1. 15.1 Electric Charge and Current pp. 340 - 345 Mr. Richter

2. Agenda  Intro to Electrostatics  Notes:  Charge and Net Charge  Measuring Charge  Electroscopes  Charging Objects Three Ways

3. Objectives: We Will Be Able To…  Distinguish between a positive and negative net charge. Identify the charge of electrons and protons.  Explain the meaning of Coulomb’s Law.  Describe the different ways of charging an object.

4. Warm-Up:  Sometimes when you scuff your feet on a carpet and then touch someone, you both experience a small shock. Why do you think this happens?  Discuss at your table, and we will discuss as a class in a few minutes.

5. Electric Charge

6. Electricity at an Atomic Level  All matter has electric charge because it contains:  protons (positive charge), and  electrons (negative charge)  When we talk about electricity, we are talking about the flow of electric charge.

7. Positive and Negative Charge  There are two types of charges, positive and negative.  Like charges will repel each other.  Unlike charges will attract each other.  Like magnets!

8. Net Charge

9.  Just like net force, the net charge of an object is the sum of the total charges within it.  When there is a perfect cancellation of positive and negative charges, there is a net charge of exactly zero. An object with a net charge of zero is called electrically neutral.

10. Net Charge  An object is charged if the net charge is not zero. Sometimes called the “excess charge”.  Positively charged if there are more positive charges than negative charges  Negatively charged if there are more negative charges than positive charges

11. Measuring Charge

12. Coulomb  The unit of charge is called a Coulomb (C), named for the French physicist who first accurately measured forces between charges.  The coulomb is a HUGE amount of charge.  A single proton has a charge of 1.602 × 10 -19 coulomb.  The charge of an electron is -1.602 × 10 -19 coulomb.  For example: the amount of charge flowing through one lightning bolt is about 15 C.

13. Coulomb’s Law  Electric forces (of attraction or repulsion) are created between charges.  These forces are incredibly strong!  If you could separate the positive and negative charges of the tip of a pencil to about 1 meter apart, the force is about 50,000,000,000,000 Newtons! (The weight of 5 billion cars!)

14. Coulomb’s Law (continued)  The magnitude of the force between charges depends on:  the amount of charge (how many coulombs)  the distance between them  The greater the charge, the greater the force  The closer the charges are to each other, the greater the force.

15. Coulomb’s Law (Continued)  Forces between charges create an action-reaction pair.  Equal in magnitude and opposite in direction.  see p342

16. Coulomb’s Law (Continued)  …explains the relationship between the amount of each charge, the distance between them, and the electrical force.  k is a constant: k = 9x10 9 N*m 2 /C 2  This is very similar to Newton’s Law of Universal Gravitation

17. Your Turn  Two steel marbles are each given a net charge of one thousandth (0.001) of a coulomb.  Calculate the magnitude of the force on the marbles if they are held 2 meters apart.  Given  q 1 = 0.001 C  q 2 = 0.001 C  r = 2m  k = 9x10 9 N*m 2 /C 2  F E = 2250 N

18. Warm-Up  When you rub a balloon against your hair for a few seconds, sometimes you can get it to stick to the wall? Why do you think that is?  Discuss at your table, and we will discuss as a class in a few minutes.

19. The Electroscope

20.  Because electrons are small, light, and are on the outside edges of atoms, they move easily.  Most electrical effects are caused by moving electrons, not protons.  Because electrons move, we can measure the charge of an object with an electroscope.

21. The Electroscope

22.  Electrons in a conductor will move until they are evenly spread around.  If the negatively charged rod is placed near the knob of the electroscope, the electrons move to the leaves.  Now that both leaves are negatively charged, they repel each other.  Using Coulomb’s Law, these forces and charges can be measured.

23. Three Ways to Charge Objects And how static electricity works!

24. Electrostatics and Electroscopes.  Charging by friction and charging by contact.  https://www.youtube.com/watch?v=yU55lXbrV0U

25. Charging Three Ways By Contact  If you touch the negatively charged rod to the electroscope, the leaves separate. Electrons transfer between objects.  When you remove the rod, the leaves remain split.  The rod is now Neutral, and the leaves are negatively charged.

26. Charging Three Ways By Friction  When one neutral object is rubbed against another.  Electrons are physically knocked off of one to the other.  One object is now negatively charged and one is positively charged.

27. Static Electricity  Static electricity (and the shock you feel as a result) is a combination:  First an object is charged by friction (scuffing one’s feet on the carpet).  Then the excess charge is transferred to another object by contact.  The moving charge makes a brief but intense flow of current.

28. Charging Three Ways By induction (indirectly)  Move a charged object close (but not touching) to the electroscope, but then ground it (remove the excess negative electrons)…  When you remove your finger the leaves stay separated because the net charge is now positive.  The charge of the original object (balloon) is not changed. Grounded with your finger  p 345

29. Electroscope: Charging by Induction  https://www.youtube.com/watch?v=cMM6hZiWnig

30. Polarization

31.  The balloon that was charged by friction will stick to a wall (not a conductor) but not a doorknob (conductor).  The atoms in a wall can’t move very well, but they can turn their positive sides toward the balloon.  This is polarization.  A doorknob will absorb some of the negative charge from the balloon.  Because they are both negative, they will repel each other.

32. Wrap-Up: Did we meet our objectives?  An object has a net negative charge if it has gained electrons, and a net positive charge if it is missing some electrons.  Coulomb’s law relates the amount of charge of two particles, the distance between them, and the electric force between them.  Objects can be charged by contact, by friction, and by induction.  An electroscope will remain charged if charged by induction (grounded).

33. Homework  p345 #1-4