Electrostatic Experiments

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Bending Water with a Balloon

What You will Need:

Small Rubber Balloon
Water Faucet

What to Do:

1) Blow up and tie off the balloon

2) Turn on the Faucet so that you have a thin, unbroken stream of water

3) Rub the balloon quickly on your hair back and forth a number of times. Notice what happens as you lift the balloon away from your head (make sure your hair is very dry and clean).

4) Hold the balloon close (1-2cm) beside the stream of water. Make sure the balloon doesn’t touch the water.

You'll see the water bend towards the balloon and be careful that it still doesn't touch the balloon as this can stop the effect.

What Happens?

When you rub the balloon on your hair, it becomes charged with static electricity.
As the rubber slides over the hair, molecules of rubber form temporary bonds with molecules in your hair by sharing some of their electrons. When the balloon moves on, the bonds break, sometimes leaving an electron or two stranded on the wrong side of the bond. As the electrons spend slightly more time on the rubber side of the bond, the balloon ends up with many more electrons than it began with and leaving it with an overall negative charge! Conversely, you hair has lost some of its complement of electrons so it is now some of its positively charged protons are not cancelled out anymore and leaving the hair with an overall positive charge. As oppositely charged objects attract each other, your hair sticks to the balloon as you lift it off.

Watch out though, these unbalanced charges repel themselves and if you put enough charge on the balloon, some of the electrons will be kicked out and jump to another object, forming a spark. Fortunately, due to the large size of the balloon's surface, it is very hard to get so much charge on the balloon that a spark will jump to an uncharged object. However, it is much easier to get a spark to jump from the balloon to a positively charged object like your hair. Also, the positive charge on the hair that touched the balloon will attract electrons from the surrounding hair making tiny sparks jump within your hair. Don't worry too much, these sparks should not be large enough to hurt at all, in fact, they're so small that if you want to see them, your best option is to charge your hair up in a very dark room in front of a mirror and then you should be able to see sparks jumping through your hair.

How does the balloon attract the water?

As water is a neutral molecule consisting of two hydrogen (H) atoms and one oxygen (O) atom - a total of 10 positively charged protons - with a complement of 10 electrons to exactly cancel out the positive nuclei so there is no net charge to cause the attractive force (there is a small amount of positively charge particles (ions) from salts and other chemicals dissolved in the water, but there is an equal number of negatively charged ions to cancel them out as well).
However, just as the rubber-hair chemical bond share the electron unevenly, the electrons in each of the H-O bonds spend more of their time close to the oxygen making the oxygen end of each bond a little negative and the hydrogen end a little positive. As the atom is arranged in a shape, with the oxygen at the pointy end, water has distinctly positive and negative ends.

Thus, it is described as being polarized having a positive electrical charge at the oxygen end and a negative charge located between the hydrogen. When you bring the negatively charged balloon close to the stream of water, the hydrogen end is attracted towards the balloon and the oxygen end is repelled away from it. This pair of forces twists the water molecule so that the line from the negative pole to the positive pole points towards the balloon.

Now once the water has rotated, the hydrogens are slightly closer to the balloon that the oxygen (about 30 angstroms), so the attractive force on the hydrogens is slightly stronger than the repulsive force on the oxygen, so each water molecule is attracted towards the balloon. However, as this force is quite small and the water is already moving downwards quite quickly, rather than being attracted up on to the balloon like your hair was, the stream is just bent.

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Two Magnets or Not Two Magnets?

How can you tell what is a magnet?
I have two identical iron bars; the only difference is that one is a bar magnet with N - S poles and the other is just an iron bar. Can you tell which one is the magnet by only touching them together once?
Hint: If you put their ends together, then they will atract each other - so you will gain no information. In fact, the permanent magnet will force the iron in to a temporary magnetic state - an effect called paramagnetism.

The answer is later, but don't cheat - see if you can think it out.
If you like, you can try playing around with a magnet and a piece of metal to see if you can work it out that way.

This is an old puzzle, that many of you will have seen before but is worth covering again. It boils down to "how can you tell what is a magnet?"

