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Transcript
8 Making use of electricity 8 Making use of electricity 8 . 1 Conditions necessary for current flow 1 For a current to flow, a complete path and a source of electrical energy are necessary. 2 A complete circuit is called a closed circuit while an incomplete circuit is called an open circuit. 8 . 2 Electrical conductors and insulators 1 Materials that conduct current are called electrical conductors. Metals, salt solutions and graphite are examples. 2 Materials that do not conduct current are called electrical insulators. Wood, cotton and plastics are examples of electrical insulators. 3 A switch is a device for opening or closing an electric circuit. 8 . 3 Current, voltage and resistance 1 Matter is made up of atoms. electron (carries negative charge) nucleus (carries positive charge) Structure of an atom Mastering Science © Oxford University Press 2011 Short notes for revision 2 -1- 8 Making use of electricity 2 Electrical conductors have a large number of free electrons. Electrical insulators have very few free electrons. 3 In a closed circuit with an energy source, there is a net flow of free electrons through the circuit from the negative terminal to the positive terminal. 4 The direction of current is opposite to that of electron flow, i.e. from the positive terminal of the cell to the negative terminal through the circuit. current Direction of electron flow 5 Direction of current Analogy of an electric circuit with a water flow model: water pump water battery current water pipe bulb wire fan 6 Parts in the water flow model pump water flow fan water pipe Parts in an electric circuit battery current bulb wire The voltage of a cell is a measure of the amount of energy supplied by the cell to the free electrons in a circuit. Mastering Science © Oxford University Press 2011 Short notes for revision 2 -2- 8 Making use of electricity 7 Measuring current and voltage: Current Voltage 8 Unit Symbol Ampere A Milliampere mA (1 mA = Volt 1 A) 1000 V Ammeter Voltmeter In circuit A, the more the cells, the larger the current in the circuit and the brighter the bulb. black knob black knob ammeter voltmeter red knob Circuit A 9 Measured with red knob Circuit B In circuit B, the more the cells, the higher the voltage across the cells and the larger the current flowing through the circuit. 10 Resistance is a measure of the opposition of a material to the flow of current. Its unit is the ohm (). 11 Resistor: has a certain value of resistance. the higher the resistance of the resistor connected to a circuit, the smaller the current flowing through the resistor. Resistors Mastering Science © Oxford University Press 2011 Short notes for revision 2 -3- 8 Making use of electricity Е 12 Factors affecting the resistance of a wire: Factor Effect on the resistance of a wire Material A copper wire has a lower resistance than a nichrome wire of the same length and thickness. Length For wires made of the same material, a longer wire has a higher resistance than a shorter wire of the same thickness. Thickness For wires made of the same material, a thinner wire has a higher resistance than a thicker wire of the same length. 13 A rheostat is a resistor that has an adjustable resistance. It can be used to adjust the size of the current in a circuit. current out current in A sliding rheostat current out current in A rotary rheostat Mastering Science © Oxford University Press 2011 Short notes for revision 2 -4- 8 Making use of electricity 8 . 4 Circuit symbols and diagrams 1 The circuit symbols of some common circuit components: Circuit component 2 Circuit component Circuit symbol Battery Resistor Connecting wire / connector Rheostat Switch Ammeter Bulb Voltmeter Circuit symbol An example of a circuit diagram: Mastering Science © Oxford University Press 2011 Short notes for revision 2 -5- 8 Making use of electricity 8.5 Series and parallel circuits Comparing the series circuit and the parallel circuit: Series circuit Parallel circuit A1 = A2 = A3 A1 +A2 = A3 Current flows in one path only. Current can flow in more than one path. Current is the same at different points in the The current in the main loop is equal to the circuit. sum of the currents in the branches. Current will not flow in the circuit once it is broken at any point. An open circuit in a branch does not affect other branches. 8 . 6 The heating effect of current and its potential hazards 1 Heat energy is given out when a current passes through a conductor. The heating effect becomes greater as the current increases. 2 Applications of the heating effect of current: Electric kettles, heaters and hair dryers The resistive wires give out a large amount of heat when a current passes through them. Mastering Science © Oxford University Press 2011 Short notes for revision 2 -6- 8 Making use of electricity Incandescent bulbs The filament becomes very hot and glows when a current passes through it. 3 The fuse and the circuit breaker are the safety devices for preventing the current in a circuit from becoming too large. The fuse It consists of a metal wire with a low melting point. The fuse rating shows the maximum current the fuse can withstand without melting. In electrical appliances, the fuse installed should have a rating slightly higher than the current passing through the appliances under normal operation. or Circuit symbol for a fuse 8.7 1 Fuses of different fuse ratings The circuit breaker It automatically ‘switches off’ if the current in a circuit exceeds its rating. We can reset the circuit breaker by switching it back on. Household electricity In Hong Kong, the mains voltage is 220 V. Mastering Science © Oxford University Press 2011 Short notes for revision 2 -7- 8 Making use of electricity 2 The parts in a three-pin plug and a mains socket: earth hole earth pin switch neutral pin live pin live hole neutral hole A three-pin plug 3 A mains socket Internal parts of a three-pin plug: earth wire fuse (green and yellow) (installed in the live wire) neutral wire live wire (blue) (brown) 4 The earth wire connects the metal case of an appliance to the earth. The importance of earthing: If a damaged live wire touches the metal case of the appliance, the earth wire completes the circuit. A large current flows to the earth and blows the fuse in the live wire. This cuts off the current from the mains and protects us from an electric shock. Mastering Science © Oxford University Press 2011 Short notes for revision 2 -8- 8 Making use of electricity 5 Overloading occurs when too many electrical appliances are connected to a single mains socket. It can cause electricity failure or even a fire. 6 Short circuits (Bonus) bulb does not light up connecting wire The connecting wire provides a path with a very low resistance. thin nichrome wire A large current flows through the nichrome wire. The nichrome wire heats up and glows (overheating of the circuit). The current flowing through the bulb reduces to nearly zero. (The bulb is short-circuited.) Mastering Science © Oxford University Press 2011 Short notes for revision 2 -9-