Page 1 SCIENCE 160 Electric Current and its Effects 14 Y ou might have tried the game ‘How steady is your hand?’ suggested in Chapter 12 of Class VI. If not, you may try it out now. Paheli and Boojho had also set up the game by connecting an electric circuit as suggested in Class VI. They had lots of fun trying it out with their families and friends. They enjoyed it so much that they decided to suggest it to a cousin of theirs who stayed in a different town. So, Paheli made a neat drawing showing how the various electric components were to be connected (Fig.14.1). Fig. 14.1 Setup to check how steady your hand is Can you draw this circuit conveniently? It made Boojho wonder if there was an easier way to represent these electric components. 14.1 SYMBOLS OF ELECTRIC COMPONENTS Some common electric components can be represented by symbols. In Table 14.1, some electric components and their symbols are shown. You may come across different symbols for these components in different books. However, in this book, we shall be using the symbols shown here. Look at the symbols carefully. In the symbol for the electric cell, notice that there is a longer line and a shorter but thicker parallel line. Do you recall that an electric cell has a positive terminal and a negative terminal? In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal. For a switch the ‘ON’ position and the ‘OFF’ position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines. In Table 14.1, a battery and its symbol are also shown. Do you know what a battery is? Look at the symbol of a battery. Can you make out what a battery could be? For some of the activities we may need more than one cell. So, we connect two or more cells together as shown in Fig.14.2. Notice 2020-21 Page 2 SCIENCE 160 Electric Current and its Effects 14 Y ou might have tried the game ‘How steady is your hand?’ suggested in Chapter 12 of Class VI. If not, you may try it out now. Paheli and Boojho had also set up the game by connecting an electric circuit as suggested in Class VI. They had lots of fun trying it out with their families and friends. They enjoyed it so much that they decided to suggest it to a cousin of theirs who stayed in a different town. So, Paheli made a neat drawing showing how the various electric components were to be connected (Fig.14.1). Fig. 14.1 Setup to check how steady your hand is Can you draw this circuit conveniently? It made Boojho wonder if there was an easier way to represent these electric components. 14.1 SYMBOLS OF ELECTRIC COMPONENTS Some common electric components can be represented by symbols. In Table 14.1, some electric components and their symbols are shown. You may come across different symbols for these components in different books. However, in this book, we shall be using the symbols shown here. Look at the symbols carefully. In the symbol for the electric cell, notice that there is a longer line and a shorter but thicker parallel line. Do you recall that an electric cell has a positive terminal and a negative terminal? In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal. For a switch the ‘ON’ position and the ‘OFF’ position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines. In Table 14.1, a battery and its symbol are also shown. Do you know what a battery is? Look at the symbol of a battery. Can you make out what a battery could be? For some of the activities we may need more than one cell. So, we connect two or more cells together as shown in Fig.14.2. Notice 2020-21 ELECTRIC CURRENT AND ITS EFFECTS 161 Many devices such as torches, transistors, toys, TV remote controls, use batteries. However, in some of these devices the electric cells are not always placed one after the other as shown in Fig. 14.2. Sometimes the cells are placed side by side. Then how are the terminals of the cells connected? Look carefully inside the battery compartment of any device. There is usually a thick wire or a metal strip connecting the positive terminal of one cell to the negative terminal of the next cell (Fig.14.3). In order to help you to place the cells correctly in the battery compartment, ‘+’ and ‘–’ symbols are usually printed there. How can we connect the cells to prepare batteries for our activities? You may make a cell holder, as shown in Fig.14.4, using a wooden block, two iron strips and rubber bands. It is necessary Fig. 14.3 Connecting two cells together to make a battery Fig. 14.2 (a) A battery of two cells (b) A battery of four cells (a) (b) Table 14.1 Symbols for some electric circuit components S.No. Electric component Symbol 1. Electric cell 2. Electric bulb 3. Switch in ‘ON’ position 4. Switch in ‘OFF’ position 5. Battery 6. Wire that the positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery. 2020-21 Page 3 SCIENCE 160 