Class 8 Science Chapter 12 Important Questions
Class 8 Science Chapter 12 Important Questions of Electricity and Circuits. These Extra Questions answers are design in such way that it covers entire NCERT Chapter 12 of grade 6th Science.The sets of important Extra Questions include very short, short and long answers questions with suitable answers. Practice these questions to score a better mark in school tests or exams.
Class 8 Science Chapter 12 Important Questions for 2020-2021
|Chapter: 12||Electricity and Circuits|
|Contents:||Important Questions with Answers|
Class 6 Science Chapter 12 Important Extra Questions Set – 1
What are the purposes for which we use in electricity?
We use electricity for many purposes to make our tasks easier. For example, we use electricity to operate pumps that lift water from wells or from ground level to the roof top tank.
There are many other purposes for which we use electricity such as for lighting home, refrigeration, for heating and cooling home, for entertainment i.e., television.
We use electricity to run many appliances in our day to day life which make our work easier. Comment.
Electricity makes it possible to light our homes, roads, offices, markets and factories even after sunset. This helps us to continue working at night. A power station provides us with electricity. However, the supply of electricity may fail or it may not be available at some places. In such situations, a torch is sometimes used for providing light.
There are so many electrical appliances which runs with the electricity such as table fan, air condition, room heater, room cooler, table lamp, mixer and grinder, washing machine, drilling machine, hair dryer, television, radio set, torch, geyser and microwave oven and lots of things.
What is an “electric cell”?
An electric cell is a device which produces a small amount of electricity. The electric cell which we use in torch, clock, radio etc., is commonly known as dry cell or just cell. Electric cells has a small metal cap on one side and a metal disc on the other side. A positive (+) sign and a negative (–) sign marked on the electric cell. The metal cap is the positive terminal of the electric cell. The metal disc is the negative terminal. All electric cells have two terminals; a positive terminal and a negative terminal.
An electric cell produces electricity from the chemicals stored inside it. When the chemicals in the electric cell are used up, the electric cell stops producing electricity. The electric cell then has to be replaced with a new one.
What is electricity? Why danger sign is shown on electric poles, substations and other places?
Electricity is a form of energy. Electricity can be converted easily into various other forms of energy such as heat energy, light energy, mechanical energy and sound energy etc. Electricity is produced at power stations or power plants. And big electric generator at power stations produce electricity. Remember, electricity is very useful but it can also be very dangerous.
The danger sign is shown on different places to warn people that electricity can be dangerous if not handled properly. Carelessness in handling electricity and electric devices can cause severe injuries and sometimes even death. Hence, you should never attempt to experiment with the electric wires and sockets. Also remember that the electricity generated by portable generators is equally dangerous.
Why should not connect the two terminals of electric cell with wire only?
Never join the two terminals of the electric cell without connecting them through a switch and a device like a bulb. If you do so, the chemicals in the electric cell get used up very fast and the cell stops working.
What is inside the glass case of the bulb?
The thin wire that gives off light is called the filament of the bulb. The filament is fixed to two thicker wires, which also provide support to it. One of these thick wires is connected to the metal case at the base of the bulb. The other thick wire is connected to the metal tip at the centre of the base. The base of the bulb and the metal tip of the base are the two terminals of the bulb. These two terminals are fixed in such a way that they do not touch each other. The electric bulbs used at home also have a similar design. Thus, both the electric cell and the bulb have two terminals each.
What do you mean by electric circuit? Explain.
An electric circuit is a path along which electricity can flow. On connecting the torch bulb between the two terminals of the cell, the bulb lights up or glows. The bulb lights up because electricity flows through its filament. Actually, electricity or electric current from one end of the cell flows along the wire through the bulb and then back to the cell along the other wire. This is called simple electric circuit.
Remember, in an electric circuit, the direction of current is taken to be from the positive to the negative terminal of the electric.
How does a bulb glows or light up with the help of electric circuit? Explain in your own words.
The wires and bulb from a continuous path between the two terminals of the cell for electricity to flow through. This path is called an electric circuit. Thus, the electric circuit provides a complete path for electricity to pass or current to flow between the two terminals of an electric cell. Remember, the electric circuit in which there is no gap in the connection between the terminals of the cell, wires and bulb etc., is called a complete circuit or closed circuit. A bulb able lights up when the circuit is complete or closed. A bulb will not light up, if there is a gap in the circuit. In other words, a bulb will not light up if a circuit is open or broken.
What do you mean by “fused bulb”? State one possible reasons for it.
Sometimes an electric bulb does not glow even if it is connected to a cell by wires without any gap in the circuit. This can happen if the bulb has got “fused”. Generally, a fused bulb refers when the filament or thin wire of a bulb gets broken, the bulb is said to be fused. An electric bulb may fuse due to many reasons. One reason for a bulb to fuse is a break in its filament. A bulb may fuse due to normal wear and tear of the filament or when too much electricity is passed through it suddenly. A break in the filament of an electric bulb means a break in the path of the current between the terminals of the electric cell. Therefore, a fused bulb does not light up as no current passes through its filament.
What is an electric switch? What is the purpose of using it? Give some examples of electrical appliances where electric switches are used.
An electric switch is a device used to complete or break an electric circuit. Usually, it is manually operated. The switches used in lighting of electric bulbs and other devices in homes work on the same principle. Purpose of using electric switch: In “on” position, the switch allows the current to flow through the circuit. Thus, the circuit becomes a closed circuit. In “off” position, the switch does not allow current to flow through the circuit. Thus, the circuit becomes an open circuit. Examples: Electrical appliances such as fans, electric lamps, washing machines, juicer and mixture, TV, radio etc., have switches.
