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I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity There is hardly any area of present day human activity that does not depend in some way on electrical or electronic devices. And most of them contain control systems, that is, circuits which control the way the device operates. 1.- ELECTRIC CHARGE Matter is made up of atoms. Atoms are made up of other smaller particles: Neutrons: they are in the nucleus. They don’t have an electric charge. Protons: they are in the nucleus. They have a positive electric charge. Electrons: they’re around the nucleus. They have a negative electric charge Normally, matter is neutral because there is the same amount of positive and negative charges. However, sometimes electrons can go away from their atoms to different ones so that they can pass from an object to another material. For example, if we rub a biro with a cloth, electrons pass from the cloth to the biro; the biro will then have a negative charge. If we bring the biro close to small pieces of paper, they will be attracted by the biro. But the negative charges in the biro are at rest, they do not move through the material. So we say there is STATIC ELECTRICITY. Remember: two different electric charges (+ and -) are attracted to each other. Electric charges of the same type repel each other. 1 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 2.- ELECTRIC CURRENT It is a flow of electrons travelling through a conductor. So electrons are not static, they are moving through the material in a specific direction, like in a water pipe. Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=8gvJzrjwjds&index=1&list=PL253772980E9A0F88 There are two kinds of materials: Conductors: They allow electric current to travel through them very easily (low electric resistance). Good conductors are metals. Copper, aluminium, gold and silver are the most important Insulators: They do not allow electric current to travel through them at all (high electric resistance). Good insulators are plastic, wood and ceramic. There are two types of current: Name Abbreviation Alternating Current AC Direct Current DC Features 110 V. or higher. No direction of current. No positive or negative terminals. Used at home (electrical appliances) and in industry. 48 V. or lower. The current has a specific direction in a circuit. There is a + and a - terminal. Used in circuits with batteries. Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=vN9aR2wKv0U 3.- ELECTRIC CIRCUIT It is a group of connected components that allow an electric current to pass through them constantly while the circuit is complete. Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=VnnpLaKsqGU To allow this, the circuit has to be complete, that is, there should be a continuous flow of electricity from the + terminal to the – terminal. A basic circuit is made up of the following components: Generator: to provide the electric energy to the circuit. For example a battery or a power source. Conductors and insulators: both allow current to travel. We use them for connecting all the elements. Receptors: to transform the electric energy into a different type of energy (buzzers-sound, bulbs-light, motors-movement, resistors-heat…). 2 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity Besides, to improve the good working of the circuit, we should add two more components: Control components: they allow us to control the flow of the current whenever and wherever we want (switches, relay, etc). Protection components: they can detect any dangerous voltage or current and break the circuit to avoid higher risks to people and the circuit itself. (Fuses, automatic switches, etc). 4.- ELECTRICAL SYMBOLS Instead of drawing the components of a circuit, we can represent them by symbols: The most used electrical symbols are: Component Battery Light bulb Electric motor Drawing Symbol Mission Generates voltage Transforms electric energy into light Transforms electric energy into spin movement Resistor Transforms electric energy into heat Switch Controls the flow of current Single pole double throw (SPDT) switch Push switch Fuse Connection Buzzer Allows to choose between two circuits Controls the flow of current for certain time Protects the circuit against high current Connects two parts of a circuit Transforms electric energy into sound (buzz) Ammeter Measures the current Voltmeter Measures the voltage 3 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 5.