Electric Current and its Effects 14 Y ou might have tried the game ‘How steady is your hand?’ suggested in Chapter 12 of Class VI. If not, you may try it out now. Paheli and Boojho had also set up the game by connecting an electric circuit as suggested in Class VI. They had lots of fun trying it out with their families and friends. They enjoyed it so much that they decided to suggest it to a cousin of theirs who stayed in a different town. So, Paheli made a neat drawing showing how the various electric components were to be connected (Fig.14.1). Fig. 14.1 Setup to check how steady your hand is Can you draw this circuit conveniently? It made Boojho wonder if there was an easier way to represent these electric components. 14.1 SYMBOLS OF ELECTRIC COMPONENTS Some common electric components can be represented by symbols. In Table 14.1, some electric components and their symbols are shown. You may come across different symbols for these components in different books. However, in this book, we shall be using the symbols shown here. Look at the symbols carefully. In the symbol for the electric cell, notice that there is a longer line and a shorter but thicker parallel line. Do you recall that an electric cell has a positive terminal and a negative terminal? In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal. For a switch the ‘ON’ position and the ‘OFF’ position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines. In Table 14.1, a battery and its symbol are also shown. Do you know what a battery is? Look at the symbol of a battery. Can you make out what a battery could be? For some of the activities we may need more than one cell. So, we connect two or more cells together as shown in Fig.14.2. Notice 2020-21 ELECTRIC CURRENT AND ITS EFFECTS 161 Many devices such as torches, transistors, toys, TV remote controls, use batteries. However, in some of these devices the electric cells are not always placed one after the other as shown in Fig. 14.2. Sometimes the cells are placed side by side. Then how are the terminals of the cells connected? Look carefully inside the battery compartment of any device. There is usually a thick wire or a metal strip connecting the positive terminal of one cell to the negative terminal of the next cell (Fig.14.3). In order to help you to place the cells correctly in the battery compartment, ‘+’ and ‘–’ symbols are usually printed there. How can we connect the cells to prepare batteries for our activities? You may make a cell holder, as shown in Fig.14.4, using a wooden block, two iron strips and rubber bands. It is necessary Fig. 14.3 Connecting two cells together to make a battery Fig. 14.2 (a) A battery of two cells (b) A battery of four cells (a) (b) Table 14.1 Symbols for some electric circuit components S.No. Electric component Symbol 1. Electric cell 2. Electric bulb 3. Switch in ‘ON’ position 4. Switch in ‘OFF’ position 5. Battery 6. Wire that the positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery. 2020-21 SCIENCE 162 that the rubber bands hold the metal strips tightly. You could also buy cell holders from the market for making batteries of two or more electric cells. Place the cells in them properly, such that the positive terminal of one cell is connected to the negative terminal of the next cell. Connect a piece of wire each to the two metal clips on the cell holder as shown in Fig.14.5. Your battery is ready for use. The symbol used for representing a battery is shown in Table.14.1. Let us now draw a circuit diagram of an electric circuit using symbols shown in Table 14.1. Activity 14.1 Make the electric circuit shown in Fig. 14.7. You used a similar circuit in Class VI to make an electric bulb glow. Do you remember that the bulb glows only when the switch is in the ‘ON’ position? The bulb glows as soon as the switch is moved to the ‘ON’ position. Copy this electric circuit in your notebook. Make also a circuit diagram of this circuit using symbols for the various electric components. Is your diagram similar to the one shown in Fig. 14.8? It is much easier to draw a circuit diagram using symbols. Therefore, we generally represent an electric circuit by its circuit diagram. Fig. 14.9 shows another circuit diagram. Is it identical to the circuit diagram shown in Fig.14.8? In which way is it different? Would the bulb glow in this electric circuit? Recall that the bulb glows only when the switch is in the ‘ON’ position and the electric circuit is closed. Fig. 14.4 A cell holder Fig. 14.5 Another type of cell holder Paheli and Boojho wonder whether the batteries used in tractors, trucks and inverters are also made from cells. Then why is it called a battery? Can you help them to find the answer to this question? Fig. 14.6 Truck battery and its cutout 2020-21 Page 4 