SET Chapter 6.pdf

Learning Outcomes  State that supply sources can be direct current (DC) or alternating current (AC)  Distinguish the differences between DC and AC  Identify the common sources of DC and AC  Define the function of a cell  Draw the symbol of a cell  Explain the difference between a...

Learning Outcomes  State that supply sources can be direct current (DC) or alternating current (AC)  Distinguish the differences between DC and AC  Identify the common sources of DC and AC  Define the function of a cell  Draw the symbol of a cell  Explain the difference between a cell and a battery 1.6.1 Direct Current and Alternating Current The current in an electric circuit can either be direct current (DC) or alternating current (AC). DC flows in only one direction and has a constant value (Fig. 1.6-1), while AC flows in both directions and has a value that changes all the time (Fig. 1.6-2). Fig. 1.6-1 shows the waveform of a DC. Fig. 1.6-1: DC waveform on current-time graph ↓ Anode cathode Anode + ↳ + - Cathode Unit 1.6 | Electric Power Sources 41 Fig. 1.6-2 shows the waveform of an AC. Current Current 0 Time One cycle Fig. 1.6-2: AC sine waveform on current-time graph DC can be obtained from equipment such as batteries and DC generators. These DC sources have fixed plus (+) and minus (–) terminals as shown in Fig. 1.6-3. Fig. 1.6-3: Battery terminals AC can be obtained from AC generators such as the ones shown in Fig. 1.6-4. Single-phase generator Three-phase generator Fig. 1.6-4: AC generators Unit 1.6 | Electric Power Sources 42  1.6.2 Single-Phase and Three-Phase Supply Two types of alternating power supply are commonly used in Singapore:  single-phase supply  three-phase supply Single-phase supply usually has two wires for connection, namely the phase (L) and neutral (N) conductors (Fig. 1.6-5). Three-phase supply has:  three wires – L1, L2 and L3 (Fig. 1.6-6); or  four wires – L1, L2, L3 and N (Fig. 1.6-7). Fig. 1.6-5: Single-phase Fig. 1.6-6: Three-phase Fig. 1.6-7: Three-phase supply supply (three wires) supply (four wires) Note: With reference to Fig. 1.6-6 and Fig. 1.6-7, neutral (N) wire in a three-phase supply is optional. Unit 1.6 | Electric Power Sources 43  1.6.3 Cells A dry cell is an electrochemical cell with a low-moisture electrolyte paste. This electrolyte paste, together with the two electrodes (the anode or positive terminal, and the cathode or negative terminal), provides the necessary chemical reactions to convert chemical energy into electrical energy. A + B = AB - Cells can be divided into two main types:  primary cells (non-rechargeable) > chemical reaction irreversible - C + D = CD  secondary cells (rechargeable) - chemical reaction reversible The symbol of an electric cell has two vertical lines (Fig. 1.6-8). The longer line indicates the positive polarity and the shorter line indicates the negative polarity. t - Fig. 1.6-8 In a primary cell, the chemical reaction is irreversible. This means that chemical energy is completely converted into electrical energy and the cell cannot be used again. Primary cells are commonly used in smaller, portable devices. In a secondary cell, the chemical reaction is reversible. This means that chemical energy is converted into electrical energy when the cell is discharging, and electrical energy is converted into chemical energy when the cell is being charged. Secondary cells can be used in portable consumer products and in larger devices and tools that take up more power. 1.6.4 Battery A battery is a number of cells connected together. Unit 1.6 | Electric Power Sources 44  1.6.5 Battery Types Here are some types of batteries available in the market. No. Battery Type Characteristics 1 Zinc-Carbon Battery  non-rechargeable  typically has emf of 1.5 V  low cost  often used in devices which do not consume too much power (e.g., torches, remote controls, toys)  packaged in a zinc can (electrode) with a carbon anode and a paste of zinc chloride and Fig. 1.6-9: Zinc-carbon batteries ammonium chloride 2 Alkaline Battery  most are non-rechargeable (Note: Charging them is likely to make the batteries leak hazardous liquids which will corrode the equipment which carries them, and the batteries may explode too.)  nominal emf of 1.5 V  often used in devices