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15.1 Electric Charge and Current

15.1 Electric Charge and Current. pp. 340 - 345 Mr. Richter. Agenda. Intro to Electrostatics Notes: Charge and Net Charge Measuring Charge Electroscopes Charging Objects Three Ways. Objectives: We Will Be Able To….

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15.1 Electric Charge and Current

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  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. Net Charge • 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 = 9x109 N*m2/C2 • 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 • q1 = 0.001 C • q2 = 0.001 C • r = 2m • k = 9x109 N*m2/C2 • FE = 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. The Electroscope • 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. The Electroscope • 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 chargedand 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) Grounded with your finger p 345 • 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.

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

  30. Polarization

  31. Polarization • 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

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