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Ch. 20 Notes – Electricity Electric Charge and Static Electricity Electric Charge – a property that causes subatomic particles such as protons and electrons to attract or repel each other. • There are two types of charge – positive and negative • Protons have a positive charge • Electrons have a negative charge QuickTime™ and a TIFF (LZW ) decompressor are needed to see this picture. LIKE CHARGES REPEL AND OPPOSITE CHARGES ATTRACT. An excess or shortage of electrons produces a net electric charge. An atom is neutral because it has an equal number of protons and electrons. If the atom were to gain an electron, it would become a negatively charged ion, and if the atom were to lose an electron, it would become a positively charged ion. The SI unit of charge is the coulomb (C) - One coulomb is equal to about 6.24 x 1018 electrons. Electrically charged objects either attract or repel each other (pull or push) – this is Electric Force The strength of electric forces depend the amount of charge and on the distance between them. • Electric force is directly proportional to the net charge on each object • Electric force is inversely proportional to the square of the distance between them. - If you double the charge of one object, the electric force between them doubles - If you double the charge of both objects, the electric force between them quadruples - If you double the distance between the objects, the electric force is one-fourth as strong Electric forces are much stronger than gravitational forces, but on a large scale, since most matter is neutral, electric forces are usually close to zero. Electric Fields – the effect an electric charge has on other charges in the space around it Static Electricity – the study of the behavior of electric charges, including how charge is transferred between objects Law of Conservation of Charge – the total charge in an isolated system is constant • if one object gains charge, another object lost an equal amount of charge Charge can be transferred between objects by friction, contact (conduction), and induction Charging by Friction – If two objects rub together, one object may transfer electrons to the other. The object that gains electrons becomes negatively charges and the object that loses electrons becomes positively charged Charging by Contact (Conduction) – If a charged object touches a second object, electrons can either leave or enter the second object, giving it a net charge Charging by Induction – If a charged object is brought near a neutral object, charge within the neutral object can move and redistribute. If a ground is provided to the neutral object, electrons can either leave or enter, giving the object a net charge grounding - a path provided between an object and “the ground” allowing charge to transfer Static Discharge occurs when a pathway through which charges can move forms suddenly - when you rub your feet on the carpet you become charged through friction. When you touch the doorknob, a path is provided for electrons to transfer. You feel a static discharge. Lightning is a more dramatic example of static discharge Electric Current and Ohm’s Law Electric Current – the continuous flow of electric charge. The SI unit of current is the ampere (A) • One ampere (A) equals one coulomb of charge per second There are two types of current – Direct Current (DC) and Alternating Current (AC) Direct Current (DC) – current where charge flows only in one direction (ex. Batteries) Alternating Current (AC) – the flow of electric charge that regularly reverses its direction (ex. outlets) Charge flows more easily in some material than others: electrical conductor – a material through which charge can flow easily (ex. most metals) electrical insulator – a material through which charge cannot flow easily (ex. wood, plastic, rubber, air) • Metals are typically good conductors because even though the ions in the metal are not free to move, each has electrons that are not tightly bound to it. These free electrons can conduct charge. Insulators do not have many free electrons so they do not conduct charge well. You have probably noticed that most current carrying wires are surrounded by a material that is a good insulator – why do you think? Resistance - opposition to the flow of charges in a material. The SI unit of resistance is the ohm (Ω) • As electrons move through a conducting wire, they collide with other particles (electrons and ions). These collisions convert some of the kinetic energy into thermal energy – which reduces the energy available to move electrons through the wire. This reduces the current. Besides how well a type of material conducts, there are other factors that affect the resistance of a material: thickness – when a wire is thicker there is less resistance because there is more room for electrons to flow length – when a wire is longer there is more resistance because the charges have to travel further temperature – as temperature increases, resistance increases because the faster moving electrons collide more often superconductor – a material that has almost zero resistance when it is cooled to very low temperatures Voltage (or Potential Difference) – the difference in electric potential energy between two places in an electric field. Because it is measured in units of joules per coulomb, also called volts (V), potential difference is called voltage • In order for charge to flow in a conducting wire, a source of electrical energy must be provided to maintain a difference in electric potential energy between two places. This is provided by a voltage source. Voltage Sources - devices that do work to increase the potential energy of electric charges Ex. batteries, generators, solar cells • A battery converts chemical energy into electrical energy. A chemical reaction inside the battery creates an electric field that maintains a difference in electric potential energy between the two terminals of the battery (one terminal being positive and one being negative). In