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Transcript
Electrical Energy, Electric Power and Circuits Colleyville Heritage High School Mr. W. Puckett Electric Current The opposite of electrostatics is electric current; the flow of electric charges like electrons. We get electric current from wall sockets primarily. It is made with generators at electric generation plants. Why Would Electrons Move in Electric Current ? Electricity is the movement of electrons. Why would it move? Because an outside force pushes it. The pressure / force that pushes electrons in electricity is the electric field Potential Energy. We call this pressure VOLTAGE The electric generator pushes the charges with voltage. Much like water is pushed with a pump. Electric Current Analogy to Hydrology A good example is to compare the flow of water in a hose to electric flow. The amount of water flow is equal to the current. The water pressure is equal to voltage potential. The resistance is the backpressure due to either the diameter of the hose or a squiring handle nozzle that regulates water flow. Potential Electricity can be described like flowing water. Current Load Example of Using Water to get Work Done that compares to Electricity The turning wheel make corn kernels into corn meal for cooking. Gris Mills use water to turn the wheel to grind the grain. Flowing charge is called current. The symbol is I and the unit is the ampere or amp (A). SIM Electrical Current The net amount of electric charge that passes through a circuit is called the Electric Current (I) and is described by the equation: I = DQ / Dt where Q is the amount of charge and t is time. The scientific unit for current is the Ampere (A) and is equal to the flow of 1 Coulomb per second. Charge flows in an Electric Circuit. Switch Charge is a quantity of electricity. Charge has the unit coulomb (C). Direct Current Conventions Direct current in electrical devices involves the movement of charges in only one direction. Definition of the current flow is a science subject specific: Chemistry defines electricity as the flowing of electrons (-) with their negative charges. Physics & Engineering define electricity as the flowing of (+) charges of potential energy. Ohm’s Law The empirical relationship that states the current is proportional to the applied voltage divided by the resistance : I = V/R or V = IR R is the resistance to electrical flow measured in Ohms, , and is equal to volts/amps. V is volts; which is electrical potential energy in Joules/Coulomb. Electrical Energy and Power Since the change corresponding to a change in potential, DPE= q DV, then the rate at which energy as charge, Q, passes through a resistor is DPE/ D t= (D Q/ D t) DV = IV. Therefore , power loss in a circuit is P = IV = I2R = V2/R = W/t Work done by Electricity Work can be done by electricity with electric motors or used by heating machines like stoves and heaters. The cost of electric power is pennies per kilowatt-hour (kWh) = 1000 watts used continuously for one hour. 1 kWh= (1000 W)(3600 s) = 3.6x106 J Potential Energy & Electricity Fields As an electric charge moves up or down an electrical field it changes potential energy. PE electric = - qEd ; where PE is in Joules, q is the charge in coulombs, E is the electric field strength in Newtons/Coulomb, and distance =meters. /charge + Potential energy =Voltage Electric Field Strength Potential Energy & 2 Charges Charges interact with each other and produce a non-uniform electric field. This requires a different formula than a force. PE electric charges = k q1q2/ r ( note the radius in NOT squared here – calculus integration between force and PE.) Parts of an Electric Circuit Electric Circuit – a complete closed path through which charges can flow. There are 3 Basic Parts to an Electric Circuit 1. Energy Source 2. Wires 3. Loads Parts of an Electric Circuit Parts of an Electric Circuit Sometimes circuits will also contain a switch. A switch is used to open and close a circuit. When the switch is “on” the circuit is closed and charges are able to flow. When the switch is “off” the circuit is open and charges are NOT able to flow. Parts of an Electric Circuit Symbol Conventions for Circuits. Schematic map keys for electronic components. House Schematic Diagram Direct Current Circuits When a continuous conducting path (wire) is connected between the terminals of a battery, it is called a circuit. A map of its paths and components is called Schematics Direct Current Circuits are those electronic pathways that have only one direction of electrical flow and are composed of : Power sources, conductive connectors, resistors, switches, fuses, GFI’s and possibly capacitors. Monitoring of the circuits energy conditions is done with ammeters, voltmeters and multipurpose-meters. Series Circuits Series Circuit – a circuit where all parts are connected in a single loop. All the loads in a series circuit must share the same current. An example of this is that if lights are wired in a series circuit they will all glow the same. If another bulb is added the bulbs