CHAPTER- 9 ELECTRICITY AND CIRCUITS
Notes
1. We use electricity to make many of our tasks easier. Electricity is supplied to us from power stations.
2. Electric cell– In an electric cell, the metal cap is called the positive terminal (+), and the metal disc is called the negative terminal (−).
- All electric cells have two terminals: one is the positive terminal and the other is the negative terminal.
- When the chemicals stored in an electric cell are used up, the cell stops producing electricity.
3. Filament- The thin wire inside a filament bulb that emits light is called the filament of the bulb.
- This filament is fixed between two thick wires.
4. Electric circuit– An electric circuit shows the complete path of electric current between the two terminals of an electric cell.
5. A bulb glows only when electric current flows through the circuit.
6. Sometimes, an electric bulb does not glow even when it is connected to an electric cell. This may happen because the bulb is fused.
- The reason a bulb gets fused is the breaking of its filament.
7. Electric switch– A switch is a simple device that can connect or disconnect an electric circuit.
- In homes, switches are used to make bulbs glow and to operate other electrical devices.
8. Electrical conductor– Materials that allow electric current to flow through them are called electrical conductors.
9. Electrical insulator– These do not allow electric current to flow through them.
10. Our body is a very good conductor of electricity. Therefore, we should always be careful while using electrical appliances.
EXERCISE
Question- 1. Fill in the blanks:
(a) A device that is used to break an electric circuit is called ______.
(b) An electric cell has ________ terminals.
Answer- (a) A device that is used to break an electric circuit is called Switch.
(b) An electric cell has Two terminals.
Question- 2. Mark 'True' or 'False' for following statements:
(a) Electric current can flow through metals.
(b) Instead of metal wires, a jute string can be used to make a circuit.
(c) Electric current can pass through a sheet of thermocol.
Answer- (a)- Yes, (b)- No, (c)- No.
Question- 3. Explain why the bulb would not glow in the arrangement shown in Fig. 9.13.
Question- 4. Complete the drawing shown in Fig. 9.14 to indicate where the free ends of the two wires should be joined to make the bulb glow.
Question- 5. What is the purpose of using an electric switch? Name some electrical gadgets that have switches built into them.
Answer- An electric switch is a simple device that connects or disconnects an electric circuit. In devices like radios, televisions, and electric toys, the switches are built inside them.
Question- 6. Would the bulb glow after completing the circuit shown in Fig. 9.14 if instead of safety pin we use an eraser?
Answer- If we replace the safety pin with rubber, the electric circuit will not be completed. Therefore, the bulb will not glow because rubber is an electrical insulator.
Question- 7. Would the bulb glow in the circuit shown in Fig. 9.15?
Answer- No, the bulb will not glow because both wires are connected to only one terminal of the bulb. As a result, electric current will not flow through the bulb, and it will not glow.
Question- 8. Using the "conduction tester" on an object it was found that the bulb begins to glow. Is that object a conductor or an insulator? Explain.
Answer- Because the bulb is glowing, it means the electric circuit is complete; therefore, the material of the object is an electrical conductor.
Question- 9. Why should an electrician use rubber gloves while repairing an electric switch at your home? Explain.
Answer- Electricians touch electric wires every day, in which electric current may often be flowing. Since the human body is a good conductor of electricity, an electrician can get an electric shock. Rubber gloves are electrical insulators. Therefore, to protect themselves from electric shock, electricians use rubber gloves while carrying out repairs.
Question- 10. The handles of the tools like screwdrivers and pliers used by electricians for repair work usually have plastic or rubber covers on them. Can you explain why?
Answer- Plastic or rubber are electrical insulators and help prevent direct contact with electric current. Without the help of insulators, a person using pliers or a screwdriver may get an electric shock. When electricians use screwdrivers and pliers covered with plastic or rubber insulation, electric current does not flow through their bodies, keeping them safe from accidents.