a 9-volt battery the Voltage Drop, or potential difference that is maintained between the two terminals is about 9 volts. When wires are connected between the two terminals, it causes charges to move. Voltage is the cause of Current and Resistance opposes current. This relationship was expressed mathematically by a German scientist – Georg Ohm, and is expressed in what is known as Ohm’s Law Ohm’s Law Ohm’s Law – the voltage (V) in a circuit equals the product of the current (I) and the resistance ® • Voltage has a direct relationship on current • Resistance has an indirect relationship on current Voltage = Current • Resistance (volts) (amps) (ohms) (V) (A) (Ω) Ex. What current is produced when 9.0 volts is applied to a circuit with 3.0 ohms of resistance? Given Formula Solution V=I•R V or I = R Ex. What is the resistance of a wire with a current of 4.5 amps when plugged into a 120-volt outlet? Given Formula Solution Electric Circuits Electric Circuit – a complete path through which charge can flow Circuit Diagrams use symbols to represent parts of a circuit Including a voltage source and devices run by the electrical energy Series Circuit – a circuit where charge has only one path through which it can flow (picture on left) • If one element stops functioning in a series circuit, all stop since current has to be the same everywhere in the circuit • Adding bulbs to a series circuit, increases the overall resistance, which decreases the overall current, and each bulb shine less brightly • The available voltage is divided among the elements (directly proportionally to the resistance) Parallel Circuit – a circuit with two or more separate paths through which charge can flow (picture on right) • If one element stops functioning in a parallel circuit, the rest of the elements operate unchanged • Adding bulbs to a parallel circuit, decreases the overall resistance, increasing the current the voltage source puts out. However, current at other branches is not affected. • Each parallel branch receives the available voltage – it is not divided among the branches. Electric Power We have defined power as the rate of doing work, which is measure in joules per second, or watts (W). Electric Power similarly, is the rate at which electrical energy is converted into another form of energy. • Electric power is often measured in thousands of watts, or kilowatts (kW) • Electric Power can be calculated by multiplying voltage by current Power = Current • Voltage (watts) (amps) (volts) (W) (A) (V) Ex. How much power is used by a calculator with a 1.5 V battery the provides .75 A of current? Given Formula Solution P=I•V Ex. How much current is drawn by a 75-watt light bulb that is plugged into a 120-volt outlet? Given Formula Solution Power is the rate at which energy is used. To find the amount of energy we need a different equation. Energy = Power • time (kilowatt-hour) (kilowatts) (hours) (kW•hr) (kW) (hr) E=P•t One kilowatt-hour equals 3,600,000 joules Ex. If you use a 350 Watt stereo for 3 hours, how much Ex. If you use 9500 W of power on average per day, and the power Energy did you use (give answer in kW-hr and joules)? company charges you $0.08 per kW-hr, how much does it cost? Given Formula Solution Given Formula Solution Electrical Safety To be able to operate appliances independently, most homes are wired with parallel circuits. As a result, as more devices are connected to a circuit, the overall current increases, which also increases the temperature of the wire. To avoid exceeding the circuits safety limit, certain devices are usually used. Fuse – a device connected in series with the entire parallel circuit. To avoid overloading the circuit with current, a wire in the center of the fuse melts at a certain current level, which stops current in the entire circuit. Circuit Breaker – similar to a fuse, but instead of melting a thin wire, a switch opens when the current is too high Insulation – the protective coating surrounding wires prevents us from directly touching the wire. It also prevents the wires from touching each other which provides a very low resistance path for current – a short circuit Three-prong plug – the third prong on many appliances provides a ground in case a charge builds up on the appliance or a short-circuit develops – providing a path for current to the ground rather than through your body Ground-fault Circuit Interrupter (GFCI) – monitors current flowing to and from an outlet or appliance. If the two currents are not equal (which they should be) it means current is escaping and the GFCI opens to prevent shocks Electronic Devices Electronics – the science of using electric current to process or transmit information Electronic Signal – information sent as patterns in the controlled flow of electrons through a circuit analog signal – smoothly varying signal produced by continuously changing the voltage or current in a circuit (diagram A) digital signal – information encoded as a string of 1’s and 0’s (diagram B). Can be done by pulsing a current on and off (off represents a 0 and on represents a 1) Digital signals are more reliable than analog signals – unless severe damage, the signal is often still readable Vacuum Tubes – used to control electron flow, change AC into DC, increase the strength of a signal, or turn a current on or off. One example is a cathode-ray tube (CRT) Capacitors – device that stores and releases charge Semiconductors – a crystalline solid that conducts only under certain conditions. Two types, n-type and ptype, that when used together can control the flow of electrons in various ways Diodes – a solid-state component (two joined semiconductors) that only allows current into one direction – converts AC into DC Transistors – a solid-state component with three layers of semiconductors that can be used as a switch or as an amplifier, Integrated Circuits (microchips) – tiny pieces of silicon with many solid-state components like diodes, transistors and capacitors. Used in communication devices to store and process information very quickly.