The simple way is to try to pick up some metal but, if you only do this once, then how can you tell whether you are holding the magnet or you have just found a really strongly magnetised paper clip? Of course, you can just get a second paper clip and see whether the magnet or the first paper clip do the attracting, but that is bringing in a second object.

Most children learn that magnets attract metal, but rarely know how they do this.

How Magnets Attract Metal

When ever a electric charge makes a loop in space, it creates a magnetic field. From the coiled wire of an electromagnet to a single electron orbiting an atom. In some atoms, the orbits of the electrons tends to cancel out, but many atoms have "a magnetic moment." When atoms are arranged in crystal structure, theie magnetic moments (or "spins") interact.

Depending on the type of atom, the structure can be Ferromagnetic or antiferromagnetic.

In a ferromagnet, the spins tend to align with their neighbours whereas in an antiferromagnet they tend to oppose their neighbours. So in a ferromagnetic material like Iron, structures called "domains" form. Domains are small regions containing spins that all point the same way - little magnets. But in "ordinary" iron, the domains are all randomly pointed, so their magnetic effects cancel out, so most pieces of iron will not be magnetic (or at least,not very magnetic - metals are quite good at picking up magnetism from other nearby magnets).

But when there is already a magnetic field, these domains will be forced to align themselves with the field, making the iron a temporary magnet. Then the north pole of the temporary magnet will be attracted one way and the south the other, causing the iron to be attracted to the magnet. (see figure)

The Answer to Our Puzzle

So, have you worked it out yet? Obviously, touching the ends of our samples together achieves nothing - the magnet will attract the metal, no matter which one we touch to the other. This is clearly a symmetrical situation and this symmetry stops us from seeing which is the the magnet.
To start with, we need a asymmetrical option, so that we have some chance of telling which bar is which. In fact, we've already seen it - in the diagram we looked at on the last page we had the magnet end touched to the middle of the iron bar. Still, we see that the magnet attracts the bar. But what if we touch the end of the bar to the middle of the magnet?

When the bar is brought near the middle of the magnet, it encounters an electrical field running parallel tothe edges of the bar.

This sets up a temporary magnet across the width of the bar - with the north near the south pole of the permanent magnet and the south closest to the permanent north pole.

As the poles that are closest together are opposing, there is a slight magnetic attraction, but:

1) The field on the edges of the magnet is very weak, so the attraction is very small.

2) The like poles are not much greater seperated than the opposing poles, so the repulsion almost cancels out the attraction.

3) The forces are along the lines between the end of the bar and the ends of the magnet - as these forces are almost antiparallel (in opposite directions), the components of the forces along the magnet will entirely cancel.

This means we are left with only a very weak force of attraction between the bar and the magnet!

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The Kelvin Water Dropper

What You Will Need To Build Your Own

Did you know that you can build a very simple high voltage generator which has no moving parts and is powered by the energy of falling water? By dribbling water through empty tin cans, thousands of volts can be “magically” generated.

The water dropper was named for its inventor, the Baron Kelvin (1824-1907). A professor at Glasgow University (from 1846), Baron Kelvin also made important contributions to experimental electromagnetism and theoretical thermodynamics. With James Joule, he discovered the Joule-Kelvin effect. His name is also given to the unit of the absolute temperature scale, the kelvin.

To make a Kelvin Water Dropper, you will need:

1) Four tin cans. Two small (about 300 ml - the size of a tin of condensed soup - should be perfect) and two large (look for cans about one liter in volume).

2) Stiff wire, about one meter, cut in half.

3) Styrofoam. one or two pieces big enough to rest the large tins on so they are as electrically isolated as possible.

4) Electrical tape. Get the thick, insulating kind. You will need this to cover over any sharp edges/points on your dropper - for safety, aesthetics and, importantly, to control where electrical discharge occurs. Charge likes to collect at sharp points on surfaces, so if there are jagged edges uncovered, they may interfere with the operation of the water dropper.

5) Sandpaper.

6) If you aren’t in a rush, some glue (make sure it is the kind that will stick to metal), will help your dropper be sturdy and long lived

7) Once again, if you want to build a very sturdy, “high performance sports model” dropper, (and if you already have the equipment/skills) some solder to ensure good electrical contact will be useful.

