Electric Charge

1. Electric Charge Making sense of the scotch tape lab, the unit vocabulary, and the static electricity lab

2. Where does charge come from? • All matter is made up of atoms • Atoms contain • Protons (+) • Neutrons (0) • Electrons (-) Charge occurs when the number of electrons are not equal to the number of protons.

3. How can we detect charge? • We see its effects. (electroscope) • Law of electric charges

4. How Can You Charge Objects? • There are 3 ways objects can be charged: • Friction • Conduction • Induction **In each of these, only the electrons move. The protons stay in the nucleus**

5. Friction • Charging by friction occurs when electrons are “wiped” from one object onto another. Ex. If you use a cloth to rub a plastic ruler, electrons move from the cloth to the ruler. The ruler gains electrons and the cloth loses electrons.

6. Conduction • Charging by conduction happens when electrons move from one object to another through direct contact (touching). Ex. Suppose you touch an uncharged piece of metal with a positively charged glass rod. Electrons from the metal will move to the glass rod. The metal loses electrons and becomes positively charged. 

7. Induction • Charging by induction happens when charges in an uncharged object are rearranged without direct contact with a charged object. Ex. If you charge up a balloon through friction and place the balloon near pieces of paper, the charges of the paper will be rearranged and the paper will be attracted to the balloon.

8. Conservation of Charge • When you charge something by any method, no charges are created or destroyed. • The numbers of electrons and protons stay the same. Electrons simply move from one atom to another, which makes areas that have different charges.

9. Where do charges come from? Rubbing materials does NOT create electric charges. It just transfers electrons from one material to the other. http://phet.colorado.edu/en/simulation/balloons

10. + + – – – + – – + – + electrons are pulled from the wool to the balloon. When a balloon rubs a piece of wool... The balloon has more electrons than usual. wool The balloon: – charged, The wool: +charged

11. - - - - - - - + + + + + - - - - - Induction: The production of a charge in an uncharged body by bringing a charged object close to it When negatively charged rod is put near a metal can... electrons of the can are pushedaway from the rod. induced charges  top of the can: positive attraction buttom of the can: negative & attraction > repulsion metal can repulsion

12. - - - - - - - – – – – – – – – + + + + + + + + Attraction of uncharged objects Similarly, when charged rod is close to paper scrap... molecules of paper align.  attraction between the rod and + charge > repulsion between the rod and - charge. attraction paper repulsion

13. How does charge move? Insulators and conductors

14. Conductors: materials that allow electrons to flow through them easily. Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape.

15. Insulators: materials that do NOT allow electrons to flow through them easily. Insulators can be easily charged by friction as the extra electrons gained CANNOT easily escape.

16. Static Electricity Phet.colorado.edu • Static electricity is the electric charge at rest on an object. • When something is static, it is not moving. • The charges of static electricity do not move away from the object that they are in. So, the object keeps its charge. • Ex. Clothes taken out of a dryer

17. Electric Discharge • The loss of static electricity as charges move off an object is called electric discharge. Sometimes, electric discharge happens quickly. Ex. wearing rubber-soled shoes on carpet, lightning Sometimes, electric discharge happens slowly. Ex: static on clothes

18. How Lightning Forms

19. Lightning • Lightning usually strikes the highest point in a charged area because that point provides the shortest path for the charges to reach the ground. • Anything that sticks up or out in an area can provide a path for lightning. • A lightning rod is a pointed rod connected to the ground by a wire. • Objects, such as a lightning rod, that are joined to Earth by a conductor, such as a wire, are “grounded.” Any object that is grounded provides a path for electric charges to move to Earth. • Because Earth is so large, it can give up or absorb charges without being damaged. • When lightning strikes a lightning rod, the electric charges are carried safely to Earth through the rod’s wire. By directing the charge to Earth, the rods prevent lightning from damaging buildings.

20. How do you calculate the charge on an object? • Elementary charge is 1.6x10-19 C • Electrons are negative • Protons are positive • Total charge = charge from electrons + charge from protons • Remember the signs on the charges

21. How do you know that charge exerts a force? • You can see the affects of the charge – items move. • If objects move, they have acceleration. • Forces cause acceleration • Therefore, electric charge exerts a force.

22. What affects this force? • The size of the charge • The distance between the charges.

23. How is this force similar to gravity? • Fg=Gm1m2 d2 • Fe=Kq1q2 d2

24. How can you calculate the electrical force between charges? • Coulomb’s Law Fe=KqAqB d2 K=9.0x109 Nm2/C2 • Example: Sphere A, with a charge of +6.0mC, is located near another charged sphere, B. Sphere B has a charge of -3.0mC and is located 4.0cm to the right of A. What is the force of sphere B on sphere A? (100N)