SCIENCE 160 Electric Current and its Effects 14 Y ou might have tried the game ‘How steady is your hand?’ suggested in Chapter 12 of Class VI. If not, you may try it out now. Paheli and Boojho had also set up the game by connecting an electric circuit as suggested in Class VI. They had lots of fun trying it out with their families and friends. They enjoyed it so much that they decided to suggest it to a cousin of theirs who stayed in a different town. So, Paheli made a neat drawing showing how the various electric components were to be connected (Fig.14.1). Fig. 14.1 Setup to check how steady your hand is Can you draw this circuit conveniently? It made Boojho wonder if there was an easier way to represent these electric components. 14.1 SYMBOLS OF ELECTRIC COMPONENTS Some common electric components can be represented by symbols. In Table 14.1, some electric components and their symbols are shown. You may come across different symbols for these components in different books. However, in this book, we shall be using the symbols shown here. Look at the symbols carefully. In the symbol for the electric cell, notice that there is a longer line and a shorter but thicker parallel line. Do you recall that an electric cell has a positive terminal and a negative terminal? In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal. For a switch the ‘ON’ position and the ‘OFF’ position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines. In Table 14.1, a battery and its symbol are also shown. Do you know what a battery is? Look at the symbol of a battery. Can you make out what a battery could be? For some of the activities we may need more than one cell. So, we connect two or more cells together as shown in Fig.14.2. Notice 2020-21 ELECTRIC CURRENT AND ITS EFFECTS 161 Many devices such as torches, transistors, toys, TV remote controls, use batteries. However, in some of these devices the electric cells are not always placed one after the other as shown in Fig. 14.2. Sometimes the cells are placed side by side. Then how are the terminals of the cells connected? Look carefully inside the battery compartment of any device. There is usually a thick wire or a metal strip connecting the positive terminal of one cell to the negative terminal of the next cell (Fig.14.3). In order to help you to place the cells correctly in the battery compartment, ‘+’ and ‘–’ symbols are usually printed there. How can we connect the cells to prepare batteries for our activities? You may make a cell holder, as shown in Fig.14.4, using a wooden block, two iron strips and rubber bands. It is necessary Fig. 14.3 Connecting two cells together to make a battery Fig. 14.2 (a) A battery of two cells (b) A battery of four cells (a) (b) Table 14.1 Symbols for some electric circuit components S.No. Electric component Symbol 1. Electric cell 2. Electric bulb 3. Switch in ‘ON’ position 4. Switch in ‘OFF’ position 5. Battery 6. Wire that the positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery. 2020-21 SCIENCE 162 that the rubber bands hold the metal strips tightly. You could also buy cell holders from the market for making batteries of two or more electric cells. Place the cells in them properly, such that the positive terminal of one cell is connected to the negative terminal of the next cell. Connect a piece of wire each to the two metal clips on the cell holder as shown in Fig.14.5. Your battery is ready for use. The symbol used for representing a battery is shown in Table.14.1. Let us now draw a circuit diagram of an electric circuit using symbols shown in Table 14.1. Activity 14.1 Make the electric circuit shown in Fig. 14.7. You used a similar circuit in Class VI to make an electric bulb glow. Do you remember that the bulb glows only when the switch is in the ‘ON’ position? The bulb glows as soon as the switch is moved to the ‘ON’ position. Copy this electric circuit in your notebook. Make also a circuit diagram of this circuit using symbols for the various electric components. Is your diagram similar to the one shown in Fig. 14.8? It is much easier to draw a circuit diagram using symbols. Therefore, we generally represent an electric circuit by its circuit diagram. Fig. 14.9 shows another circuit diagram. Is it identical to the circuit diagram shown in Fig.14.8? In which way is it different? Would the bulb glow in this electric circuit? Recall that the bulb glows only when the switch is in the ‘ON’ position and the electric circuit is closed. Fig. 14.4 A cell holder Fig. 14.5 Another type of cell holder Paheli and Boojho wonder whether the batteries used in tractors, trucks and inverters are also made from cells. Then why is it called a battery? Can you help them to find the answer to this question? Fig. 14.6 Truck battery and its cutout 2020-21 