How do you make a simple switch? Explain with an activity.
We can make a switch using two drawing pins, a safety pin (or a paper clip), two wires and a small sheet of thermo Col or a wooden board. Insert a drawing pin into the ring at one end of the safety pin and fix it on the thermo Col sheet. Make sure that the safety pin can be rotated freely. Now, fix the other drawing pin on the thermo Col sheet in a way that the free end of the safety pin can touch it. Now, make a circuit by connecting an electric cell and a bulb with this switch. Rotate the safety pin so that its free end touches the other drawing pin. The safety pin covered the gap between the drawing pins when you made it touch two of them. In this position the switch is said to be ‘on’. Since the material of the safety pin allows the current to pass through it, the circuit was complete. Hence, the bulb glows. On the other hand, the bulb did not glow when the safety pin was not in touch with the other drawing pin. The circuit was not complete as there was a gap between the two drawing pins. In this position, the switch is said to be ‘off’.
What do you mean by “electric conductors”?
Those material which allow electric current to pass through them are called conductors of electricity. The flow of electric current is called electricity, so we can say that conductors are those materials which allow electricity to pass through them very easily.
“We use always metals (copper and aluminium) in electric wires instead of using non-metals”. Justify your answer.
All the metals are conductor of electricity. Silver metal is the best conductor of electricity. Copper and Aluminium metals are also good conductors. Electric wires are made of copper and Aluminium because they are conductors of electricity. All the objects made of metals or metal alloys are also conductors. And non-metal called carbon in the form of graphite is also a conductor of electricity. But remember, we do not use all the non-metals in as conductor.
What do you mean by insulators?
Material which do not allow electric current to pass through them are called insulators. Insulators are actually the non-conductor of electricity. Such as rubber, plastics, wood, cotton, jute asbestos, glass, thermocol, wax, paper, ceramic, air etc., most of the glass are also insulators. They do not allow electric current to pass through them.
Why is it compulsory to keep a fire extinguisher near the metre room of housing societies?
It is compulsory to keep a fire extinguisher near the metre room of housing society so that it can be used easily to “douse electrical fire” caused by electrical sparks.
Explain why all the light bulbs and fans go off if the main switch is turned off in your house.
Current flows in our house appliances through the main switch. When it is turned off, flow of current stops and our lights and fans go off.
What type of cells are used in calculators? What are the main advantages of using such type of cells?
The cells used in calculators are known as button cells, as they resemble the shape of a button. The plates of these cells are made from compounds of nickle and cadmium. The main advantages of these cells is that they can stay in an active stage for months without any leakage.
How does an electric cell produce electricity?
The chemical energy stored in the chemical stored inside a cell converts into electric energy. When the chemicals of the cells are used up, it stops producing electricity.
The handles of the tools like screwdrivers and pliers used by electrician for repair work usually have plastic or rubber covers on them. Can you explain why?
Without the help of insulators, the use of electrical tool like pliers and screwdrivers will give electric shock to user. Wood and plastic are insulators and help in avoiding direct contact with electric current. When electrician touch these live electric wires with these tools covered with plastic or rubber, current does not flow in their body making safer them from any accident.
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.
Since the object allows current to flow and complete the circuit and makes bulb glow. This means the object is a conductor of electricity.
Why should an electrician use rubber gloves while repairing an electric switch at your home? Explain.
Electrician frequently touch copper wires or they may accidentally come in contact with live wire or wires carrying current. Since human body is a good conductor, electrician may get shock. Rubber is an insulator. To prevent themselves from electric shock or flow of current, the electrician uses rubber gloves while repairing.
A fused bulb does not glow. Why? A torch is not functioning, through contact points in the torch are in working condition. What can be the possible reason for this? Mention any three.
A fused bulb does not glow because in a fused bulb, the filament is broken and the circuit is incomplete. Therefore, the current does not flow through it.
The possible reason for not functioning of torch are:
(i) The torch bulb may be fused.
(ii) The cells may be discharged.
(iii) The connecting wires may be broken up.
(iv) The cells may not be placed in the correct order.
(v) The switch is faulty.
Why do bulbs have two terminals? Paheli wanted to glow a torch bulb using a cell. She could not get connecting wires, instead, she got to stripes of aluminium foil. Will she succeed? Explain how?
Bulb has two terminals to connect the filament with the terminals of a cell or a battery so that the current can pass through the filament. Yes, she will succeed. Aluminium foils can act as connecting wires because Aluminium is a conductor of electricity, it will complete the circuit and therefore, the bulb will glow.
How does a light bulb produce light? How does bulb fuse?
A light bulb consists of a thin brittle wire called as the filament. That filament is connected to thick wires at both ends. One of the thick wires is connected to the metal case at the base of the bulb and the other thick wire is connected to the metal tip at the centre of the base. These are the two terminals of the bulb and do not come into contact with each other. When electricity pass through, the filament it heats up and begins to glow. And bulb produces or emit light. Bulb fuses when the bulb does not glow due to some defects within it. When the filament in the bulb is broken, it cannot complete the circuit to allow the flow of electricity. This breaks the circuit and therefore the filament does not have electricity to flow through it. As a result, the filament does not heat up and therefore the bulb does not glow.
Out of aluminium foil, paper, dry cloth and rubber sheet, which is the good conductor? Tell by using a torch.
Aluminium is a good conductor of heat and electricity. For this explanation, take a Torch with cell. Switch on to see that it lights up. Now, in the place between bulb and the cell, put all things like aluminium foil, dry cloth and rubber sheet, one by one and check whether the torch lights up. It lights up with Aluminium foil only. So, we can say that Aluminium is a good conductor.