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Solutions to Exercises (Page No 119) of NCERT Class 7 Science Chapter 10 Electric Current and its Effects:
1. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.
Answer:
Find the appropriate symbols below:
2. Draw the circuit diagram to represent the circuit shown in Fig. 10.21.
Answer:
The circuit diagram to represent the circuit is shown below:
3. Fig. 10.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.
Answer:
To make the battery work, connect the positive terminal of one cell to the negative terminal of the next cell. You can even do the reverse: you can connect the negative terminal of one cell to the positive terminal of the next cell. The setup is shown the figure below:
4. The bulb in the circuit shown in Fig. 10.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.
Answer:
The bulb does not glow because the positive terminals of both cells are adjacent to each other and hence the battery does not work. To make the battery work, connect the positive terminal of one cell to the negative terminal of the second cell and the bulb will glow.
5. Name any two effects of electric current.
Answer:
The two effects of electric current are:
- Heating effect due to which a conductor such as a wireheats upwhen current is passed through it.
- Magnetic effect due to which the wire behaves like a magnet when current is passed through it.
6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Answer:
When current flows through a wire it behaves like a magnet due to magnetic effect of current. Since the needle of the compass kept near the wire is a tiny magnet, it gets deflected from its original north-south position due to the magnetic effect of the wire.
7. Will the compass needle show deflection when the switch in the circuit shown by Fig. 10.24 is closed?
Answer:
The compass needle will not show any deflection because there is no battery which is the energy source in the circuit. Without the battery current will not flow in the circuit and there will be no magnetic effect.
8. Fill in the blanks:
(a) Longer line in the symbol for a cell represents its ________ terminal.
(b) The combination of two or more cells is called a _________.
(c) When the current is switched ‘on’ in a room heater, it _________.
(d) The safety device based on the heating effect of electric current is called a _________.
Answer:
(a) Longer line in the symbol for a cell represents its positive terminal.
(b) The combination of two or more cells is called a battery.
(c) When the current is switched ‘on’ in a room heater, it produces heat.
(d) The safety device based on the heating effect of electric current is called a fuse.
9. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
(c) An electromagnet does not attract a piece of iron. (T/F)
(d) An electric bell has an electromagnet. (T/F)
Answers:
(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
False. To make the battery work the negative terminal of one cell should be connected to the positive terminal of the other cell. If the negative terminals of the two cells are connected together the battery will not work and current will not flow.
(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
True. The melting of the fuse wire happens due to the heating effect of electric current. The fuse wires are made from special materials which melt quickly due to heating effect and break the circuit when large electric currents pass through them.
(c) An electromagnet does not attract a piece of iron. (T/F)
False. An electromagnet uses the principle of magnetic effect of current to attract iron. When current flows the electromagnet coil behaves like a magnet and attracts iron which is a magnetic material.
(d) An electric bell has an electromagnet. (T/F)
True. An electric bell contains an electromagnet which is a coil of wire wound on an iron piece. Current flows through the wire and makes it behave like a magnet. The attraction of the electromagnet causes the to and fro motion of the iron hammer striking the gong, which produces the ringing sound.
10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer:
Electromagnets cannot be used for separating plastic bags from a garbage heap because they can only attract magnetic materials (like iron) using magnetic effect of current. Plastic bags are non-magnetic and hence cannot be separated from a garbage heap using an electromagnet.
11. An electrician is carrying out some repairs in your house. He wants to replace a fuse with a piece of wire. Would you agree? Give reasons for your response.
Answer:
No. A wire cannot replace a fuse because it is made of different material. The fuse is made of a special material with a lower melting point which melts and breaks the circuit when large currents flow through it. The wire has a higher melting point and does not melt, thereby posing a danger.
12. Zubeda made an electric circuit using a cell holder shown in Fig. 10.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.
Answer:
The defects might be that: the terminals of cells are in wrong sequence (positive terminal of one cell is not connected to the negative terminal of the second), the cells themselves might be spent, loose connections between cells and strips due to loose rubber band and loosely connected wires, fused bulb and faulty switch.
13. In the circuit shown in Fig. 10.25.
(i) Would any of the bulb glow when the switch is in the ‘OFF’ position?
