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. I repeat steps 2 and 3 using the ebonite rod.

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 ebonite wool We rub

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.

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.

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

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

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

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

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

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 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 C).

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). +q’+q rFF

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 = m). Electron charge = -1.6 x C.

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

Problems 1. Two electric charges + 2 C and -0.5 C are 2mm apart. a) Calculate the electric force that exist between these two charges. b) Is this force an attraction or repulsion? Explain.

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

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

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.