ELECTRIC CURRENT AND ITS EFFECTS 163 electric current passes through it. When the bulb gets fused, its filament is broken. Fig. 14.8 Circuit diagram of electric circuit shown in Fig. 14.7 Fig. 14.9 Another circuit diagram If the filament of the bulb is broken, would the circuit be complete? Would the bulb still glow? You might have noticed that a glowing electric bulb become warm. Do you know why? 14.2 HEATING EFFECT OF ELECTRIC CURRENT Activity 14.2 Take an electric cell, a bulb, a switch and connecting wires. Make an electric circuit as shown in Fig.14.9. This activity has to be done using only one cell. Keep the switch in the ‘OFF’ Fig. 14.7 An electric circuit In the bulb there is a thin wire, called the filament, which glows when an CAUTION Never touch a lighted electric bulb connected to the mains. It may be very hot and your hand may get burnt badly. Do not experiment with the electric supply from the mains or a generator or an inverter. You may get an electric shock, which may be dangerous. Use only electric cells for all the activities suggested here. § Notice that the key or switch can be placed anywhere in the circuit. § When the switch is in the ‘ON’ position, the circuit from the positive terminal of the battery to the negative terminal is complete. The circuit is then said to be closed and the current flows throughout the circuit instantly. § When the switch is in the ‘OFF’ position, the circuit is incomplete. It is said to be open. No current flows through any part of the circuit. 2020-21 Page 5 SCIENCE 160 Electric Current and its Effects 14 Y ou might have tried the game ‘How steady is your hand?’ suggested in Chapter 12 of Class VI. If not, you may try it out now. Paheli and Boojho had also set up the game by connecting an electric circuit as suggested in Class VI. They had lots of fun trying it out with their families and friends. They enjoyed it so much that they decided to suggest it to a cousin of theirs who stayed in a different town. So, Paheli made a neat drawing showing how the various electric components were to be connected (Fig.14.1). Fig. 14.1 Setup to check how steady your hand is Can you draw this circuit conveniently? It made Boojho wonder if there was an easier way to represent these electric components. 14.1 SYMBOLS OF ELECTRIC COMPONENTS Some common electric components can be represented by symbols. In Table 14.1, some electric components and their symbols are shown. You may come across different symbols for these components in different books. However, in this book, we shall be using the symbols shown here. Look at the symbols carefully. In the symbol for the electric cell, notice that there is a longer line and a shorter but thicker parallel line. Do you recall that an electric cell has a positive terminal and a negative terminal? In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal. For a switch the ‘ON’ position and the ‘OFF’ position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines. In Table 14.1, a battery and its symbol are also shown. Do you know what a battery is? Look at the symbol of a battery. Can you make out what a battery could be? For some of the activities we may need more than one cell. So, we connect two or more cells together as shown in Fig.14.2. Notice 2020-21 ELECTRIC CURRENT AND ITS EFFECTS 161 Many devices such as torches, transistors, toys, TV remote controls, use batteries. However, in some of these devices the electric cells are not always placed one after the other as shown in Fig. 14.2. Sometimes the cells are placed side by side. Then how are the terminals of the cells connected? Look carefully inside the battery compartment of any device. There is usually a thick wire or a metal strip connecting the positive terminal of one cell to the negative terminal of the next cell (Fig.14.3). In order to help you to place the cells correctly in the battery compartment, ‘+’ and ‘–’ symbols are usually printed there. How can we connect the cells to prepare batteries for our activities? You may make a cell holder, as shown in Fig.14.4, using a wooden block, two iron strips and rubber bands. It is necessary Fig. 14.3 Connecting two cells together to make a battery Fig. 14.2 (a) A battery of two cells (b) A battery of four cells (a) (b) Table 14.1 Symbols for some electric circuit components S.No. Electric component Symbol 1. Electric cell 2. Electric bulb 3. Switch in ‘ON’ position 4. Switch in ‘OFF’ position 5. Battery 6. Wire that the positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or more cells is called a battery. 