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
Answers:
(i) Would any of the bulb glow when the switch is in the ‘OFF’ position?
None of the bulbs will glow when the switch is in the ‘OFF’ position because the circuit is incomplete. Current will only flow if the circuit offers a closed path which is not the case when the switch is ‘OFF’. Therefore, no current will flow through the circuit and the bulbs will not glow.
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
When the switch is moved to ‘ON’ all the bulbs A, B and C will glow at the same time. When the circuit is complete current flows through it instantly at an extremely high speed and thus all the bulbs which are separated by a small distance will glow at the same time.
Solutions to Extended Learning – Activities and Projects (Page No 121) of NCERT Class 7 Science Chapter 10 Electric Current and its Effects:
1. Set up the circuit shown in Fig. 10.17 again. Move the key to ‘ON’ position and watch carefully in which direction the compass needle gets deflected. Switch ‘OFF’ the current. Now keeping rest of the circuit intact, reverse the connections at the terminal of the cell. Again switch ‘on’ the current. Note the direction in which the needle gets deflected. Think of an explanation.
Answer:
Solution to Extended Learning Problem 1
2. Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets.
Answer:
Solution to Extended Learning Problem 2
3. Using an electromagnet, you can make a working model of a railway signal as shown as in Fig. 10.26.
Answer:
Solution to Extended Learning Problem 3
4. Visit an electric shop. Request an electrician to show you the various types of fuses and MCB and to explain how they work.
Answer:
Solution to Extended Learning Problem 4
Solutions to In Text Questions of NCERT Class 7 Science Chapter 10 Electric Current and its Effects:
1. (Page 109) You 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. Paheli made a neat drawing showing how the various electric components were to be connected.
Can you draw this circuit conveniently? It made Boojho wonder if there was an easier way to represent these electric components.
Answer:
You can draw a circuit diagram for the above electric circuit using the symbols for the various electric components as shown below:
Note that the key serves the function of a switch because every time the key touches the wire the circuit gets completed and the bulb glows. When the key does not touch the wire, the circuit remains open and the bulb does not glow.
2. (Page 109) Do you know what a battery is? Look at the symbol of a battery. Can you make out what a battery could be?
Answer: The symbol for a battery is shown below:
The symbol for an electric cell is shown below:
It is easy to see that the battery consists of two or more electric cells connected together where the positive terminal of one cell is connected to the negative terminal of the next cell.
3. (Page 111) 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?
Answer:
It is called a battery because it is made up of a series of cells arranged in a row inside a casing. Each cell consists of a positive plate and a negative plate stacked next to each other and a certain number of these cells forms the batteries of the tractors, trucks.
4. (Page 112) If the filament of the bulb is broken, would the circuit be complete? Would the bulb still glow?
Answer:
If the filament is broken there will be a gap in the circuit rendering it incomplete. The filament is the part of the bulb that emits light when current passes through it, so the bulb will not glow because current cannot pass through the broken filament.
5. (Page 112) You might have noticed that a glowing electric bulb becomes warm. Do you know why?
Answer:
A glowing electric bulb becomes warm due to the heating effect of current. When current passes through the filament it heats the filament up (heating effect) and in turn the hot filament emits light. This is the heat you can feel when you touch the bulb.
6. (Page 113) Can you think of any electric appliance where the heating effect of the electric current is used? Make a list of such appliances.
Answer:
A list of appliances which use the heating effect of electric current are: electric heater, electric stove, electric iron, electric kettle, electric toaster, electric hair dryer, electric blanket, electric oven etc.
7. (Page 121) Paheli and Boojho saw a magic trick sometime back. The magician placed an iron box on a stand. He then called Boojho and asked him to lift the box. Boojho could easily lift the box. Now the magician made a show of moving his stick around the box while muttering some thing. He again asked Boojho to lift the box. This time Boojho could not even move it. The magician again muttered some thing and now Boojho could lift the box.
The audience, including Paheli and Boojho, were very impressed with the show and felt that the magician had some supernatural powers. However, after reading this chapter Paheli is wondering if the trick was indeed some magic or some science was involved? Can you guess what science might be involved?