will glow dimmer. Series Circuits Uses for Series Circuits Series Circuits are useful in wiring burglar alarms. If any part of the circuit fails there will be no current in the system and the alarm will sound. Series circuits are not very useful in homes since all the parts of the circuit must be turned on for any part to work. Resistors in Series Circuits Resistors in Series In a series circuit the sum of the voltage drops equal the voltage drop across the entire circuit. V = V1 + V2 + V3 etc.... Vi and also V = IR1 + IR2 + IR3 …. The current stays the same throughout the circuit. The effective resistance in a series circuit is the sum of all the resistors. R = R1 + R2 + R3 etc.... Ri The current through a series circuit is found by calculating the effective resistance and then using Ohms law in the form of I = V/R Parallel Circuits A parallel circuit is a circuit in which the loads are attached side by side. There is more than one path for charges to flow within the circuit. We call the parallel paths BRANCHES. Loads connected in parallel can take advantage of a power source’s full voltage. This allows all the bulbs connected in parallel to glow at full brightness. Also, if one of the bulbs goes out the rest will keep working. Parallel Circuits Resistors in Parallel Circuits Uses of Parallel Circuits Useful in homes because you can turn off one appliance on a circuit and are still able to use the other appliances on that same circuit. Resistors in Parallel Circuits In a PARALLEL circuit each resistor provides a new path for electrons to flow. Each resistor can be operated independently and if one path is open it will not affect any other parallel pathway. The total current in a parallel circuit is the sum of the currents in its branches. The voltage remains the same throughout the circuit. I = I1 + I2 + I3 OR I= V = V + V + V R R1 R2 R3 If you divide each side of the equation by V:::: ∑Rparallel = 1 = 1 + 1 + 1 R eq R1 R2 R3 Take the inverse of 1/Req to find the equivalent resistance. First: Add the Inverses of the Resistors Values Parallel Circuit Resistor Addition Adding parallel resistors requires that you add the inverses of the individual resistors and then invert the final sum. Ex: look at the top circuit: 1/6 + 1/6 + 1/6 = 0.5 Then take the inverse of the sum: 1/0.5 = 2 Ώ Effective Resistance Summation in a Circuit In analyzing a combination of series and parallel circuits, you must first combine parallel resistors into a single resistor. Then combine series resistors to find the Total Effective Resistance of the Circuit. Ohm's law can then be used on each separate part of a series - parallel circuit. Total Effective Resistance in a Circuit Add parallels with inverse summation. Then add series with normal addition. = Total Resistance Circuit Resistor Summation Water Flow Circuit Pattern: Compare this to the electricity Volume in = Volume out through the path of least resistance. Parallel Circuit Current Flow Patterns Parallel Circuit Current Addition Batteries convert chemical energy into electrical energy! douglasbattery.com 1. Dry Cell Battery creative-science.org 2. Wet Cell Battery channel4.com Batteries The parts of the battery ( electric cell) are the electrodes (anode and cathode), a connecting part (ion/salt bridge) for the electrodes and the electrolyte. The simplest battery is just two different types of metal divided by an electrolyte. Technically, a series of electric cells makes up a battery. For Example, a series of six 2 volt cells makes up a 12 volt car battery. Battery Mechanism When the anode (usually zinc or other electrically active metal) dissolves in the acidic electrolyte and the cation goes out and leaves the electrons (oxidation). Those electrons flow to the cathode through the connecting bridge to the cathode (reduction site) at a rate and potential difference based upon how different the electrochemistry is between them. This is the Electro Motive Force (emf) in the battery. AC/DC Direct current (DC) is charge moving in one direction •Alternating current (AC) is charge moving back and forth. •Batteries produce direct current (DC). •The power company uses electrical generators to produce alternating current (AC). •120V of AC is available at your house’s wall sockets. •It is more efficient for power companies to send AC to your house, but most electrical devices need DC. •These devices have components called capacitors inside them that convert AC into DC. 120V AC 9V DC 1.5V DC Electrical Resistance. – This is when current flow is slowed down. – Current seems to lose energy. Actually, the electrical energy is converted into heat and/or light. – The symbol is R. – The unit is the ohm (). Water Heater Element Stove Top Burners Light bulb Resistivity and Temperature The resistivity in most metals increases linearly with increasing temperature. The hotter the wire / electronic; the more the resistance. A conductor’s electrical resistance depends on Four Variables. The length of the conductor. The cross-sectional area of the conductor. The material of which the conductor is made. The temperature of the conductor. Problem Types 1. Ohms Law with voltage, current, resistance. 