- EFFECTS OF AN ELECTRIC CURRENT Receptors can transform the energy in an electric current into other type of energy that we can use, such as heat, light, sound and motion. Heat Electrons crash into the atoms of the material they are flowing through. Part of the energy that they are carrying is transformed into heat when these collisions happen. This is called the Joule effect. The higher the resistance of a conductor wire, the higher the electrical energy is transformed into heat. : Conductivity (depends on the material) R L S L: Length S: Section The components used to transform electrical energy into heat are called resistors. They are commonly used in appliances such as toasters, hair dryers, ovens and washing machines. Light Take a look at this website: http://www.leroymerlin.es/productos/iluminacion/bombillas_y_tubos.html Electrical energy can be transformed into light using different types of components: Light bulbs: o Filament bulb: No longer manufactured due to its low efficiency (20 %) and life (1,000 hours). o Halogen bulb: Low efficiency. o Low-energy bulb: A kind of fluorescent tube. Low power consumption and long life (5,000 h). o LED bulbs: They are expensive, but worth because of their very low consumption and very long life (20,000 h). Fluorescent tubes: Some gases emit light when they carry an electric current. Motion Electric motors transform electrical energy into mechanical energy. They are based on the forces of attraction and repulsion between a magnet and a conductor wire placed in one or more loops inside it. Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=d_aTC0iKO68 4 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 6.- BASIC ELECTRIC QUANTITIES There are three main electrical magnitudes and they are related to each other: Voltage (V) It is the force that causes the electrons to move in an electrical circuit. Just as water needs some pressure to force it through a pipe, electrical current needs some force to make it flow. Voltage is typically supplied by either a generator or a battery. Voltage is measured in Volts (V). A Voltmeter is the instrument to measure the difference of voltage between two points of a circuit. Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=zYS9kdS56l8 Current (I) Current is determined by the number of electrons that pass through a cross-section of a conductor in one second. The electric current is measured in amperes or amps (A). An ammeter is the instrument we use to measure the flow of electrons (e-), that is, how many amps of current are flowing in a specific point of an electrical circuit. A current of one amp means that in one second about 6.24 x 1018 electrons move through a cross-section of a conductor. Resistance (R) Electrical resistance is defined as the resistance to flow of electricity through a material. Even the best conductors, such as gold, have some resistance. Talking about conductors, resistance increases with an increase of length or a decrease of cross-section, but all the components in a circuit (lights, motors, buzzers…) impede the flow of current The electrical resistance is measured in ohms (Ω). Remember: the lower the resistance of a material, the better the material conducts. Magnitude Unit Instrument Amperage A Amperes A Ammeter Voltage V Volts V Voltmeter Resistance R Ohms Ω Ohmmeter The multimeter is an instrument that can measure all of these electrical quantities Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=THP4GsECIyw 5 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=-mHLvtGjum4 7.- OHM’S LAW The relationship between current, voltage and resistance was studied by the 19th century German mathematician, George Simon Ohm. Ohm formulated a law which states that current varies directly with voltage and inversely with resistance. From this law the following formula is derived: I V R There is an easy way to remember which formula to use. By arranging current, voltage and resistance in a triangle, one can quickly determine the correct formula: Using the simple circuit below, assume that the voltage supplied by the battery is 10 volts, and the resistance is 5 W. To find how much current is flowing through the circuit, cover the I in the triangle and use the resulting equation. I V ; R I 10volts ; I= 2 A 5 8.- ELECTRIC POWER. ELECTRICAL ENERGY 8.1.- Electric Power Watch this video (or others) on Youtube: https://www.youtube.com/watch?v=p8JQTLkV5C8 The Electric Power consumed in a resistor depends on the amount of current that passes through the resistor for a given voltage. This is expressed as voltage times current: P V ·I Power is measured in watts (w) and is represented by the letter “P”. The watt is defined as the rate work is done in a circuit when 1 amp flows with 1 volt applied. 6 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 8.2.- Electrical Energy The electrical energy is one of the items we have to pay in the invoice to the provider company. The more energy you consume at home, the more you pay. The energy transformed into a different type of energy by an appliance (resistor) depends on two factors: the power that is being consumed in a resistor and the time that lasts: E P·t Electric Energy is measured in watts per second (w·s), or kilowatts per hour (Kw·h) and is represented by the letter “E”. 7.3.- Cost of electricity The variable part of an invoice (if you do not use electricity at home you will pay anyway a fix amount of money) depends on how much electric energy has been consumed and the price of it. Cost E·price For instance, imagine that you have a LED light bulb of 10 W on for 5 hours and the price of electricity is 0.15 €/Kwh: E P·t ; E 10w·5h 50wh ; Cost E·price ; E 0.05Kwh Cost 0.05Kwh·0.15€ / Kwh ; Cost 0.0075€ A real invoice: 7 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 9.- CIRCUITS 9.1.