Answer:
There was no magic involved, instead the science of magnetic effect of current was involved. The magician hid an electromagnet which strongly attracted the iron box when he switched it on by moving his stick and so Boojho could not move it. When the magician turned it off again, the iron box could be moved.
Solutions to All Activities of NCERT Class 7 Science Chapter 10 Electric Current and its Effects:
1. Complete Activity 10.1 (Page 111). Make the electric circuit shown in Fig. 10.7. 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. 10.8. Fig. 10.9 shows another circuit diagram. Is it identical to the circuit diagram shown in Fig. 10.8? Would the bulb glow in this electric circuit?
Answer:
2. Complete Activity 10.2 (Page 112). Take an electric cell, a bulb, a switch and connecting wires. Make an electric circuit as shown in Fig. 10.9. This activity has to done using only one cell. Keep the switch in the ‘OFF’ 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. Make the necessary conclusions.
Answer:
3. Complete Activity 10.3 (Page 113). Make a circuit as shown in Fig. 10.10. Take about 10 cm long piece of nichrome wire and tie it between the nails. Touch the wire. Now switch on the current in the circuit by moving the switch to the ‘ON position. After a few seconds touch the wire. (Do not hold it for a long time.) Switch off the current. Touch the wire again after a few minutes. Make the necessary conclusions.
Answer:
4. Complete Activity 10.4 (Page 115). Make the circuit we used for Activity 10.3 again. However, replace the cell with a battery of four cells. Also, in place of the nichrome wire, tie a thin strand of steel wool. If there are any fans in the room, switch them off. Now pass the current through the circuit for sometime. Observe the strand of steel wool carefully. Note what happens. Does the strand of steel wool melt and break?
Answer:
5. Complete Activity 10.5 (Page 116). Take the cardboard tray from inside a discarded matchbox. Wrap an electric wire a few times around the cardboard tray. Place a small compass needle inside it. Now connect the free ends of this wire to an electric cell through a switch as shown in Fig. 10.17. Note the direction in which the compass needle is pointing. Bring a bar magnet near the compass needle. Observe what happens. Now, while watching the compass needle carefully, move the switch to the ‘ON’ position. What do you observe? Does the compass needle deflect? Move the switch back to the ‘OFF’ position. Does the compass needle come back to its initial position? Repeat the experiment a few times. What does this experiment indicate?
Answer:
6. Complete Activity 10.6 (Page 117). Take around 75 cm long piece of insulated flexible wire and an iron nail, say about 6 – 10 cm long. Wind the wire tightly around the nail in the form of a coil. Connect the free ends of the wire to the terminals of a cell through a switch. Place some pins on or near the end of the nail. Now switch on the current. What happens? Do the pins cling to the tip of the nail? Switch off the current. Are the pins still clinging to the end of the nail?
Answer:
Extra Questions to Complement Solutions to NCERT Class 7 Science Chapter 10 Electric Current and its Effects:
Very Short Answer Type Questions:
1. Why is it necessary to draw a circuit diagram?
Answer:
It is much easier and more convenient to draw a circuit diagram using conventional symbols. Therefore, we generally represent an electric circuit by its circuit diagram.
2. Which component of the circuit controls the flow of current?
Answer:
Switch is the component of the circuit that controls the flow of current.
3. What do the lines in a circuit diagram represent?
Answer:
The lines in a circuit diagram represent wires that connect the various components.
4. How is a battery different from an electric cell?
Answer:
A battery is a combination of two or more electric cells such that the positive terminal of one cell is connected to the negative terminal of the next cell.
5. What is the source of current for devices such as torches, transistors, toys, TC remote controls etc?
Answer:
The source of current for devices such as torches, transistors, toys, TC remote controls etc is a battery or a set of batteries.
6. What material is used to make electric wires?
Answer:
Copper metal is used to make electric wires.
7. If you touch a copper wire, will you get electric shock?
Answer:
Copper wires are insulated with plastic which prevents the person touching it from getting an electric shock.
8. Can any magnitude of current flow through an electric circuit?
Answer:
No, there is a maximum limit on the current which can safely flow through an electric circuit.
9. How does an electric cell produce electricity?
Answer:
An electric cell produces electricity via chemical reactions occurring inside.
