Unit 5: Electricity Chapter 14: Electric Charges and Forces 14.1 Electric Charge and Current 14.2 Electric Current, Resistance, and Voltage 14.3 Capacitors

SWBAT… describe how charges are transferred. Objective Date 3/9/15 SWBAT… describe how charges are transferred.

Electric charge Electric charge, like mass, is also fundamental property of matter. Inside atoms found in matter, attraction between positive and negative charges holds the atoms together. The unit of charge is the coulomb (C). 1

Electric charge and current The direction of current was historically defined as the direction that positive charges move. Benjamin Franklin first used the terms “positive” and “negative” to describe charge. 2

Conventional current Because of Franklin’s work, the direction of electric current is defined as going from positive to negative. Long after Franklin’s work, scientists discovered that current in wires is the flow of electrons which move in a circuit from negative to positive. 3 & 4

Electric Charge Objects can lose or gain electric charges. The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges. A tiny imbalance in either positive or negative charge on an object is the cause of static electricity. 5

Electrostatics Electrostatics is the part of physics that studies the forces created by unmoving charges. A photocopier uses electrostatic forces.

Electric forces The forces between the two kinds of charge can be observed with an electroscope. 6 An electroscope contains two very thin leaves of metal that can swing from a central rod connected to a metal ball. Charges can flow freely between the ball and the leaves. Suppose a positively charged rod touches the metal ball of an electroscope. Some negative electrons are attracted to the rod. The metal ball and leaves of the electroscope are left with a net positive charge. Since both leaves have the same positive charge, the leaves repel each other and spread apart. Once an electroscope is charged, it can be used to test other charged objects. The leaves spread farther apart if another positively charged rod is brought near the metal ball. This happens because the positive rod attracts some negative electrons from the leaves toward the ball, increasing the positive charge on the leaves. 8

Charging by friction Under dry conditions, a balloon rubbed on hair will transfer electrons from hair to the balloon. This is called charging by friction. Objects charged by this method will attract each other. Once the balloon is charged, it can also stick to a neutral wall through “polarization”

Charging by induction Charging by induction is a method of using one object to charge another without changing the net charge on the first object. 7

Conductor: Electrons in random motion If a copper wire is not connected to a battery, the free electrons move around at high speeds. They have no net motion because as many move one way as the other way.

Conductors and insulators A semiconductor has a few free electrons and atoms with bound electrons that act as insulators. 8

Capacitors A capacitor is a storage device for electric charge. Capacitors can be connected in series or parallel in circuits, just like resistors. 9

Capacitors A capacitor can be charged by connecting it to a battery or any other source of current. A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

Capacitance Capacitance is measured in farads (F). A one-farad capacitor can store one coulomb of charge when the voltage across its plates is one volt. One farad is a large amount of capacitance, so the microfarad (μF) is frequently used in place of the farad.

Introductory Electrostatics Notes Name: 3 Sub-Atomic Particles in the Atom: ___________ which have _____________ charge. KEY Electrons negative Protons positive Neutrons no Electric Forces Opposite charges ________________ each other. Like charges ____________ each other. The closer the charges are together, the _________ the force. The further the charges are together, the __________ the force. attract repel stronger weaker

electrons protons fussion electrons protons fission Introductory Electrostatics Notes Name: KEY Charging Atoms An atom can have a Positive charge by … SUBTRACTING ___________________ ADDING ________________ (also called nuclear ___________) An atom can have a Negative charge by … ADDING _________________ SUBTRACTING _____________ (also called nuclear _______________________________) electrons protons fussion electrons protons fission