1 Electrostatics (static electricity)

2 Electric Charges: Electric charge is a fundamental quantity that is responsible for all electric phenomena. Charge is a property of all atomic particles. Charge can be positive, negative, or neutral.

3 Atoms: In general, the heavy nuclei of atoms have a positive charge. The small, light, negative electrons reside in shells or orbits outside the nucleus.

4 Atoms: In general, the heavy nuclei of atoms have a positive charge. Atomic structure

5 Electrons and You: Your body contains an astronomical number of electrons. (Far more than would be needed to electrocute a person.)

6 However, the negative electrons are balanced by an approximately equal number of positive protons. This balance gives a net charge of zero (neutral)

7 Conservation of Charge: When one item has “extra” electrons, this means that something else must have “lost” electrons.

8 Just as mass, energy, and momentum are conserved, so is electric charge. You can’t create or destroy electric charge, it is just transferred or moved.

9 Electrical Forces: Opposites attract. This is due to a force arising from the electric charge on particles. We will learn more about this force later... Likes repel.

10Review: ++? Repulsive + -? Attractive

11 Coulomb’s Law: k = 9 x 10 9 Nm 2 /C 2 k = 9 x 10 9 Nm 2 /C 2 q = one charge q’ = another charge d = distance between the charges This is the law that quantitatively relates the attraction or repulsion of electric charges. k q q’ k q q’ F E = d 2 d 2

12 Charge Unit: Since electrons are much too small and numerous to be counted individually, they are counted in groups called Coulombs. 1 Coulomb, C, = 6.25 x electrons.

13 Q: Which is stronger, gravitational or electric force? A: For atoms and objects on our scale, the electric force is much stronger. On a planetary scale, gravity is much stronger. ?

14 Conductors : Conductors: Material where electrons are loosely bound and are able to flow throughout (free electrons). Ex: metals, impure water, humans Don’t try this at home...

15 Insulators: Insulators: Materials where electrons are bound and don’t flow easily. Ex: glass, rubber, plastic Conductors & Insulators

16 Semi Conductors: Semiconductors: Materials in between insulator and conductor. Ex: silicon, carbon, germanium Used in transistors and other electronic components.

17 Super Conductors: Super conductor: a material where electrons flow without any resistance. Generally, superconductivity only occurs at very low temperatures. Magnetically levitated superfast trains are one application of superconductivity.

18 Charging by Contact Simply rubbing one object against another can transfer electrons. This makes one positive and one negatively charged object _ _ _ __ _ Before: both neutral __ _ _ _ _ After: Silk positive, glass negative friction

19 Charging by Contact Simply rubbing one object against another can transfer electrons. This makes one positive and one negatively charged object. Charging by friction

20 Charging by Induction: Even without direct contact, you can induce the electrons to move due to the electric force acting on them. 1. Both neutral at first 2. Negative rod separates charges. 3. Separate the spheres 4. Remove rod, spheres are now charged.

21 Lightning: When one mass of air moves over another, electrons are transferred. This is just like moving your feet on carpet to build a charge. A large negative charge on the bottom of a cloud induces a positive charge on the ground below. ( they are attracted)

22 If the charge imbalance becomes large enough, lightning occurs. This discharges the accumulated charge Lightning

23 Charge Polarization: Just as a charged cloud causes charge to separate in the ground, this can happen in other objects. Notice how the wall is still neutral, but the charge is separated... Balloon & Wall Another Balloon

24 Why Does it Cling? Although the paper is still neutral, since its polarized, the (-) is now closer, and the (+) is farther. This means the attractive force is greater, and the repulsive force is less. ( It sticks!)

25 Electric Field: Imagine there is a cluster of – charge as shown. - If a small + charge was placed, what force would it feel?

26 This “map of force lines” shows what a positive test charge will do when exposed to any particular field. When lines are closer together, that means the force is stronger. Here are some pictures of various fields:

27 Electric Field Strength/Intensity Electric potential describes how strong the field is per amount of charge. Electric Field Strength = Electrostatic Force / charge E = F e / q

28Question: Q: Suppose you had a charged rod, and an oppositely charged hoop as pictured Describe the electric field between them. Describe the electric field inside the hoop.

29 A: Between, the field points directly from the (+) to the (-). That’s the direction a positive test charge would want to go. However, inside the hoop, there is surprisingly no field at all!!!

30 Shielding: This last question points out an odd, but important fact: The electric field inside a conducting surface is zero!

31 Anywhere inside a conducting surface, the forces on you cancel out, giving no electric field. Even though you may be closer to one end ( less distance), on the other end, there is more charge pulling on you. These effects cancel out and give 0 field.

32 Lightning Strike? Q: If you are inside your car when it is struck by lightning, you will survive. Why?

33 A: The electrons repel themselves to the extreme outside of the car. Two electrons wouldn’t want to be near each other on the inside by you. The electricity flows around the outside of the car, not through you. This would happen even if there were no tires at all on the car!

34 Work and the Electric Field Work is done by the electric field in moving the positive charge. If the charge were moved counter to the field, work would be required.

35 Field between plates is uniform - (same strength all over)

36 Electric Potential How much work is done, depends on the amount of charge. Electric potential describes how much work is done, per amount of charge. Electric potential = potential energy / charge

37 Electric Potential Unit: A Joule per Coulomb is defined as a Volt. 1 V = 1 J / 1C A volt is the basic unit of electric potential. Named after Alessandro Volta, he invented the electric battery. DieHard

38 Mechanical and Electrical Energy Gravitational potential energy is derived from the earth’s gravitational field. Electrical potential energy, is derived from an electric field. Despite the name similarity, electric potential is NOT the exact same as potential energy. Electric potential describes how much work could be done per amount of charge.

39 Mechanical / Electrical Energy Two rocks are at the same height, the larger one has more PE. Two charges have the same electric potential, the larger charge has more PE.

40 V=W( Joules ) V=W( Joules ) q( Coulombs ) q( Coulombs ) W = work done against field or energy acquired working with field (Joules or eV) q - amount of charge moving through field (Coulombs) q - amount of charge moving through field (Coulombs) V - Potential Difference (volts) Ex) It takes 6 Joules of work to move 2 Coulombs of charge between 2 points in an electric field. What is the potential energy difference (voltage) between these 2 points?

41 Ex) It takes 6 Joules of work to move 2 coulombs of charge between 2 points in an electric field. What is the potential energy difference (voltage) between these 2 points? V=W( Joules ) V=W( Joules ) q( Coulombs ) q( Coulombs ) V =6 Joules / 2 coulombs V = 3 Volts or J/C

42 Both groups of charges on the side are at the same electric potential (voltage). However, it would take much more work to move the lower one closer since it has a larger charge...