10. Paheli wonders if the switch should be put in a specific position in the circuit?
Answer:
No, the switch can be placed anywhere in the circuit.
11. When the circuit is open does current flow through a specific part of the circuit?
Answer:
No,the current does not flow through any part of the circuit when the circuit is open.
12. What material is a fuse wire made of?
Answer:
A fuse wire is made of an alloy of lead and tin.
13. You may have noticed that when an electric heater is switched on the coil becomes red hot and gives out heat. What is this coil called and why does this happen?
Answer:
The coil is called the element and it gets heated up due to heating effect of current.
14. Who is credited with discovering the magnetic effect of current?
Answer:
A scientist called Hans Christian Oersted discovered the magnetic effect of current.
15. Name a device which behaves like a fuse and does not need to replaced when a fault occurs.
Answer:
Miniature circuit breaker (MCB) is the device which automatically breaks the circuit and does not need to be replaced.
16. When current stops flowing in a coil will the coil remain an electromagnet?
Answer:
No, the coil will not remain an electromagnet because once the current is turned off the magnetic effect of current stops working.
Short Answer Type Questions:
1. Paheli does not have a night lamp in her room. She covered the bulb of her room with a towel in the night to get dim light. Has she taken the right step? Give one reason to justify your answer. (NCERT Exemplar)
Answer:
No, Paheli has not taken the right step because the towel might get hot and burn due to heating effect of current. Also, most of the light emitted by the bulb is blocked and absorbed by the towel and this results in wastage of energy.
2. Can we use the same fuse in a geyser and a television set? Explain. (NCERT Exemplar)
Answer:
No, the fuse to be used depends on the electrical requirements of each appliance. In order to function a geyser typically requires higher power and draws more current than a television set. So, a geyser will require a fuse with a higher current rating that can safely handle a higher current without burning.
3. We know that heat is generated by a current flowing through a wire. Name two factors which the amount of heat generated depends on.
Answer:
The amount of heat generated depends on the length of the wire: the greater the length, the higher the heat generated. The amount of heat generated also depends on the cross-sectional area of the wire: the greater the area, the lesser the heat generated.
4. Why should you not use a metal strip instead of a fuse?
Answer:
You cannot use a metal strip because it has a high melting point and unlike a fuse, it will not melt and effectively break the circuit when high currents flow through it. Thus, electric circuits can get damaged due to short circuit and overloading and fires may start.
5. An electric bell rings 60 times in 1 minute. How many times did the circuit break in 1 minute?
Answer:
In an electric bell, the ringing sound is produced by the circuit repeatedly breaking and closing in quick succession. Each time the circuit breaks, the iron strip gets pulled towards the electromagnet and the bell rings once. So, the number of times the circuit breaks is the same as the number of times the bell rings, that is 60 times in 1 minute.
6. How do short circuits happen?
Answer:
Short circuits happen when excessive currents flow due to direct touching of wires when the insulation on the wires come off due to wear and tear.
7. The wires used in making electric circuits do not become hot while the elements of some electric appliances become so hot – why?
Answer:
This is because different materials are used to make the wire used in making electric circuits and the elements of some electric appliances. Heating elements in appliances such as toasters or electric stoves, are specifically designed to generate heat unlike a copper wire used in a circuit.
8. You are given an incandescent electric bulb, a CFL and a LED bulb to use in your house. Which one should you use?
Answer:
You should use the LED bulb because it is the most electricity efficient among the three. In all three, electricity is used to produce light and also to produce heat. An LED bulb gives out the least amount of heat and therefore consumes the least electricity for producing a given intensity of light, followed by a CFL and then an incandescent bulb.
Long Answer Type Questions:
1. Describe how the different components in an electric circuit contribute to the flow of current.
Answer: The roles of the different components are as follows:
Battery: The battery is the power source inside the circuit. Chemical reactions take place inside the battery to generate electric current. Current flows from the positive terminal to the negative terminal of the battery through the circuit.
Switch: The switch controls the flow of current in the circuit. Current will only flow if the switch is closed and the circuit is complete.
Wires: Electricity travels through the conducting wires and reaches all the different components of the circuit. Without the wires electricity would not be able to flow through the circuit.
