Electric and Magnetic Forces and Properties of Materials

1. Electric and Magnetic Forcesand Properties of Materials Chapter 16 and 24

2. Static Electricity • Better known as Electrostatics comes from the roots electro and statics. • “Electro” is Greek for amber, a petrified tree resin which when rubbed will attract other objects. When it was discovered that the movement of a sub-atomic particle was responsible for this attraction, the particles were called electrons; the force was called electric. • “Stati” is Greek for standing or place. • Thus, electrostatics is the study of electrical forces at rest.

3. What do the forces do? • First – they are not like gravity • The electrical forces can either attract or repel one another. • Ben Franklin named the two types of forces positive and negative.

4. Like charges repel each other, unlike charges attract each other. Opposites attract!

5. Coulomb’s Law The force between two electrically charged particles (q) is proportional to the product of their charges divided by the square of the distances between them. K is the universal electrostatic constant. It is equal to 9.0 x 109 N-m2/C2. Which is stronger, gravitational forces or electric forces? Problem: Assume that you have two objects, one with a mass of 10 kg and the other with a mass of 15 kg, each with a charge of –3.0 x 10-2 C and separated by a distance of 2 meters. Compare the electrical and gravitational forces, which is greater?

6. The Solution

7. What causes the charge? • The movement of electrons (remember back to sixth grade at JGMS) • Atom basics • The atom is made from protons and neutrons in the nucleus and electrons that rotate around the nucleus. • Electrons have a negative charge, protons have a positive charge. • If an atom loses a negative electron or gains one, it becomes an ion (a charged particle)

8. Conductors permit electrons to flow freely from atom to atom and molecule to molecule permit charge to be transferred across the entire surface of the object Conductors allow for charge transfer through the free movement of electrons. Insulators materials which impede the free flow of electrons from atom to atom and molecule to molecule The particles of the insulator do not permit the free flow of electrons; subsequently charge is seldom distributed evenly across the surface of an insulator How are materials classified?

10. How can an object be charged? • By friction, when electrons are transferred by friction from one object to another. • By conduction, when electrons are transferred from one object to another by direct contact without rubbing. • By induction, when electrons are caused to gather or disperse by the presence of nearby charge.

11. Law of Conservation of Electric Charge • In a closed system, the net amountof charge produced in any process is zero • The strength of charged particles is measured in coulombs. An electron and a proton have the same magnitude of charge, just opposite signs. The magnitude of the charge of either of these two particles is: • 1.602x10–19 coulombs So, we can say that an electron has a charge of –1.602x10–19Coulombs, and a proton has a charge of +1.602x10–19Coulombs.

12. A little more on the conservation of charge How much energy required to tear away electrons varies from substance to substance. • Rubber holds electrons more firmly than fur. When rubber and fur are rubbed together, electrons transfer from the fur to the rubber rod. • The rubber has excess electrons and is negatively charged. • A glass or plastic rod rubbed with pure silk will transfer its electrons to the silk. Giving the silk a negative charge. • The electrons are not created or destroyed, they are simply transferred from one object to another.

13. Polarization • Charge polarization occurs in insulators that are in the presence of a charged object. • A realignment of charge rather than a migration of charge occurs.

14. Electric Fields and Potentials The Van de Graaff is coming!!!

15. Electric Field & Field Lines • The space around every electrical charge • Has both magnitude and direction, a vector quantity • Exploring Electrical Fields

16. Charge Distribution How does charge distribute itself on an object? • Charges spread out as much as they can. • In order to be in equilibrium, charges will bunch up at corners. (they gather so the net force in the center of a conductor is zero)

17. Charge distribution car What should you do if a broken power lands on top of your car?

18. Electric Shielding • Static charge occupies only the outer surface of a conductor; inside the conductor the electric field is zero.

19. Electrical Potential Energy • The work required to push a charged particle against the electric field of a charged object increases the particle’s electrical potential energy. • Work done is equal to the energy gained. • Similar to how a mass’s PE depends on its location within the Earth’s gravitational force field

20. Electric Potential • Electrical potential is the electrical potential energy per charge. • Electric potential = electrical potential energy/charge • Other terms for Electric Potential: • Voltage • EMF (electromotive force) • 1 volt = 1 joule/coulomb

21. What is Lightning? Lightning on Science Joy Wagon