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Electricity & Magnetism Ch 9 6th grade How do charges interact? • Atoms contain charged particles called electrons (-) and protons (+). • If two electrons come close together, they repel each other. Protons act the same way. • If a proton and an electron come together, they attract one another because the two have opposite electric charges. • The interaction between electric charges is called electricity. • The force between charged objects is called electric force. • An electrical field is a region around a charged object where the object’s electric force is exerted on other charged objects. Electric Fields • Electrical fields and forces get weaker the farther away from the charge. • An electrical field is invisible. • Field lines point away from positive charges and toward negative charges. • Single charges have straight field lines, but if multiple charges are present, these forces combine to make more complicated field lines. How does charge build up? • Most objects have no charge because each atom’s positive charge is balanced by a negative charge. • An uncharged object can become charged by gaining or losing electrons – If it looses electrons, it will have an overall negative charge. – If it gains electrons, it will have an overall positive charge. Static electricity • This build up is called static electricity. • Charges build up on an object, but they do not flow continuously. • Charges are neither created nor destroyed. This is known as the law of conservation of charge. • If one object loses electrons, another object must pick them up. Methods • Friction: uncharged objects are rubbed together and moves electrons from one to the other • Conduction: charged object touches another object and transfers electrons • Induction: reaction to the electric field of a charged object without touching the object* • Polarization: electrons react to electric fields resulting in individual atoms having charged ends that are attracted to charged objects* • *parts of objects end up charged Static discharge • Positively charged objects gradually gain electrons and negatively charged gradually loose electrons and the objects eventually become neutral. • Objects hold on to charge better in dry air. • Lightening bolts are also discharge from the charge that builds up within the clouds. Electric current • Static electric charges do not flow continuously, but when electric charges are made to flow through a material, they produce an electric current. • The amount of charges that passes through a wire in a given period of time is the rate of electric current, called an ampere, or amp (A). Circuit • In order to maintain an electric current, charges must be able to flow continuously in a loop, called an electric circuit. How do conductors differ from insulators? • The atoms in conductors have loosely bound electrons that can move freely. • Electrons in insulators cannot move freely among atoms. What affects current flow? • Current flow is affected by the energy of the charges and the properties of the objects that the charges flow through. • Electric currents flow through wires like water through pipes and changes into different forms of energy. • Voltage is the difference in electric potential energy per charge between two points in a circuit. • The amount of current in a circuit also depends on the resistance of the circuit. • The greater the resistance, the less current there is for a given voltage. • The unit of measure of resistance is the ohm (Ω) Factors that determine resistance • Diameter: Current flows more easily through a wide wire than through a narrow wire. • Length: Short wires have less resistance than long wires. • Material: Electrons loosely held to their atoms have a low resistance. • Temperature: The electrical resistance of most materials increases as temperature increases.