Static Electricity Lecture

Transcription

1 Static Electricity Lecture Atoms are made of protons, which have a positive charge, and electrons, that have a negative charge. This little electron is what carries the electric current in wires. Charges these two particles carry are opposite. Which brings me to a fundamental rule of electricity: Like charges attract Opposite charges repel A cool thing to think about is that, in a simple way, we can think of the hydrogen atom as a system like our solar system. The proton is like the sun and the electron is like a planet. What is the force that attracts the sun to the planet? Gravitational force Think about the hydrogen atom. What is the force that holds an electron in orbit around the nucleus? Electrical force We say that that atom has NO net charge. What does that mean? An atom that is neutral may have a positive proton and a negative electron, but it is essentially neutral if you are far enough away from it. For example, if you stick a negative charge far enough away from it the negative charge doesnʼt notice the charges inside the atom (the positive nucleus and the electron orbiting it). The electron just sits there in space. Letʼs analyze why this is true... The electron feels an attractive force towards the positive proton (draw a vector/arrow), and it feels an equal negative repulsive force from the negative electron (draw a vector/ arrow). Notice that the strength of these forces is essentially the same so they cancel each other out. So, for all intents and purposes, the atom has no net charge, it is a neutral atom.

2 However, what happens if you bring the electron closer to the atom? Things change. The electron is closer to the positive and experiences a greater force of attraction; it is further from the electron so it experiences a weaker repulsion. There will therefore be a net force of attraction towards the atom and bonding occurs! A molecule is formed. When you studied chemistry you learned about all these bonding forces. All these chemical bonding forces are electrical forces! You are held together by electrical forces!! These forces are very much like gravitational forces.

3 Do you remember learning about the force of attraction between masses? We have a model that explains it, that predicts the strength of the force. F~ m1m2/d 2 A similar model works for the electrical force. This is why physics is so cool. F~ q1q2/d 2 Notice, they are very similar to one another! IN the electric force equation, q is the amount of charge and it is measured in Coulombs. The only real difference between these two models is that the gravitational force only attracts, but the electrical force can attract and repel. The way to read the equation is to say: The amount of electrical force depends on the amount of the two charges, and the distance they are apart from one another. It makes intuitive sense. The more charge, the greater the force. The less charge, the weaker the force. The further apart the weaker the force. The closer together, the stronger the force. You can convert these from proportionalities to equations by finding the constant of proportionality. If you take all the variables and put them on one side of the equation you can calculate the constant. F d 2 /m1m2= G F d 2 /q1q2= k No matter what numbers you use you will always get the same number, that is why it is called a constant! So now the equations look like this: F= Gm1m2/d 2 F= kq1q2/d 2 If you have a helium atom you have two protons so they pull harder on the electron and it comes into a closer orbit. There are actually two electrons so there is a bit of a repulsion between them both and therefore the two electrons have a little bit bigger orbit then if there were just one electron. What keeps the two protons inside the nucleus? First, letʼs be in awe over how powerful the electrical force is compared with the gravitational force. It is a billion, billion, billion, billion times stronger.

4 The electrical force is 2 x greater than the gravitational force!!! Well, there is a force greater than even the electrical force, but it acts over very short distances. It is called the strong nuclear force. It is what holds the protons and neutrons inside of the nucleus of atoms. If the nucleus gets big, too big, then the electrical force overwhelms the strong nuclear force and the atom comes flying apart. This is known as radioactivity. Demo: Cat fur and rubber rod Rub rubber rod with cat fur. Do you think all substances hold onto their electrons equally? It turns out, no, they donʼt. The rubber holds onto them tighter than the cats fur so they are leaving the cats fur and moving onto the rubber rod. The rod has more electrons than before. I say, this is charged. Is it negatively or positively charged? Negatively? Ben Franklin was the I. Newton of electricity. The cats fur is positively charged. This leads to: Conservation of Charge. The number of electrons now on the rubber rod are equal to the electrons that left the catʼs fur. Itʼs like a brick road. If you take bricks out of the road and put them on the sidewalk, you still have the same number of bricks as before. The number of bricks on the sidewalk match the number of holes in the road. So, if I put a millionth of a Coulomb (the unit of charge) on the rubber rod and it is negative. Then I have a millionth of a Coulomnb, of positive charge on the cats fur. Demo: Ping Pong ball with metallic paint suspended from a set up. Charge up rubber rod, touch the ping pong ball. Then try to touch it again. It repels. Like charges repel. Now rub glass rod against silk. It attracts. Opposite charges attract.

5 Now use a non-charged material- suspend a piece of styrofoam packing material. Use the rubber rod: It attracts. And then it repels. Use the glass rod: It attracts. And then it repels. Why? Due to charge polarization. If you bring a negative rod up to the packing material the positive part of the molecules will flip towards the negative rod. When a bunch of these do this, the packing material is attracted to the rubber rod. Demo: Little Pieces of Paper On the other hand, if you bring the glass rod which is positively charged, the negative part of the molecules will flip towards the glass rod and the packing material is attracted to the glass rod. This is what happens when you can pick up little pieces of paper with a charged rod. What is happening is I am polarizing the charge distribution in the paper. Demo: Electrophorus Wooden handle (insulator) and a metal pie tin (conductor). Charge the insulated surface (in our rooms, the seats!), place electrophorus on the seat, touch the metal pie tin, lift pie tin up with handle and show it is charged by moving it towards the piece of suspended packing material. The packing material will be attracted towards the pie tin. Question: are there any charges in the pie tin? Of course, everything has charges! Can they move? You bet, it is a conductor!

6 When you put the electrophorus on the table top the charges separate. The negatives go to the top, the positives are at the bottom (it is actually the negatives that move). Notice that there are equal numbers of positive and negatives. Why? Conservation of charge, remember?! When I put my finger on the top, it provides a place for the electrons to go so now the electrophorus becomes charged! Demo: Whimhurst machine Large radius of curvature vs. point Which will hold charge better? Large radius No charge can build up on a sharp point. This is the principle behind a lightening rod. Lightening The charge builds up in the cloud. The response on the ground is to cause separation of charge. This builds up enough that eventually the charge crosses the air and this is a lightening bolt. Golfers, large houses, etc. provide a shorter path.

7 Ben Franklin put the first lightening rod on the Unitarian Church in Philadelphia Purpose of lightening rod: goal is to prevent lightening rod from hitting the building, because it canʼt build up a charge. Common misconception: A lightening rod attracts the lightening bolt so it hits the lightening rod instead of the structure or people nearby. In reality, the sharp point of the lightening rod simply prevents the buildup of sufficient charge so a lightening bolt never had an opportunity to jump through the air to the building. Demo: Van de Graff Generator Build up a charge on the top. Pie Tins Hair Human chain Lighting a lamp in dark- depends on the orientation When I held the lamp it lit up, when I held it the other way, it didnʼt Dome is surrounded by an electric field. Where is it strongest? Close to the dome. Energy per charge we are talking about voltage. Where is voltage greatest? Where is the voltage difference the greatest? Where did a current flow? Why? Next time!

8

9

10

11