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Electrical charges

Electrical charges. Attraction and repulsion. +. +. -. +. Charge It. How does a neutral object react to a charged object? I place several pieces of paper on the desk. I rub a glass rod with wool. I move the rod close to the pieces of paper. I observe that the rod attracts the pieces.

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Electrical charges

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  1. Electrical charges Attraction and repulsion + + - +

  2. Charge It • How does a neutral object react to a charged object? • I place several pieces of paper on the desk. • I rub a glass rod with wool. • I move the rod close to the pieces of paper. • I observe that the rod attracts the pieces. • I repeat steps 2 and 3 using the ebonite rod.

  3. ebonite We rub wool Conclusion • When we rub a glass rod on the wool, the rod gives electrons to the wool. The rod becomes positively charged. • When we rub an ebonite rod on the wool, the rod takes electrons from the wool. The rod becomes negatively charged. We rub

  4. Gaining and losing electrons • Atoms of some objects hold their outer electrons more loosely than others. • Glass hold electrons more loosely than wool and wool hold electrons more loosely than ebonite. • Rubbing removes electrons from glass to wool. The protons in glass remain in place. Glass, that loses electrons, becomes positively charged. • Rubbing removes electrons from wool to ebonite. Ebonite, that gains electrons, becomes negativeley charged.

  5. Explanation • A small piece of paper is neutral. It contains positive charges as much as negative charges. • Positive charges on a rubbed glass rod attract the negative charges on the paper. • Negative charges on a rubbed ebonite rod attract the positive charges on the paper.

  6. Glass rods rubbed with wool Repulsion + + Two charges with the same sign repel • The two rods are charged positively. • A positive charge repels another positive charge.

  7. Repulsion + - + - Repulsion Repulsion of electric charges • A positive charge repels another positive charge. • A negative charge repels another negative charge.

  8. + + - - Repulsion of electric charges The charges repel. Repulsion Repulsion

  9. + - Attraction Attraction of electric charges • Two charges with the same sign attract. • The unit of the electric charge is « coulomb», symbol C.

  10. + - Attraction of electric charges • The charges attract each other. Attraction

  11. Electric force between two charges • You have seen in chemistry that: • Any atom, that loses electrons, becomes a positive ion. • Electric charge of positive ion, or positively charged object, is a multiple of the proton charge (p+ = + 1,6 X 10-19 C). • Any atom, that gains electrons, becomes a negative ion. • Electric charge of negative ion, or negatively charged object, is a multiple of the electron charge (e- = - 1,6 X 10-19 C).

  12. +q’ F +q r F Electric force between two charges • Between two electric charges q and q’, there is an electric force. We can calculate this force by using Coulomb’s formula. • Coulomb’s Formula • q and q’ are the charges values in coulombs (C) without sign. • r is the distance between the two charges in meters (m). • F is the force value in newtons (N).

  13. Example • The ionic bond between the ions Na+ and Cl- is an electric force between two opposite electric charges. • What is the electric charge q of sodium ion? • What is the electric charge q’ of chloride ion? • Calculate the value of this electric force? Given : The distance between the two ions is 276 pm (1 pm = 1 picometer = 10-12 m). Electron charge = -1.6 x 10-19 C.

  14. Aswers • Sodium ion has 11 protons et 10 electrons. It has one more proton. Its charge q = +1.6 x 10-19 C. • Chloride ion has 17 protons et 18 electrons. It has one more electron. Its charge is q’ = -1.6 x 10-19 C. • Electric force Electric force F = (N = newton)

  15. Problems • Two electric charges + 2 C and -0.5 C are 2mm apart. • Calculate the electric force that exist between these two charges. • Is this force an attraction or repulsion? Explain.

  16. Problems 2. The two ions Ca2+ and O2- bond together by ionic bond. They are 240 pm apart. • calculate the charge of each ion if the electron charge is -1.6 x 10-19 C. • Calculate the electric force that define this ionic bond (1 pm = 10-12 m).

  17. Answers 1. The electric force is attractive. The two charges have opposite signs. 2. Charge of Calcium ion = 2 x (+1.6 x10-19) = +3.2x 10-19 C Charge of Oxide ion = 2 x (-1.6 x10-19) = -3,2 x 10-19 C electric force F =

  18. Did you know? • A conductor is a substance that allows charges to move freely. Examples : • Copper, aluminum, iron… • These metals hold their electrons loosely. Electrons are the charges that move through metals. • An insulator is a substance that does not allow charges to move through it. Examples : • Glass, ebonite, wood… • These insulators hold tightly their electrons.

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