- Series circuits A series circuit is formed when any number of electric components is connected end-to-end so that there is only one path for current to flow. a) Voltage sources connected in series: b) Resistors connected in series: It I1 I 2 I 3... Rt R1 R2 R3 ... The resistors can be actual resistors or other devices that have resistance (motors, bulbs, speakers…). Calculations 1.- Draw the equivalent circuit: 2.- Calculate the Total Resistance (Rt). Rt = R1 + R2 + R3 + R4 Rt = 5 + 1 + 2 + 2 = 10 Ω 3.- Calculate the total current (It), that is the same every where: It = Vt / Rt = 12 V / 10 Ω = 1.2 A 4.- Calculate the voltage on every resistor. V1 = I1 · R1 = 1.2 A · 5 Ω = 6 V V3 = I3 · R3 = 1.2 A · 2 Ω = 2.4 V 8 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity V2 = I2 · R2 = 1.2 A · 1 Ω = 1.2 V V4 = I4 · R4 = 1.2 A · 2 Ω = 2.4 V 9.2.- Parallel circuits A parallel circuit is formed when two or more resistances are placed in a circuit side-by-side so that current can flow through more than one path. a) Voltage sources connected in parallel: Vt V1 V 2 V 3 All of them must have the same voltage and be connected the same way (positive to positive and negative to negative). This arrangement is used to provide the voltage to the resistances for more time. b) Resistors connected in parallel: 1 1 1 1 Rt R1 R 2 R3 Calculations 1.- Draw the equivalent circuit: 2.- Calculate the Total Resistance (Rt). 1/Rt = 1/R1 + 1/R2 1/Rt = 1/10 + 1/15 1/Rt = 15 + 10 / 150 = 25 / 150 Rt = 150 / 25 = 6 Ω 3.- Calculate the total current (It) in the equivalent circuit, that is the maximum current in the original one. It Vt 24V 4A Rt 6 4.- The total voltage (Vt) is given, and is the same for all the resistors (24 V): Vt = V 1 = V 2 5.- Calculate the current that flows through every resistor: I1 V 1 24V 2.4 A R1 10 I2 V 2 24V 1.6 A R 2 15 It = I 1 + I 2 9 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 9.3.- Series-parallel circuits Series-parallel circuits are also known as compound circuits. At least three resistors are required to form a series-parallel circuit. The following illustrations show two ways a series-parallel combination could be found: Some examples of compound circuits could be the following: Calculations There are two methods to know all the electric magnitudes in a series-parallel circuit: A) Using equations: 10 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity B) Using the equivalent circuit: 1) Draw the equivalent circuit and calculate the total resistance RT 2) Calculate the total intensity of current IT = I1 It Vt 9V 0.0036 A 3.6mA Rt 2500 3) Calculate the voltage dropped across the first resistor VAB VAB = I1 · R1 = 0,0036 A · 2000 Ω = 7.2 V 4) The voltage dropped across the other two resistors VBC will be the rest of the voltage VBC = VT – VAB = 9 V – 7.2 V = 1.8 V 5) Calculate the intensity of current flowing through the two branches, I2 and I3 I2 VBC 1.8V 0.0018 A 1.8mA R2 1000 I3 VBC 1.8V 0.0018 A 1.8mA R3 1000 8.4.- Common circuits 11 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 9.- DANGERS OF ELECTRICITY Always . . . Use shielded plugs. Switch off the socket before inserting or removing a plug. Switch off the appliance before inserting or removing a plug. Select and use the correct fuses. Unplug a kettle whilst filling with water. Replace all cut, frayed and damaged cables immediately. Take care when using mains electricity. Never . . . Touch mains plugs and sockets with wet hands. Remove covers from appliances when they are plugged in. Connect appliances with covers removed. Connect two many appliances to the same socket. "Test" by probing with a screwdriver. Be tempted to "play" with mains voltages - it could be fatal! 10.- ENERGY CONSUMPTION. EFFICIENCY There are several ways to save electricity at home through the careful use of home appliances, lights, electronic gadgets and heating/cooling systems: Home appliances: o Use the washing machine, dishwasher and tumble dryer only when they are full, and use low temperature and eco programs if possible. o Fridge: place it far away from the oven. Check that the doors close properly. Open the doors just for the necessary time. Keep the freezer at an average of -15 ºC. o Induction and ceramic cookers: turn cookers off a few minutes before you finish cooking to make use of the residual heat they keep. Lights: o Make the best of natural light. o Turn off lights you are not using. o Use low-energy or LED bulbs. They consume less energy and last longer. o Electronic devices: switch off the devices you are not using, even ones that have a stand-by mode as they continue using some electricity. Heating and cooling systems: o Keep the temperature between 19 and 21 ºC in winter, and between 22 and 25 ºC in summer. 12 I.E.S. Emilio Alarcos Technology 3º E.S.O. Electricity 11.- RELAYS A relay is a switch which is turned on and off by an electromagnet. When a small current flows through the coil it produces a magnetic field that magnetizes an iron core inside. This attracts the armature which forces the switch contacts to touch. When the current is turned off, the switch contacts open again. Normally, the small current through the coil controls the high current through the contacts. Relays allow the design of automated circuits that work on their own, so that nobody has to operate any switch. Some examples are: 13