2. Power calculations 3. Circuit analysis – Simple series or parallel circuits – Complex circuits with multiple branches 4. Capacitance 5. Daily Household electrical costs. 6. Transformer calculations Complex Circuit Analysis When a circuit has both parallel and series circuits, the analysis of the total resistance is done by – adding all possible series resistors – Then convert parallels to series – Then finish adding all resistors in series. Complex Circuit Analysis Try this circuit: Your Turn to Find Total Resistance The relationship between voltage, current, and resistance is V = I · R or Ohm’s Law fm-transmitter.com V = Voltage I = Current R = Resistance (SIM1) (SIM2) esitest.com/img Digital Multimeters can measure voltage, current, resistance, and more. Electrical Work. A voltage source does work when releasing charge. The unit of electrical work is joules (J). Electrical Power. The unit of power is called the watt (W). power = current · voltage P=I ·V Paying for Electricity. The electric company charges you based on the amount of power and the amount of time. Energy (electrical) = Power x Time The unit is kilowatt-hour (kW-hr). 1 kW = 1,000 Watts Transformer Equation Vs = Ns = I p Vp Np Is Where V is voltage of secondary / primary = the number of wire wraps N of the secondary / primary. Voltmeters and Ammeters An Ammeter is used to measure current in a circuit. It is hooked in series within the circuit to measure the entire amount of electricity being transmitted. It should have a very small restriction effect upon the circuit. A Voltmeter is used to measure the voltage potential within a circuit. It is hooked in parallel and has a high resistance to measure the electrical “pressure” pushing the current. Voltmeters and Ammeters Multi Meter This meter has multiple uses including as a voltmeter and an ammeter Parallel Plate Capacitor For a parallel plate capacitor the voltage V is given by the equation: V= Ed; A capacitor can change AC to DC inside of an electrical device and adjust radio freq. Capacitance A capacitor is a device that stores energy temporarily. It is used to change AC to DC current and tuning frequencies on radios (and several other electric functions.) The SI unit for capacitance is the Farad (F). C = Q / ΔV where Q is coulomb and V is volts and C = ε0 A/d ; where ε0 is permittivity of free space vacuum; A is area in m2 and d is distance between plates in meters. Dielectric = Insulation between Capacitor Plates Capacitors in a Circuit Capacitors are shown by 2 equal size parallel lines in schematics. Potential Energy in a Capacitor A charged capacitor stores electrical PE because it requires work to move charges through a circuit to the opposite plates of a capacitor. PE electric capacitor = ½ QV = ½ CV2 = Q2/2C Capacitors eere.energy.gov Several things in our homes consume electricity. eere.energy.gov The hot water heater uses electricity to heat water. Look for this sticker to find out how your appliance compares to others that are similar. eere.energy.gov This meter can usually be found outside our home. It keeps track of the amount of electrical power that your home uses over time. It is illegal to tamper with it!!! Where does electricity come from? Power plants use generators to produce power. This generator is called a turbine. Electric Generator Electricity is not entirely efficient. Household Circuits In the home, your circuits are wired in parallel to allow for all electric appliances and motors to have the same voltage. Circuit breakers, fuses and Ground Fault Interrupters (GFI’s next to water sources) are used in household circuitry to prevent shocking and overheating of the electric circuitry. Common voltages are 120V and 240 V. Normal appliances use 120V while larger ones require 240V ( electric stoves and clothes dryers). Household Circuit Safety Circuit Failure - Broken wires or water can cause a short circuit. In a short circuit, charges do not go through one or more loads in the circuit. This can cause heat to build up and fires to start. Fuses - A fuse has a thin strip of metal. Fuses keep charges from flowing if the current is too high. If the current does get too high the strip of metal will melt and the circuit will be broken. Household Circuit Safety Circuit Breakers - a switch that automatically opens if the current is too high. Charges stop flowing. Electrical Safety Tips - Do not overload circuits by plugging in too many electrical devices. Do not use electrical devices near water. Electrical Safety Safety should always be foremost in everyone’s mind. A small electrical shock produces “tingling” and a large one can burn skin, stop the heart, brain or both. We use electricity to restart hearts in a direct shocking procedure called “defibrillation” The first line of defense against electrical shock is the insulation on the wires. Fuses, Circuit breakers and GFI’s are used to prevent shocking. A circuit breaker is a resetable / reusable fuse. Electrical Fuses and Circuit Breakers