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Transcript
Current Electricity What do you know about Electricity? Draw a Spider Diagram Electricity Electricity was first discovered in 600B.C. by Thales de Miletus a Greek Philosopher. He noticed that when amber, a hard dry yellow substance, is rubbed with wool or fur it attracted light material e.g. hair or dust. Facts about Electricity Benjamin Franklin demonstrated that lightning is electricity. He tied a key to a kite string during a thunderstorm, and proved that static electricity and lightning were the same thing. In 1879 Thomas Edison invented the first long-lasting incandescent light bulb that could be used for about 40 hours without burning out. By 1880, his bulbs could be used for 1,200 hours. Electricity travels at the speed of light - more than 186,000 miles per second! A spark of static electricity can measure up to three thousand (3,000) volts. A bolt of lightning can measure up to three million (3,000,000) volts, and it lasts less than one second! Electricity always tries to find the easiest path to the ground. Electricity can be made from wind, water, the sun and even animal poo. A 600 megawatt natural gas plant can power 220,000 homes. Electrical conductors and insulators • In some materials, like metals, charge can move easily. In other materials, charge cannot move easily. • A conductor allows charge to move. • An insulator holds a charge. Insulators • Insulators are substances which do not normally allow charge to flow through them. • E.g. perspex, glass, polythene Conductors Conductors are substances which allow charge to flow through them freely. e.g. metals like copper Current Electricity The first battery (or cell) was discovered by Luigi Galvani. He was dissecting a frog at the time. He happened to be using two dissecting tools each of different metals. He touched a nerve and the dead frog's leg twitched! He had accidentally produced an electric current. This lead to the development of what we now call the battery Electrical circuits • An electrical circuit is an arrangement of conductors connected together so that charge can move around in a loop. Battery has a positive and negative terminal What is an electric current? An electric current is a flow of microscopic particles called electrons through wires and components. + - In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell. Electric Current A flow of electric charge is called an electric current Insulators & Electrons In some atoms there is a strong force of attraction between nucleus and outer electrons. These atoms do not tend to lose electrons and have nearly no free electrons These materials are known as insulators Conductors & Electrons Electrons are negatively charged. Outer electrons in some atoms are only loosely attached to nucleus These electrons can easily escape from the atoms Materials containing free electrons like these are known as conductors Mandatory Experiment To test if a substance is a conductor or Insulator Pg. 348 Method. 1. Make a list of all the materials you are going to test 2. Divide them into two columns predicting whether they are insulators or conductors Table predicting insulators or conductors Material Insulator Conductor Method 3. Set up the circuit as shown in diagram Method 4. At the beginning there is a gap between the crocodile clips X and Y. The bulb does not light because the circuit is not complete and electrical charge cannot flow through it. 5. Now bridge the gap from X to Y with one of the materials you want to test. Method Observe if the bulb lights up or does not light up. If the bulb lights up the material you are testing is a Conductor. If the bulb does not light up the material you are testing is an insulator. Result When _________ attached at clip X and Clip Y the bulb lighted up and the circuit was complete When ___________ was attached at clip X and Clip Y the bulb did not light and the circuit was not complete Conclusion ___________ are conductors and ______ are insultators Current Electricity Current Electricity is the flow of electrons from the negative terminal of a battery to the positive terminal Simple Electrical Circuits When charge moves as in current electricity these charges flow around a path called an electric circuit. bulbs Battery wire Points to remember about electric circuits 1. Circuits must be complete for current to flow if there is a gap or break in the circuit current will not flow. A switch provides a gap in the circuit. Current flows when the switch is closed or “on”. Current cannot flow when the switch is open “off”. 2. Potential Difference A circuit must have a source of potential difference (p.d) This is also called voltage (electron moving force e.m.f.) This provides the energy to push the electrons around the circuit. Electrons cannot move around a circuit without this energy. Voltmeter A voltmeter is used to measure potential difference or voltage between two points in a circuit multimeter 3. Current The potential difference or e.m.f. in a circuit causes electrons to flow ie. a current To measure electric current an Ammeter is used. This measures the amount of charge passing a particular point in one second 4. Resistance Moving electrons may meet a part of a circuit that is difficult to pass through e.g. the thin wire in a bulb. The wire in the bulb offers a resistance to the electrons – it slows the electrons down. In the bulb electrical energy is turned into heat energy and light energy. Resistors are often used in circuits to produce heat or light or reduce the current flowing in the circuit. A rheostat is a resistor whose resistance can be changed. They are also called variable resistors. They can be used to control the volume of the radio. Ohmmeter An ohmmeter is the instrument used to measure the resistance of a resistor Units used in Electricity Different units are used in measuring various quantities Unit Symbol of electricity Information Volt V Ampere (AMP) A Unit of voltage or potential difference. It is a measure of push or strength of a battery e.g. 1.5 V batteries, 6V batteries Unit of Current. It gives a measure of the amount of charge flowing past a point in a wire per second. A current of 0.05 going across your heart could kill you. Ohms Symbol Ω Unit of resistance It measures the opposition of part of a circuit to the flow of charge. The greater the resistance the smaller the current. Scientists Alessandro Volta from Italy invented the battery. Andre Marie Ampere He devised a method of measuring current and put forward a theory to explain magnetism Circuit Vocabulary and diagrams Symbols for a circuit Relationship between Voltage, Current and Resistance You can use an electric circuit to show a relationship between voltage, current and resistance. 