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Static Electricity

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Justin Heller

on 22 March 2011

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Transcript of Static Electricity

Like most people, I am sure you all have been in a crazy storm where the sky is lit up by lightning strikes. Did you know though, that these lightning strikes are related to everyday happenings, such as when you shock a friend by rubbing your feet across carpete before touching them? Benjamin Franklin discovered this relationship with his famous kite experiment in which he flew a kite with a key attached to the string during a thunderstorm. The loose threads of the string stood up as to avoid one another and was shocked when he got close to the key. This showed that lightning is similar to electricity caused by friction and are therefore called static electricity. In physics, we call this electrostatics, the study of electrical charges that can be collected and held in one place. Charged Objects: Have you ever combed your hair on a dry day or had socks stick together after taking them out of the dryer? If not, try this simple experiment at home (on a dry day, of course...) - run a plastic comb of ballpoint pen on your clothing (wool works best) and hold it close to a pile of paper bits. The paper will be attracted to the comb/pen because the new, relatively strong force causing the upward acceleration is larger than the downward acceleration of gravity. This force will go away over time, unlike gravity which is always constant, causing the paper to fall. Any object that has an electrical interation after rubbing is said to be charged. Like charges: We know they repel each other. Let's do a demo, though, to demonstrate this! Take a piece of tape and fold over about 5 mm at the end (to use as a 'handle'). Then stick the rest of it to your desk (a smooth dry surface). Take a second similar piece and do the same. Quickly remove both pieces and bring them close to each other and observe what happens! They were prepared the same way and, thus, must have the same charge. What happens if you bring the tape close to your hand? Is there a quick way to remove the charge? Opposite charges: Now stick one strip of tape on the desk and place the 2nd strip on top of the first. Use the handle of the bottom strip to pull the two off the deks together. Rub them with your fingers until they are no longer attracted to your hand. Pull the two strips of tape apart at the handles quickly now. They once again are attracted to your hands, but do they still repel each other? No, because they are oppositely charged which means that they attract each other. A Microscopic View of Charge: Electrical charges exist in atoms. J.J. Thompson discovered this in 1890, when he found that all materials contain light, negatively charged particles he called 'electrons'. From 1909-1911, Ernest Rutherford discovered that atoms have a 'massive' positively charged nucleus, that if balanced by the negative electrons, would give a neutral charge to the atom. Using energy, electrons can be removed from an atom, giving it a positive charge (since you are taking away negative charge). These newly freed electrons can remain un-bound or can go to another atom that wants them, in which case, the new atom would become negatively charged (because it added negative charge to it). Separation of Charge: If two neutral objects are rubbed together, each can become charged when electrons from atoms on one object get moved to the other object The combined charge is still neutral, but one is more positively charged and the other is more negatively charged. This confirms the that individual charges are never created nor destroyed! This can happen inside the top and bottom of a cloud or even between the tires of your car and the road. Conductors and Insulators: Hold a plastic rod at the middle and rub only one end and only that end would become charged. The charges you transferred to the plastic stayed put. Materials in which charges will not easily move are called insulators. Examples are: Glass Dry Wood most plastics cloth dry air If you support a metal rod on an insulator so that it is completely surrounded by insulators, then touch a charged object to the metal rod, you would find that the charge spreads very quickly over the entire rod. This is because it is a conductor or a material that allows charges to move to it easily. Metals are good conductors because at least one electron on each atom of the metal can be easily removed. The electrons act as if they don't belong to any one atom but the metal as a whole. Air is an insulator but inder certain conditions lightning occurs because charge is allowed to move through the air as if it were a conductor. Conductors must have charges that are free to move, and for a spark or lightning to occur, free moving charged particles must be formed in the normally neutral air. With lightning, excess charges in the cloud and on the ground are great enough to remove electrons from the molecules in the air. The electrons and positively or negatively charged atoms become free to move, forming a conductor that is a plasma. Chapter 20: Static Electricity Electrical Force: You can produce the same results you saw with the tape with a negatively charged rubber rod that will turn when you bring a second negatively charged rubber rod near it without touching it because the electrical force acts over a distance. There are 4 main points that can be summed up by these types of experiments: 1. There