8) You will also need a bucket (and, for a deluxe, professional dropper a couple of little taps/valves)

How To Build It

1) To Open and empty all the cans. Unless you bought them yourself, or have collected used ones, you should probably save the contents for the rightful owner. Using a can opener, remove the tops from all four cans, and the bottoms from the smaller two.

2) Using either the glue or just the electrical tape, connect a large and a small can to either end of each of the wires. In the diagram, the red wire connects the large and the small red cans, and the green wire connects the green cans. Ensure that there is a good metal to metal contact so that the electricity can flow, by removing paper wrappings, sanding gently the exposed surfaces to remove and insulating oxidization and, if you want, using a coating of solder to make the joint.

3) Cover over the ends of the wire and any rough surfaces with the electrical tape.

At this point you should check that your wires are bent correctly, so that you can arrange each small can directly over the other large can. When you do this, the wires should be roughly 2-3 cm (about 1 inch) apart. Once it’s running, you will need to fine tune the separation, so don’t stress over the exact distances here.

4) Drill or punch two holes in the bucket, spaced correctly so that the water falls through them and then through the small cans. If you are making the deluxe version, make these holes so that you can fit the taps to them, then you can easily control the flow of water. However, I found that the first dropper I built worked fine without the valves.

5) Now that everything is constructed, you will need to arrange the components so that you can generate the electricity. Place the large cans on the styrofoam, with the small cans over them. Now, using a shelf or a table, hold the bucket over the cans so that the streams of water falling through the holes go through the small cans into the large cans.

What You Will See

As the water is falling, observe the space between the wires (where they are closest) and the water as it passes through the small cans. If things are arranged correctly, you should see two things.

1) If the wires are close enough together, you will soon see sparks jumping between them. If you don't see sparks almost straight away, the wires are probably too far apart. Being careful not to touch the metal directly (so you don't risk a shock) or with anything conducting, push the wires slightly closer together. When you get them close enough that the field between them is higher than the breakdown strength of air, you should see sparks jumping between them regularly.

2) Near the small cans, you will probably see small droplets turning around in mid air and flying sideways or even upwards away from the cans

These droplets are charged the same as the cans and the Coulomb force between them repels the drops.

How it Works

Although water is full of charged particles - ions from dissolved salts and from the breakdown of water itself, the water that initially falls through the holes in the bucket will be uncharged on the average. However, the random ness of the universe will soon cause there to be a slight charge on one the can/wire systems. Perhaps a drop will fall that is slightly charged, or maybe a cosmic ray will hit the wire and cause a small charge to form on one of the little cans.
Let's say that the little can on the left (and hence the large can on the right) is slightly positive. Then, this positive charge will have three effects on the falling streams

1) Negative ions will be slightly attracted towards the left most hole, so that the water falling through it will, on average, be slightly negatively charged.

Thus, the left most large can (and the right side small can) will become negatively charged

2) Conversely, positive ions will move to the vicinity of the right hand side whole, making the right hand side stream positively charged.

3) The positive charge on the can will repel any slightly positively charged drops on the left hand side - this will tend to increase the total negative charge falling into the left hand large can.

This spontaneous process will cause a negative charge to form on one wire and a positive charge on the other. What's more, this spontaneous charge separation will tend to reinforce itself - the positive charge on one small can will cause negative charge to collect on the large can below, which will charge up the other small can, which will attract positive charge to the big can below it - increasing the positive charge on the first small can, which in turn increases the rate negative charge falls through it.

The stronger this charge is, the more it tends to reinforce itself, in a process called positive feedback, that only ends when the charge becomes so great that a spark jumps between the wires, removing the charge difference (until it spontaneously builds up again).

Instead of using the charge to generate a spark, there are a variety of ways the droppers charge can be harnessed. The coulomb attraction can be used to cause a metal rod to strike a bell (you will need conducting wires to suspend the components), or the voltage might be sufficient to light a small bulb.

If you build a water dropper I would love to see pictures of them (perhaps I might put the best ones up for all to see. I would especially love to see any innovative ways you use the generated voltage.

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