1. The resistance of a coil will be measured. 2. The voltage across the coil and the current going through the coil will be measured 3. We will establish a ling between resistance, voltage and current Instead of using a battery we can use low voltage power supply. This allows us to vary the supply of voltage to the circuit. We use a voltmeter to measure voltage (potential difference) in a circuit. A voltmeter must be connected to two different points in the circuit. It is connected in parallel in a circuit. An ammeter is used in the circuit to measure current. The ammeter is connected in series in the circuit. Simple Circuits • Series circuit – All in a row – 1 path for electricity – 1 light goes out and the circuit is broken • Parallel circuit – Many paths for electricity – 1 light goes out and the others stay on Ohm’s Law • The ohm (symbol: Ω) is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Ohm’s Law • The current decreases because the resistance increases. • Ohm’s Law says that I=V/R. The voltage in the system is constant, resistance increases. The relationship between current, potential difference and resistance in metal conductors • If we measure the current passed through a metal conductor which has a certain resistance and also measure the voltage (potential difference) needed to push the current through the resistor, we find that as the voltage is increased, the current increases. We can write this relationship in a formula. • Voltage = resistance x current, or • V = RI Experiment to Measure the Resistance of a coil or a resistor (Mandatory Experiment) Results Table Voltage (V) Current (A) 2 4 6 8 10 12 Ohms Law Calculations based on Ohms Law Relationship between potential difference, current and resistance Georg Ohm showed for a given conductor at constant temperature the voltage and the Current are proportional to each other Ohms Law Triangle Simple calculations on V=RI • Calculate the voltage across a resistance of 15 ohms when a current of 0.5 amps flows through it. • Solution: We are given R = 15 ohms and I = 0.5 amps. Using V = RI V = (15)(0.5)=7.5 volts Effects of an Electric Current We cannot see electric charge moving. We will examine 3 effects of electricity. These are when electrical energy is converted to (1) heat energy (2) magnetic energy (3) chemical energy 1. Heating Effect Some metals e.g. nichrome have a high resistance to the flow of current. More energy is need to force current through the nichrome wire. As a result heat is given off. That is why nichrome is used in the element of an electric fire. Nichrome Wire Tungsten Wire Tungsten wire is used in light bulbs.The bulb is filled with argon or nitrogen gas. Experiment to demonstrate the heating effect of an electric current Expt 45.4 pg 354 Apparatus: Beaker, water, coil of nichrome wire, low voltage power supply, calorimeter, labquest & temperature sensor, timer Method: 1. set up apparatus as shown in diagram 2. Take the temperature of the water 2. Turn on the power supply Result: Conclusion Diagram: Time Temp of water 0 mins 20.9 2 mins 22.8 4 mins 23.3 6mins 23.5 8 mins 24 To demonstrate simple series and parallel circuits containing a switch and bulb • Date: • Apparatus: Low voltage power supply, switch, 2 bulbs, connecting wires, crocodile clips • Method: • Diagrams • Result Simple series and parallel circuits Result: a) Bulbs in series b) Bulbs in Parallel Conclusion Note: Old Christmas tree lights are sometimes arranged in series – if one bulb blows all the lights go out. Most lights in your house are arranged in parallel the big advantage of this is that they can individually be switched on and off. Different ways of wiring a circuit • There are two ways of wiring light bulbs in a circuit. • In series: The bulbs are connected so that the current through one passes through the next. Different ways of wiring a circuit • In parallel • The current is divided into different branches. • The advantage is that if one bulb blows, the others remain lit A Fuse A fuse is thin piece of wire enclosed in a porcelain case. It is a weak link which is deliberately put into a circuit to make it safer. If a current above the expected level flows the fuse wire will get hot and melt safely in its case and the circuit will be broken. A fuse Draw diagram on pg. 355 fig 45.10 Everyday applications of heating effect of an electric current 1. Electric current producing heat in an electric fire. 2. Metal coil in an electric kettle 2. Magnetic Effect A compass needle moves if placed near wire that had current flowing through it. When a current flows in a wire that is wound around a nail the nail becomes a strong magnet called an electromagnet. Everyday applications of the magnetic effect of an electric current 1. Electromagnets are used in electric bells 2. Electromagnets are used as circuit breakers. 3. Chemical Effect The chemical effect of an electric current can be used to separate water into hydrogen and oxygen This is called electrolysis of water Everyday Applications of the Chemical Effect of an Electric Current 1. Electrolysis can be used in beauty treatments to remove hair from the body. 2. Electroplating is used in making cutlery eg forks. A cheap metal is covered with a layer of another metal to protect it from corrosion and to make it look better. Electroplating Coating a cheap metal with a thin layer of another more expensive metal. E.g. cutlery Direct and Alternating Current 1. Direct Current Current that travels in one direction only is known as direct current or d.c. for short. e.g. a battery pushes current in one direction only. Alternating Current The current from the mains supply changes direction 100 times per second. This type of current is called alternating current or a.c. AC to DC For most appliances it does not matter whether the current is alternating or direct. A bulb will work if the current is flowing through it in one direction only or if it is constantly changing direction. Some appliances like a T.V. need a d.c. supply. A device called a rectifier is needed to convert a.c. to d.c. Summary: Key points • Charge exerts a force. There are two types: positive and negative. • Unlike charges attract, like charges repel. • Conductors are materials that allow charge to move through them. Insulators are materials that hold a charge. • All charge tries to go to earth. The earth is a source and a sink for charge. Summary (continued) • Electrical current is the movement of charge through a material. • Potential difference is the energy needed to move charge between points. • When current moves through a device, there is resistance to this movement. • The relationship between current, potential difference and resistance in a metal resistor is V = RI. • Devices can be connected in a circuit in series or in parallel.