2020-21 SCIENCE 162 that the rubber bands hold the metal strips tightly. You could also buy cell holders from the market for making batteries of two or more electric cells. Place the cells in them properly, such that the positive terminal of one cell is connected to the negative terminal of the next cell. Connect a piece of wire each to the two metal clips on the cell holder as shown in Fig.14.5. Your battery is ready for use. The symbol used for representing a battery is shown in Table.14.1. Let us now draw a circuit diagram of an electric circuit using symbols shown in Table 14.1. Activity 14.1 Make the electric circuit shown in Fig. 14.7. You used a similar circuit in Class VI to make an electric bulb glow. Do you remember that the bulb glows only when the switch is in the ‘ON’ position? The bulb glows as soon as the switch is moved to the ‘ON’ position. Copy this electric circuit in your notebook. Make also a circuit diagram of this circuit using symbols for the various electric components. Is your diagram similar to the one shown in Fig. 14.8? It is much easier to draw a circuit diagram using symbols. Therefore, we generally represent an electric circuit by its circuit diagram. Fig. 14.9 shows another circuit diagram. Is it identical to the circuit diagram shown in Fig.14.8? In which way is it different? Would the bulb glow in this electric circuit? Recall that the bulb glows only when the switch is in the ‘ON’ position and the electric circuit is closed. Fig. 14.4 A cell holder Fig. 14.5 Another type of cell holder Paheli and Boojho wonder whether the batteries used in tractors, trucks and inverters are also made from cells. Then why is it called a battery? Can you help them to find the answer to this question? Fig. 14.6 Truck battery and its cutout 2020-21 ELECTRIC CURRENT AND ITS EFFECTS 163 electric current passes through it. When the bulb gets fused, its filament is broken. Fig. 14.8 Circuit diagram of electric circuit shown in Fig. 14.7 Fig. 14.9 Another circuit diagram If the filament of the bulb is broken, would the circuit be complete? Would the bulb still glow? You might have noticed that a glowing electric bulb become warm. Do you know why? 14.2 HEATING EFFECT OF ELECTRIC CURRENT Activity 14.2 Take an electric cell, a bulb, a switch and connecting wires. Make an electric circuit as shown in Fig.14.9. This activity has to be done using only one cell. Keep the switch in the ‘OFF’ Fig. 14.7 An electric circuit In the bulb there is a thin wire, called the filament, which glows when an CAUTION Never touch a lighted electric bulb connected to the mains. It may be very hot and your hand may get burnt badly. Do not experiment with the electric supply from the mains or a generator or an inverter. You may get an electric shock, which may be dangerous. Use only electric cells for all the activities suggested here. § Notice that the key or switch can be placed anywhere in the circuit. § When the switch is in the ‘ON’ position, the circuit from the positive terminal of the battery to the negative terminal is complete. The circuit is then said to be closed and the current flows throughout the circuit instantly. § When the switch is in the ‘OFF’ position, the circuit is incomplete. It is said to be open. No current flows through any part of the circuit. 2020-21 SCIENCE 164 position. Does the bulb glow? Touch the bulb. Now move the electric switch to the ‘ON’ position and let the bulb glow for a minute or so. Again touch the bulb. Do you feel any difference? After moving the switch back to the ‘OFF’ position, touch the bulb again. Activity 14.3 Make a circuit as shown in Fig.14.10. Take about 10 cm long piece of nichrome wire and tie it between the nails. (You can get nichrome wire from an electric repair shop or you can use a piece of discarded coil of an electric heater.) Touch the wire. Now switch on the current in the circuit by moving the switch to the ‘ON’ position. After a few The wire gets hot when an electric current passes through it. This is the heating effect of the electric current. Can you think of any electric appliance where the heating effect of the electric current is used? Make a list of such appliances. Fig. 14.10 seconds touch the wire. (Do not hold it for a long time.) Switch off the current. Touch the wire again after a few minutes. You might have seen an electric room heater or an electric heater used for cooking. All these contain a coil of wire. This coil of wire is called an element. You may have noticed that when these appliances are switched on CAUTION Do not keep the switch in the ‘ON’ position for a long time, otherwise the cell may become weak very quickly. Boojho could not see element in an electric iron. Paheli told him that electrical appliances, such as immersion heaters, hotplates, irons, geysers, electric kettles, hair dryers, have elements inside them. Have you ever seen the element in any appliance? Fig. 14.11 Element of electric iron 2020-21Read More
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