Electricity

2. What is static electricity? You walk across the rug and reach for a door knob ZAP! You come in from the cold and remove your hat Boing! your hair stands up. Why does this happen? The answer is Static Electricity To understand what static electricity is, we have to learn a little bit about the nature of matter. Or in other words, what is all the stuff around us made of? To understand what static electricity is, we have to learn a little bit about the nature of matter. Or in other words, what is all the stuff around us made of?

3. History of electricity?

4. The build up or lack of electrons. Called electrostatics

5. Electrostatics – Static Electricity Electricity at rest, a build up of charge. Static electricity is the imbalance of positive and negative charge. involves: Electric charges The force between them Their behavior in materials, and The aura that surrounds electric charge Overview of ElectorstaticsOverview of Electorstatics

6. The Atom Positive nucleus “+” protons (all same) No charge neutrons Negative clouds “-”electrons (all same) Move around Usually same # of “+” and “-” Net charge is zero Everything around us is made of atoms. Scientists so far have found only 115 different kinds of atoms. Everything you see is made of different combinations of these atoms. Everything around us is made of atoms. Scientists so far have found only 115 different kinds of atoms. Everything you see is made of different combinations of these atoms.

7. Electrons move. Protons and neutrons are held tightly. Outer electrons can move Can move atom to atom. If atom loses “-” electron then the atom is net “+“ charge called a “+” ion If atom gains “-” electron then the atom is net “-” charge called a “-” ion Materials that hold electrons tightly are called insulators – rubber, cloth, glass, and dry air Materials that hold electrons loosely are called conductors – most metals, water, human body Only electrons move Ionic/convalent bonding Light Balloon demo Metal rod demo

8. Conservation of Charge When we charge something with static electricity, no electrons are made or destroyed. Electrons are just moved from one place to another. The net, or total, electric charge stays the same. This is called the principle of conservation of charge. 

9. Electric Charge Is an integer multiple of the elementary charge. Elementary charge – the charge of one electron – 1.6 x 10 -19 C Symbolized with “q” ie. q = 3e = 4.8 x 10 -19 C Measured in units called Coulombs (C) Detect small amounts with an electroscope

10. How are electrons moved? Rubbing materials together – Friction - electrons move Conduction – contact between two charged objects – electrons move Induction – no contact between objects. Charge polarization – when electrons move from one side of an object to the other.

11. Electric Force Is a vector. #, unit, and direction Opposite Charges attract – pull toward each other So what does all this have to do with shocks? Or hair full of static? When you take off your wool hat, it rubs against your hair. Electrons move from your hair to the hat. Now each of the hairs has the same positive charge. Remember, things with the same charge repel each other. So the hairs try to get as far from each other as possible. The farthest they can get is by standing up and away from the others. Bad hair day!  As you walk across a carpet, electrons move from the rug to you. Now you have extra electrons. Touch a door knob and ZAP! The door knob is a conductor. The electrons move from you to the knob. You get a shock. We usually only notice static electricity in the winter when the air is very dry. During the summer, the air is more humid. The water in the air helps electrons move off you more quickly, so you can not build up as big a charge. So what does all this have to do with shocks? Or hair full of static? When you take off your wool hat, it rubs against your hair. Electrons move from your hair to the hat. Now each of the hairs has the same positive charge. Remember, things with the same charge repel each other. So the hairs try to get as far from each other as possible. The farthest they can get is by standing up and away from the others. Bad hair day! As you walk across a carpet, electrons move from the rug to you. Now you have extra electrons. Touch a door knob and ZAP! The door knob is a conductor. The electrons move from you to the knob. You get a shock. We usually only notice static electricity in the winter when the air is very dry. During the summer, the air is more humid. The water in the air helps electrons move off you more quickly, so you can not build up as big a charge.

12. The electroscope A device used to detect small charge Uncharged Charged

13. How the electroscope works Bring “-” object near Negatives push other negatives away Pointer repels because of build up of like charge

14. Charles Coulomb First described electric field strengths in the 1780's Found that for point charges, the electrical force varies directly with the product of the charges And the field varies inversely with the square of the distance between the charges.

15. Example Problem What is the force between a proton and electron that are separated by a distance of 6.0 x 10 -10 m? qp=+1.6 x 10 -19 C qe=-1.6 x 10 -19 C K = 9.0 x 10 9 Nm2/C2

16. Summary Electrons are responsible for charge Charge is positive or negative Charges exert forces of attraction and repulsion + force is repulsion - force is attraction Force is a vector

17. Coulomb’s Law Charged objects create an invisible electric force field around themselves The strength of this field depends on many things, amount of charge, distance involved, and shape of the objects This can become very complicated. We can simplify things by working with "point sources" of charge. This can become very complicated. We can simplify things by working with "point sources" of charge.

18. Term was created by Michael Faraday Charge creates a field in all directions Depends on distance, charge and shape The region around a charged object that has the ability to exert a force.

19. Can’t see it unless a very large charge.

20. Place a small test charge near and observe the electric force of attraction or repulsion. The collection of all forces is the electric field.

21. Use electric field lines to represent field. Direction is the same as the force acting on a “+” test charge.

22. Use electric field lines to represent field. Number of lines represents the size of charge.

23. Use electric field lines to represent field. Spacing of lines indicates field strength.

24. Unlike charges

25. Depends on the force exerted. means it depends on the charge and distance from the charge. Measured in N/C Is a vector – direction is the same as the direction of the force vector.

27. Similar to gravitational Potential Energy Zero level surface of earth G field is toward earth Mass at h1 has PEg If mass is at h2 it will have more PEg To change PEg force is applied over a distance. Work is done W=DPE

28. Directly related Force (F) and charge (q) are directly related.