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CHAPTER 17: ELECTRICITY 1 17.1 ELECTRIC CHARGE AND FORCE CHAPTER 17: ELECTRICITY ELECTRIC CHARGE • Electricity: force created by a difference in charges (+, -) due to gained or lost electrons. • Protons (+) are trapped in the nucleus, so moving electrons (-) are responsible for electricity • Electric Charge: an electrical property of matter that creates electric and magnetic forces and interactions • Like energy, electric charge is never created or destroyed • The SI unit of electric charge is the coulomb, (C) 3 TRANSFER OF ELECTRIC CHARGE • When charges between two points differ because electrons build up, the electric charge can be transferred • This can happen through induction, conduction, or friction 4 TRANSFER OF ELECTRIC CHARGE • Induction: presence of (-) charge forces electrons to move away (no contact involved) • Conduction: when a charged object touches a neutral object and the charges transfer. Often called charging by contact 5 TRANSFER OF ELECTRIC CHARGE • Friction: when objects are rubbed together and one material gains electrons, becoming negatively charged, and the other loses electrons, becoming positively charged • Static electricity happens when the charges build up, but don’t move 6 TRANSFER OF ELECTRIC CHARGE • Although charge can jump between points, it often travels through a conductor • Electrical Conductor: a material in which charges can move freely, allows charges to flow • The best conductors are metals because their electrons are not tightly bound to the nucleus • Electrical Insulator: a material in which charges cannot move freely, does not allow charges to flow • The electrons are tightly bound in insulators, most plastics are insulators 7 ELECTRIC FORCE • Electric charges exert a force • Electric Force: the attractive or repulsive interaction between any two charged objects • Like charges repel, opposites attract • This force decreases as the distance between charges increases 8 ELECTRIC FIELD • Electric Field: the space around a charged object in which another charged object experiences an electric force (this is like gravitational forces!) • Any charge that is placed in an electric field will be pushed or pulled by the field • For example, if you have a + charge, and place a + charge near it, they will repel 9 17.2 CURRENT CHAPTER 17: ELECTRICITY LET’S REVIEW! • Potential Energy: energy stored in an object due to its position • Kinetic Energy: the energy an object possesses due to its motion 11 CURRENT • Electric Current: rate at which charges pass through a given point (flow of electrons from point A to B) • Related more to the amount than the speed • The SI unit of current is the ampere (A) 1 amp = 6.25 x 1018 electrons per sec. (6,250,000,000,000,000,000!) • When charges flow, they carry energy that can be used to do work 12 ELECTRICAL POTENTIAL ENERGY • Electrical Potential Energy: the ability to move an electric charge from one point to another • Charges naturally move from an area with high electrical PE to an area with low electric PE • Electrical PE is measured in volts (V) 13 ELECTRICAL POWER & ENERGY • When electrons flow, they transfer energy and transform electrical energy into other forms of energy (sound, light, heat) • Electrical Power: The rate at which electrical energy is converted into other forms of energy • The SI unit for power is the watt (W) • Electric power is calculated by multiplying the total current, I, by the voltage, V, in a circuit 14 power current voltage P = IV VOLTAGE • Voltage: The difference in charges between two points, also called potential difference • Voltage sets charges in motion • Think of a battery - there is voltage across the terminals • One terminal is positive and the other is negative • Electrons will always flow from (-) to (+) when they are connected 15 DIRECT CURRENT (DC) • Flow of a charge always in one direction • The amount of current can change, but it will always flow in the same direction • For example – a battery • In metals, moving electrons make up the current 16 DIRECT CURRENT (DC) • Lightning is a form of DC caused by static electricity in clouds. • Static is formed when air molecules move past each other (like clothes in a dryer), and negative charges group at the bottom of the cloud and transfer electrons to the ground, which has taken on a positive charge. 17 DIRECT CURRENT (DC) • Battery: device which produces a continual direct current (DC), through a chemical reaction. • When the (+) and (-) terminals are connected through a device, they produce a flow of electrons from negative to positive, which powers the device. 18 ALTERNATING CURRENT (AC) • Flow of charge back and forth, changing its direction many times in one second. • AC power is used all over the world because it’s cheaper and easier to make devices with AC power • All of the appliances and lights in your house probably run off of AC power 19 CONVENTIONAL CURRENT • Electrons move from areas of negative charge to positive • Conventional current moves in the other direction – positive to negative • The direction of current is opposite to the direction that the electrons move 20 ELECTRICAL RESISTANCE • Resistance: a material’s opposition to the flow of electrons • Resistance is caused by internal friction, which slows the movement of charges through a conducting material • All materials have some resistance • Conductors are low, insulators are high • Resistor: certain type of conductor used to control current 21 ELECTRICAL RESISTANCE • Resistance is measured in Ohms (Ω) • 1 Ω = 1 V/A • Ohm’s Law: resistance in a circuit equals voltage difference, divided by current • Ohm’s Law provides a way to measure the resistance of objects and materials voltage resistance = current V R= I 22 ELECTRICAL RESISTANCE • Semiconductors: materials that have electrical properties between those of insulators and conductor, and conduct under certain conditions. • Remember your periodic table! • Superconductors: some metals and compounds that have zero resistance when their temperature falls below the critical temperature • Critical temps range based on the superconductor – can be as low 23 as –272 ºC (–458 ºF) and as high as –123 ºC (–189 ºF) ELECTRICAL RESISTANCE • Once current is established in a superconductor, it will continue even if voltage is removed, making them very useful • Superconducting magnets are so strong they are used to levitate trains (MagLev Trains) which reduces the friction between a normal train and the track • For any conductor, an increase in temperature will increase resistance BILL NYE 24 17.3 CIRCUITS CHAPTER 17: ELECTRICITY 25 CIRCUITS • Electric Circuit: pathway along which an electric current can travel, en route to a device. • Closed Circuit: Complete electrical path in which current circulates • Open Circuit: An incomplete electrical path with no charge flow and therefore no current • Switches interrupt the flow of charges in a circuit. • You can use a switch to open and close a circuit. 26 SERIES CIRCUITS • Series Circuit: only one path from the source through all of the loads and back to the source • Current in each device is the same, but resistances and the voltage across each device may differ • If one element along the path in a series circuit is removed, the circuit will not work • A string of old Christmas lights is an example of a series circuit 27 PARALLEL CIRCUIT • Parallel Circuit: two or more paths for current to flow through • Voltage is the same across each device, and the sum of all currents equals the total current • A break in any one path does not interrupt the flow of electric charges • Newer Christmas lights and household circuits are an example 28 CIRCUIT BREAKERS • When electrical wires carry more than a safe level of current, the circuit is overloaded. • Overloaded circuits can cause fires • Short Circuit: When current stops short of the intended device • Circuit Breaker: device which prevents too much electricity from flowing through a circuit, can be reset usually with the flip of a switch. 29 FUSES • Fuse: an electrical device that contains a metal strip that melts when current in the circuit becomes too great • Fuses melt to prevent circuit overloads. 30 SCHEMATIC DIAGRAMS • Schematic Diagram: graphical representation of a circuit • Uses lines to represent wires • Uses symbols to represent components • Schematic diagrams use standard symbols and can be used to describe all electrical devices 31 32