Load: The load attached to the circuit such as light bulbs, motors, fans etc utilizes the electric current to produce light, power a motor etc. In the process the load completes the circuit and helps facilitate the flow of current through the circuit.
2. Give the differences between wire and fuse.
Answer: The differences between wire and fuse are as follows:
| Wire | Fuse |
| (i) The function of the conducting wire is to enable electric current to flow through the circuit. | (i) The function of the fuse is to act as a protective device that melts and breaks the circuit in case of short circuit or overloading. |
| (ii) Wires are mostly made of copper. | (ii) Fuse wire is made of an alloy of lead and tin. |
| (iii) Have a high melting point. | (iii) Have a lower melting point. |
| (iv) Wires are typically thicker. | (iv) Fuses are typically thinner. |
| (v) In case of excessive currents wires may heat up and cannot prevent damage. | (v) In case of excessive currents fuses heat up, melt and break the circuit, thereby preventing damage. |
| (vi) Wires can be used repeatedly if they are damaged. | (vi) Fuses need to be replaced once it has blown. |
Fill in the Blanks:
positive, electric bell, fuse, overloading, cartridge, open, negative
(a) In the symbol of the electric cell the longer line represents the __________ terminal and the shorter line represents the __________ terminal.
(b) When a circuit is __________ no current flows through it.
(c) A __________ can prevent damage due to short circuit.
(d) If too many devices are attached to a single socket it may cause __________.
(e) The __________ is a device that makes use of the concepts of the magnetic effect of current.
(f) __________ fuses are used in electrical appliances.
Answers:
(a) In the symbol of the electric cell the longer line represents the positive terminal and the shorter line represents the negative terminal.
(b) When a circuit is open no current flows through it.
(c) A fuse can prevent damage due to short circuit.
(d) If too many devices are attached to a single socket it may cause overloading.
(e) The electric bell is a device that makes use of the concepts of the magnetic effect of current.
(f) Cartridge fuses are used in electrical appliances.
Match and Pair:
| Column I | Column II |
| (a) Electric Bulb | (i) Low melting point |
| (b) Fuse | (ii) ISI mark |
| (c) Bureau of Indian Standards | (iii) Heating effect of current |
| (d) Short circuit | (iv) Thomas Edison |
| (e) Electric kettle | (v) Touching of wires |
Answer: The correct table is shown below:
| Column I | Column II |
| (a) Electric Bulb | (iv) Thomas Edison |
| (b) Fuse | (i) Low melting point |
| (c) Bureau of Indian Standards | (ii) ISI mark |
| (d) Short circuit | (v) Touching of wires |
| (e) Electric kettle | (iii) Heating effect of current |
++++++++++++++
Frequently Asked Questions (FAQs) on NCERT Solutions to Class 7 Science Chapter 10 Electric Current and its Effects:
Our expert team of Indian and foreign-educated teachers, engineers and scientists have taken care to answer all the exercise questions, extended learning activities, in-text questions and activities thoroughly along with answers to self-designed extra questions to help you develop a well-rounded knowledge of the topic. The explanations have been complemented with beautiful images to aid in visual learning.
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2. What are the main topics/concepts covered NCERT Solutions to Class 7 Science Chapter 10 Electric Current and its Effects?
The following topics are covered:
10.1 – Symbols of Electric Components
10.2 – Heating Effect of Electric Current
10.3 – Magnetic Effect of Electric Current
10.4 – Electromagnet
10.5 – Electric Bell
Here are the number of problems for the chapter:
(i) 3 Drawing Based Questions (Question 1, 2, 3)
(ii) 8 Short Answer Type Questions (Questions 4, 5, 6, 7, 10, 11, 12, 13)
(iii) 1 Fill in the Blanks Type Question (Question 8)
(iv) 1 True/False Type Question (Question 9)
Start by learning to draw circuit diagrams (using symbols) for different circuits and how to identify problems in a given circuit by visual inspection. Then in the second part of the chapter understand the concepts of heating effect and magnetic effect of current and how they are used to operate different devices. We suggest that alongside studying our solutions, practise drawing the different circuits until you are comfortable with them. To learn and retain the material better we advise that you study our images closely as well while studying the material – we call this visual learning.
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