are two kinds of electrical charges, positive and negative 2. Charges exert force on other charges over a distance 3. The force is stronger when the charges are closer together 4. Like charges repel; opposite charges attract A device called an electroscope is used to determine electrical charge (tape isn't very scientific...) It consists of a metal knob connected by a metal stem to two thin, light weight pieces of metal foil called leaves When a negatively charged rod is touched to the knob of an electroscope, electrons are added to the knob and the charges are spread over all the metal surfaces. What then, happens to the leaves inside the electroscope? That's right! They spread apart because the negative electrons travel down and the like charges on each leaf repel each other This is called charging by conduction (touch)! It also stands to reason that the leaves will spread out if the electroscope is positively charged. What happens if the electroscope has a charge different to that of the rod? Yup! The leaves will fall (come together). Let's revisit the tape experiment... When you brought your finger close to the tape, the tape was attracted to your finger, but your finger wasn't charged. If you move your any uncharged object, like your finger, to a positively charged object, the negative charges in the object (again, your finger) will be attracted to the positively charged object, and the positive charges will be repelled Your fingers remain uncharged but the positive and negative charges will be seaparated on your finger. It is this separation that allows a charged object and uncharged object to be attracted. In the case of lightning, negative charges at the bottom of thunderclouds cause charge separation on Earth where the positive charges in the ground are attracted to Earth's surface under the cloud. The forces of the charges in the cloud and those on Earth's surface break molecules apart, separating positively and negatively charged particles. These charged particles, now free to move create a conducting path from the ground to cloud and lightning is created as a way to discharge the cloud. Coulomb's Law: We can use this to charge objects without touching them! Get two identical neutrally charged spheres and put them together so they are touching. If you bring a negatively charged rod close to one of the spheres, electrons from the first sphere will be forced onto the spehere farther from the rod and make it negatively charged, thus making the sphere close to the rod positively charged. This is called charging by induction! In your tape experiment, you demonstrated that the force depended on the distance, such that the closer you brought the charged rod to tape, the stronger the force. Coulomb took an insulating rod with small conducting spheres at each end and was suspended by a thin wire. A similar charged sphere was placed in contact with one of the spheres, allowing charge to flow over both spheres. The symbol for charge is q Coulomb found that the force depended on the distance by showing that the amount of deflection (distance the sphere twists the rod) at different distances. When Coulomb charged the first two spheres and added a third identical uncharged sphere, he was able to charge sphere 3 with sphere 2 causing each to have half the original charge of sphere 2. When he put the second sphere back close to sphere 1, it only had half the force The magnitude of the force between charge qa and qb, separated by a distance d, is proportional to the magnitude of the charges and inversely proportional to the square of the distance The unit of charge is called the Coulomb (C), is a standard quantity of charge in terms of the amount of force it produces and is equal to the charge of 6.25e18 electrons or protons The magnitude of the charge of an electron is called the elementary charge. Without this principle of static electricity, soot wouldn't be collected in smokestacks (thus adding pollution to the environment), cars and other objects wouldn't be painted uniformly, and copy machines wouldn't make precise reproductions. Take laser printers, in which static charges transfer black powder onto paper by heat-bonding tiny dots of toner onto the paper The computer sends instructions to the printer that controls either a laser or LED. The surface of the metal drum is coated with a photosensitive semiconductor and functions as an insulator in the dark that is sprayed with electrons as the drum rotates, giving it a negative charge. According to the processor, a mean of light sweps over the surface of the drum and the drum becomes a conductor wherever the light strikes. Uncharged areas on the drum correspond to white portions of the page. The drum is rotated so that it contacts uncharged toner particles made up of small plastic beads coated in graphite and the beads are attracted to the charged areas. A sheet of paper being pushed by rollers receives a small opposite charge and electrically attracts the toner. After picking up toner from the drum, the paper moves into the fusing system where the paper iis exposed to heat and pressure that melts the grains of toner and binds them to the paper The drum is then returned to the dark where is is an insulator and the surfaces are recharged and ready to receive the image of another page. describe and calculate how the magnitude of the electrical force between two objects depends on their charges and the distance between them characterize materials as conductors or insulators based on their electrical properties
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