3. Electric Fields Review of gravitation Review of gravitation Gravitational field strength g gives the ratio of force to mass. Gravitational field strength g gives the ratio of force to mass. F g = G m earth M body F g = G m earth M body d 2 d 2 Where F g is the force of gravity (weight) and Where F g is the force of gravity (weight) and d is the distance from the center of the earth. d is the distance from the center of the earth. F g = m body g, we can think of m earth as creating the field and m body “experiencing the force” in response to the field. F g = m body g, we can think of m earth as creating the field and m body “experiencing the force” in response to the field. g id the “field strength” of the earth’s gravitational field (9.8 N/kg at the surface ) g id the “field strength” of the earth’s gravitational field (9.8 N/kg at the surface ) We can calculate g = G m earth: this gives the field strength at any ddistance from the earth We can calculate g = G m earth: this gives the field strength at any ddistance from the earth

4. What Gives an Electric Charge? An imbalance of protons and electrons. An imbalance of protons and electrons. Neutral objects have equal numbers of electrons and protons. Neutral objects have equal numbers of electrons and protons. Positively charged objects have more protons than electrons. Positively charged objects have more protons than electrons. Negatively charged objects have more electrons than protons. Negatively charged objects have more electrons than protons. When you rub a rubber rod with rabbit fur electrons are transferred to the rubber rod and it becomes negatively charged. When you rub a rubber rod with rabbit fur electrons are transferred to the rubber rod and it becomes negatively charged. Conservations of charge Conservations of charge When electrons are transferred from one substance to another the overall charge is conserved. When electrons are transferred from one substance to another the overall charge is conserved.

6. The SI Unit for Charge is the Coulomb The charge of a proton is +1.6 x 10 -16 Coulombs. The charge of a proton is +1.6 x 10 -16 Coulombs. The charge of a electron is -1.6 x 10 -16 Coulombs. The charge of a electron is -1.6 x 10 -16 Coulombs. This is why an atom is neutral when there are equal numbers of protons and neutrons. This is why an atom is neutral when there are equal numbers of protons and neutrons.

7. Conductors Conductors have electrons that are more loosely held and therefore allow charges to flow more easily. Conductors have electrons that are more loosely held and therefore allow charges to flow more easily. Good conductors are: Good conductors are: Metals Metals Water and solutions with ions present. Water and solutions with ions present. Wires are usually composed of copper wire. Wires are usually composed of copper wire.

8. Insulators Insulators have more tightly held electrons and are not good conductors of electricity. Insulators have more tightly held electrons and are not good conductors of electricity. Good Insulators are: Good Insulators are: Rubber Rubber Plastic Plastic Air Air Silk Silk Good electrical insulators are usually also good thermal insulators. Good electrical insulators are usually also good thermal insulators. Copper wires are usually encased in rubber to prevent shock. Copper wires are usually encased in rubber to prevent shock.

9. Millikan In 1909 Robert Millikan a physicist at the University of Chicago ran an experiment that revealed that charges occur in discrete quantities. In 1909 Robert Millikan a physicist at the University of Chicago ran an experiment that revealed that charges occur in discrete quantities. These quantities always occur in whole number intervals and are said to be “quantized”. These quantities always occur in whole number intervals and are said to be “quantized”. These quantities are known as electrons which have a charge of - 1.6 x 10 -19 Coulombs. These quantities are known as electrons which have a charge of - 1.6 x 10 -19 Coulombs. Electrons cannot be divided into fractions. Any object that is charges has a surplus or deficit of some whole number of electrons. Electrons cannot be divided into fractions. Any object that is charges has a surplus or deficit of some whole number of electrons. # of electrons = total charge/charge of an electron. # of electrons = total charge/charge of an electron. If something has a - 1C of charge it contains 6.2 x 10 18 electrons. If something has a - 1C of charge it contains 6.2 x 10 18 electrons.

10. Millikan’s Oil Drop Experiemnt

11. Charging Objects Charging by Friction Charging by Friction Electrical charges can be transferred from one object to another. Electrical charges can be transferred from one object to another. Two substances can be rubbed together. Two substances can be rubbed together. Rub a balloon with wool. Rub a balloon with wool. The wool will transfer electrons to the balloon and the balloon will become negatively charged. The wool will transfer electrons to the balloon and the balloon will become negatively charged. Static Cling Static Cling Clothing in the dryer rub electrons off each other and they become oppositely charged and stick together. Clothing in the dryer rub electrons off each other and they become oppositely charged and stick together.

12. Conduction Objects can be charged by contact. Objects can be charged by contact. Since electrons can move from one place to another they will move into another object to even out the overall charge of the two objects. Since electrons can move from one place to another they will move into another object to even out the overall charge of the two objects. Conservation of charge. Conservation of charge. - -

13. Induction Charges within a neutral conductor can move. Charges within a neutral conductor can move. Objects can gain an induced charged when a charges object is brought near a neutral object. Objects can gain an induced charged when a charges object is brought near a neutral object. Charges become polarized. Charges become polarized. This induced charge is temporary if the objects never touch. This induced charge is temporary if the objects never touch. Once the charged object is removed the charges evenly spread out again. Once the charged object is removed the charges evenly spread out again.

14. Grounding Grounding an object provides a conduit in which the excess electrons can leave the object. Grounding an object provides a conduit in which the excess electrons can leave the object. Objects that are charged by induction can gain a permanent charge by grounding. Objects that are charged by induction can gain a permanent charge by grounding. Objects that are charged by induction and then grounded end up with an opposite charge than the conductor. Objects that are charged by induction and then grounded end up with an opposite charge than the conductor. Objects charged by conduction end up with the same charge as the original conductor. Objects charged by conduction end up with the same charge as the original conductor.

15. Grounding

16. Grounding

18. Charging by Induction

20. Electric Forces Electric Force Electric Force Force between charges Force between charges Repulsion between like charges and attraction between opposite charges. Repulsion between like charges and attraction between opposite charges. Coulomb’s Law Coulomb’s Law F e = k q 1 q 2 F e = k q 1 q 2 d 2 d 2 The electric force decreases with the square of the distance between the charges

21. Look Familiar? Remember F g = G m 1 m 2 Remember F g = G m 1 m 2 d 2 d 2 Quantity of charge is like the mass of an object Let’s compare G and k G = 6.67 x 10 -11 N m/kg 2 k = 8.99 x 10 9 N m/C 2 Which force is stronger? A pair of positively charged particles of 1 C each 1 meter apart would experience a repulsive force of 9.0 x 10 9 N The weight of a battleship!

22. Law of Superposition The resultant force on any given charge is the vector sum of all the forces acting on that charge. The resultant force on any given charge is the vector sum of all the forces acting on that charge. Applied to three or more charges with a certain distance to each other. Applied to three or more charges with a certain distance to each other. Must consider whether or not the force is attractive or repulsive. Must consider whether or not the force is attractive or repulsive.