STD 7 Basic Science: Chapter 05 When Current Flows - Questions and Answers | Teaching Manual 


Textbooks Solution for Class 7 Basic Science (English Medium) വൈദ്യുതി പ്രവഹിക്കുമ്പോൾ | Text Books Solution Basic Science Chapter 05 When current flows | Teaching Manual | Teachers Handbook
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Chapter 05: When Current Flows - Questions and Answers
1. What are the problems that you face at home on a day when there is no power supply?
Answer:
• We cant switch on the motor and fans
• We cant glow bulbs
• We cant preserve food in the refrigerator.
• We cant run grinders and washing machines.
• We cant charge mobile phones and computers.

2. Have you observed what happens to a bulb when it fuses? Is the circuit closed or open then?
Answer: When the filament of a bulb is broken then the bulb is said to be fused. When it gets fused it does not emit light. When the bulb gets fused, the electric current is not allowed to pass through the filament and the bulb does not glow because the filament of the bulb is broken. Then the circuit is open.

3. i) Complete the circuit by replacing the bulb with a buzzer. Doesn't the buzzer work? Observe the illustration of the circuit of this arrangement.
Answer: Yes, the buzzer will work.

ii)  complete the circuit replacing the buzzer with a mini motor and make it work. Illustrate how this circuit is arranged.
Answer:
iii) Haven't you familiarised yourself with three electric circuits? Try to write down the components in each circuit after examining all three.
Answer: 
iv) Which of these components are similar?
Answer:
• Source of electricity
• Device that uses electricity
• Wire that conducts electricity

4. Some electric circuits are given below. None of the bulbs in these circuits is glowing. Identify its reason and note it down.
Answer: None of the bulbs in these circuits is glowing. Because these circuits are open.

5. Make an arrangement as shown below and try to complete the circuit using different materials. Which materials made the bulb glow?
6. What are called conductors?
Answer: Substances that conduct electricity are called conductors.

7. What are called insulators?
Answer: Substances that do not conduct electricity are called insulators.

8. Draw the circuits you have seen earlier using these symbols.
Answer:
9. i) Note down in your science diary what 1,2,3 and 4 in the given circuit indicate.
Answer:
1) Glowing Bulb
2) Conducting Wire
3) Battery
4) Switch in the 'on' mode

ii) Is the circuit closed or open? What change should be made in the circuit to show it as an open circuit?
Answer: The circuit is closed. To show the circuit as an open circuit, we should connect it to Switch in the off’ mode.

10. What is a switch?
Answer: A switch is a device used to make a circuit work only when necessary.

11. Which are the conducting wires usually used in circuits?
Answer: Copper and aluminium wires are the conducting wires usually used in circuits.

12. You might have seen lines carrying electricity and also the wires that carry electricity from electric posts to houses. Find out the peculiarities of such conducting wires and write them down in your science diary.
Answer: The wires through which electricity passes are non-insulated conductors. But the wire that carries the electricity from the electric post to the house is insulated.

13. Let's do an activity.
i) Operate the electric circuits arranging them as shown in the figures. As you know, the fuse is a very thin wire. Doesn't the bulb glow now?
Answer: The bulb will glow.

ii) What do you observe when the circuit is operated using more cells? Doesn't the fuse wire get burnt off throwing the circuit open?
Answer: Yes, the fuse wire get burnt off throwing the circuit open. The fuse wire gets heated when current flows through it. It was the flow of excess current that burnt off the thin fuse wire.

14. What is a safety fuse?
Answer: A safety fuse is a device used in circuits as a precaution to protect the circuit and electric devices from damage caused by the flow of excess current.

15. A special type of thin wire is used in the safety fuse. What could be the reason for this?
Answer: Fuse wire is generally made from an alloy of lead and tin. It has high resistance and a low melting point. When excessive current flows through the fuse wire, it will melt and protect various other electric appliances.

16. Write a short note on MCB (Miniature Circuit Breaker)?
Answer: Nowadays MCB is used instead of a safety fuse in houses. This device breaks a circuit whenever there is an excessive flow of Current.

17. Who discovered that a magnetic field is produced around a current-carrying conductor?
Answer: The scientist Christian Oersted discovered that a magnetic field is produced around a current-carrying conductor.

