To get the angle between the balls to be cut in half again (9 degrees), he had to twist the wire 567 degrees, so the total separation was 567 + 9 = 576 degrees which was 16x the original 36 degrees – so cutting the separation to ¼ of its initial value took 16x the force.

Coulomb’s studies led him to 3 characteristics of the forces between electric charge:

• the force between electric charges obeys an inverse square law (F = k/r^2)
• electric forces add up as independent vectors
• each charge both exerts and experiences a force

To further quantify the behavior of electric charges, people needed a unit to use to measure them. This proved non-trivial. The SI (System Internationale) unit of electrical charge (the coulomb, C), wasn’t defined until 1881 – 75 years after Coulomb’s death!

One coulomb is defined to be the amount of charge that must flow each second through two 1 meter long parallel wires that are separated by 1 meter in order to cause a force between them of 0.0000002 Newtons. This amount of charge flow, or “current,” is defined as 1 ampere (1 amp). (The amp is actually one of the System Internationale’s 7 fundamental units.)

To see how this works you need to know 2 things:

1) An electric current generates a magnetic field around it described by “the right hand rule” illustrated to the left below: if you point the thumb of your right hand in the direction of a current, your fingers show the direction of the magnetic field

2) A magnetic field generates a force on any wire which is carrying an electric current – in a way given by the “left hand rule” illustrated in the central picture below: if you arrange the thumb, index & middle finger of your left hand to be perpendicular to each other, and align the middle finger with the current & the index finger with the magnetic field, the thumb points to the force

Combining these two facts means that a wire with a current in it will create a magnetic field that will exert a force on another wire carrying a current (right-hand picture below)

current forcesOne coulomb is a LOT of electricity. A normal household circuit breaker trips at 15 amps = 15 Coulombs/sec – which is enough to kill people if shocked by it. The charges created by rubbing balloons on your hair, etc. are less than a micro-coulomb = a millionth of a Coulomb. But the charge on 1 electron is just 1.60218E-19 C, so even even creating a charge of 1microcoulomb takes the transfer of 6,250,000,000,000 (6.25 trillion electrons)!

Once people had units with which to measure charge, they could quantify Coulomb’s law: the force (in Newtons) between two electrical charges (Q1 and Q2, in coulombs) separated by a distance R (in meters) is given by

F = kQ1Q2/R^2 where k = 9.0E9 Nm^2/C^2.

For example, the attractive force between charges of +1C and -1C separated by a distance of one meter, would be 9×10^9 Newtons = 2 billion lbs! Yikes. The force between two 0.1-micro-coulomb charges separated by 1mm (=.001m) would be about 20 lbs.

There are two interesting things about electrical (and also gravitational) forces:

1) Electrical forces are not conserved. If charge “A” is attracting charge “B” with a strength of “2″ and another charge (“C”) comes near A and is attracted to A with a strength of “1″ , A’s attraction for B doesn’t decrease to “1″ – it remains “2.” It’s not like there’s only a certain amount of attraction to go around and that the greater the number of things attracted, the less strength there is in each attraction. (A hint: when in a serious relationship with another person, don’t try explaining that your attraction for a 3rd person doesn’t lessen your attraction for your significant other. It seems to work better for charges than for people.)

additivity

2) Electrical forces don’t diminish over time. If charge “A” is attracting charge “B” but they are prevented from moving toward each other, the attraction will continue indefinitely. It’s not like a charge only has a limited amount of attractiveness that gets used up over time. (How sweet… charges never get tired of each other!)

ACTIVITY 5.1

To convince yourself, that, in chemistry, electrical forces are more important than gravitational forces, compare the electrical and gravitational attractions of an electron (charge = -1.60218E-19 coulombs, mass = 9.10939E-31 kg) and a proton (charge = +1.60218E-19 coulombs, mass = 1.67262E-27 kg) when they are separated by a distance of 100E-12m. (Reminder: The gravitational force between two masses M1 and M2 is given by Fgrav = GM1M2/R2 where G = 6.67E-11 Nm2/kg2.)

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June 20, 2009

Sat June 20 2009

Filed under: Uncategorized — doncass @ 9:19 am

Well, Sean Murphy helped me get started with this way of getting online to replace a COA web page that I made up over 10 years ago and then never got back to….

I must say that this WordPress method is a lot easier than however I did it before! It’s a dreary Saturday morning here in Maine and wife Suzy’s off at a writing workshop, so I’m taking a bit of time to explore what wordpress has to offer, update some posted class info and to test out this blog stuff…. So far so good…. I wonder how long it’ll be before laptops come with a ‘post this to every site that I belong to’ button? And how long it’ll be before it becomes physically impossible for everyone to post everything that they want to post everywhere that they want to post it? What is the environmental impact of all this “virtual” stuff that so many think has “no” impact? Materials to produce all these  servers? Power to power them? I just don’t know… it’s all very cool, but also pretty scary… but maybe that’s what people always think when they get to be my age!

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