which do not consume Fig. 1.6-10: Alkaline batteries too much power (e.g., torches, remote controls, toys)  higher energy density and longer shelf-life than zinc-carbon batteries  has an alkaline electrolyte of potassium hydroxide, unlike the acidic electrolyte of zinc- carbon batteries 3 Nickel-Cadmium Battery  rechargeable  nominal voltage of 1.2 V  often used in devices which do not consume too much power (e.g., portable electronics, cordless telephones, power tools)  has electrodes using nickel oxide hydroxide and Fig. 1.6-11: NiCd batteries metallic cadmium Unit 1.6 | Electric Power Sources 45  No. Battery Type Characteristics 4 Nickel-Metal Hydride Battery  rechargeable  nominal voltage of 1.2 V  often used in consumer electronics (e.g., alarm clocks, shavers, portable electronics) Fig. 1.6-12: NiMH batteries  has electrodes using nickel oxide hydroxide and hydrogen-absorbing alloy, also known as metal hydride 5 Lithium-Ion Battery  rechargeable  nominal voltage of 3.6 V  often used in consumer electronics (e.g., digital cameras, watches, portable electronics)  has an electrolyte that allows lithium ions to move between the two electrodes Fig. 1.6-13: Lithium-ion battery 6 Lead-Acid Battery  rechargeable  nominal voltage of 3.6 V  often used to power motor vehicles  must be properly disposed of due to its toxic nature.  has electrodes made of lead dioxide (cathode) Fig. 1.6-14: Lead-acid battery and sponge metallic lead (anode), with a sulphuric acid solution as an electrolyte 1.6.6 Recycling Batteries Many types of batteries, such as those used in digital cameras and mobile phones, can be recycled to conserve the limited resources of earth for sustainability. NiMH batteries are environmentally friendly because they are made from non-toxic metals. However, NiCd batteries and other types of batteries contain highly toxic heavy metals that are harmful to the environment. Hence, these batteries must be properly disposed. Visit the website of the National Environment Agency (www.nea.gov.sg) to find out more about e-waste recycling. Unit 1.6 | Electric Power Sources 46  1.6.7 Emerging and Alternative Sources of Energy – Renewable Energy For a long time, humans have been relying on fossil fuels as the main source of energy. The burning of fossil fuels releases air pollutants and greenhouse gases (e.g., carbon dioxide) into the environment. Greenhouse gases contribute to global warming and speed up climate change which can cause natural disasters to be more intense. Hence, many countries including Singapore hope to reduce the use of fossil fuels by harnessing alternative sources of energy like solar power, wind power and hydropower. (a) Solar Power Solar power converts sunlight into useful electricity. Light energy is captured on solar or photovoltaic (PV) cells, which converts it into electrical energy. Initially, the PV cells were used to power small devices such as calculators and watches. Now, they are also used in larger devices, such as rooftop panels, to power water heaters. Today, you can find PV panels at chalets in East Coast Park and at various public-sector buildings in Singapore. Other countries have used this technology to build solar power stations capable of supplying megawatts of electrical energy. Anti-reflective coating N-type semiconductor Current flow P-type Back contact semiconductor Fig. 1.6-15: How solar cells work Fig. 1.6-16: Solar cell made from a monocrystalline silicon wafer Unit 1.6 | Electric Power Sources 47 (b) Wind Power As wind power is cleaner than fossil fuels, increasingly, more countries are using wind turbines to generate power for electricity. The United States, India, Germany, France, Denmark and China are some of the countries that have invested much in this technology. Fig. 1.6-17: Wind turbines harnessing wind power (c) Hydropower Hydropower uses water as a source of energy and is usually available in countries, such as China, India and the United States, which have large rivers. In a hydroelectric power station, specially designed turbines convert the kinetic energy of falling or running water into mechanical energy of turbines, which is in turn converted into useful electrical energy. Fig. 1.6-18: Hydroelectric power station Unit 1.6 | Electric Power Sources 48