18. i) Arrange an electric circuit and keep it switched off. Bring a pivoted magnetic needle just below the copper wire. Observe the direction of the magnetic needle. Now switch on the circuit and observe the magnetic needle. What do you see?
Answer: The magnetic needle deflects

ii) Now repeat the experiment after placing the magnetic needle above and also on the sides of the copper wire. What do you observe?
Answer: We can see the magnet deflects on each side.

iii) Bring another magnet near the pivoted magnetic needle. Does it move instantly?
Answer: Yes, it moves instantly.

19. How can we make an electro magnet at home?
Answer: Take an insulated copper wire of 75cm length. Wind it closely over an iron nail from one end to the other. Remove the insulation from both ends of the wire and connect them to a battery.

20. Find out examples for devices that make use of electromagnets.
Answer: Loudspeakers, headphones, generators, tape recorders, hard disks etc...

21. From where do we get the electricity needed to operate the electric devices in our homes?
Answer: From hydroelectric power stations.

22. What is the relation between the availability of rain and load-shedding?
Answer: In hydroelectric power stations, the electricity is produced from the water falling from a big height. So, the decrease in water level will lead to a shortage of electricity. So, to overcome this load shedding is introduced.

23. How electricity is produced from hydroelectric power stations?
Answer: Electricity is produced making use of the energy of the water stored in dams. The water stored at a height in a dam is made to flow through penstock pipes to reach the turbine installed at a very low level. The turbine rotates using the energy of flowing water. It makes the generator work and generates electricity.

24. How does the electricity generated at a power station reach our homes?
Answer: The electrical charge goes through high-voltage transmission lines that stretch across the country. lt reaches a substation, where the voltage is lowered so it can be sent on smaller power lines. It travels through distribution lines to your neighbourhood.

25. i) Write some situations when electricity is wasted in our houses.
Answer:
• Bulb and fan switched on even when there is no one in the room.
• TV switched on even when there is no one to watch it.
• Bulbs switched on even during daytime.
• Refrigerator left open.

ii) What steps can we take to avoid wastage of electricity in such situations?
Answer:
• Turn off unnecessary lights and use natural light.
• Install LED lights
• Unplug appliances when not using them.
• Avoid using old electronic appliances.
• Buy Energy Star-rated appliance

26. Star symbols are marked on electric appliances. What does the star indicate?
Answer: Star symbols are marked on electric appliances. The number of stars on them indicates the energy efficiency of the device. See that you select electric devices with more number of Stars.

27. Write some situations leading to an electric shock?
Answer:
• Operating a switch with a wet hand.
• Removing plug pins when the switch is on.
• Using wires without insulation
• Changing bulb when the switch is on.
• Electrical appliances coming in contact with water.

28. What immediate action should we take to save a person who gets electric shock?
Answer: The first thing to do is to cut off the electric connection. For this, either switch off the electric supply or remove the fuse. In case this is not possible, push away the affected person using materials that do not conduct electricity. The person should be taken to the nearest hospital immediately if the shock is severe. Give artificial respiration, if necessary. Warm the body up by massaging softly. If the heartbeat ceases, immediately place your palms one over the other on the person's chest and start pressing down and then release. Continue this process until the heart starts beating on its own.

29. i) Which is the biggest hydroelectric power station in Kerala?
Answer: Idukki power station is the biggest hydroelectric power station in Kerala.

ii) Do you know which are the other power stations in Kerala?
Answer: Kuttiadi, Sholayar, Peringalkuthu, Pallivasal, Sengulam etc...

Let us assess

1. Though an electric circuit was made using a torch cell, a bulb and a copper wire, the bulb didn't glow. What may be the reason?
a) The torch cell may be connected in reverse order.
b) The copper wire does not touch the cell.
c) The copper wire is longer than required.
d) The hand is placed on the torch cell.
Answer
b) The copper wire does not touch the cell.

2. From an experiment, it was found that copper, aluminium, iron, steel etc. are conductors of electricity. From this, we understand that
a) Heavy materials are conductors.
b) All materials are conductors.
c) Metals are insulators.
d) Metals are conductors.
Answer:
d) Metals are conductors.

3. Is it during the summer or the monsoon season that load-shedding is normally implemented in Kerala? Why?
Answer: It is during the summer season load-shedding is normally implemented in Kerala. We mostly depend on hydroelectric power plants for electricity. During the summer season, there will be a decrease in water levels in hydroelectric power plants, which will lead to a shortage in electricity. So, to overcome this load shedding is introduced.

4. What safety precautions should be taken while handling an electric iron?
Answer:
• Never touch electric iron with wet hands
• Position the Power Cord Safely
• Avoid Ironing with Kids Around
• Always check if the cord is damaged before using it.
 




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