Answer: (a) converts chemical energy into electrical energy
Explanation:
In a cell, chemicals are used to produce potential difference.

Answer: (a) two terminals and one filament
Explanation:
A bulb has a filament which glows and two terminals which are joined to two terminals of the battery.

Answer: (d) tungsten
Explanation:
Tungsten has high melting point and high resistance.

Answer: (a) closed circuit
Explanation:
Only a closed circuit is a complete circuit.

Answer: (b) a combination of cells in which cells are joined (+) to (-)
Explanation:
In a battery two or more cells are combined in such a way that (+) terminal of one cell is joined to (-) terminal of other.

Answer: (a) a conductor
Explanation:
Substances which allow electric current to pass through them.

Answer: (a) bakelite
Explanation:
Aluminium and tap water are conductors of heat.

Answer: (d) all of these
Explanation:
Tap water, salt solution and metal wire are the examples of conductors.

Answer: (b) Two
Answer:
Dry cell has two terminals (+ve) and (-ve).

Answer: (d) plastic
Explanation:
Plastic is an insulator of electricity.

Answer: negative

Answer: conductors

Answer: insulators

Answer: good

Answer: open

Answer: conduction tester

Answer: completes

Answer: power house

Important Questions

Question 1.
What is an electric cell?
Answer:
Electric cell is a source of energy. It produces a small amount of electricity from chemicals stored inside it.

Question 2.
What is meant by a battery?
Answer:
When two or more cells are joined together, it is called a battery.

Question 3.
Give any difference between a cell and a battery.
Answer:
A cell has only one plate as a positive and only one plate as a negative electrode, while a battery, which is a combination of cells in a series, can have many plates.

Question 4.
Why does a cell stop producing electricity after some time?
Answer:
An electric cell produces electricity from chemicals stored inside it. When the chemicals inside the cell are used up, the cell stops producing electricity.

Question 5.
Why should we not join the wires connected to the two terminals of the electric cell?
Answer:
We should never join the wires with two terminals of the electric cell. If we do so, the chemicals in the electric cell get used up very fast and the cell stops working.

Question 6.
Name some devices in which we use an electric cell.
Answer:
We use electric cell in an alarm clock, wrist watch, transistor, camera, torch, etc.

Question 7.
Two cells can be joined in two ways in series and in parallel. What do you mean by these two ways? In which way the cells are joined in a torch?
Answer:
In series: When (+) terminal of one cell is joined to (-) terminal of the other, the arrangement is called in series.
Parallel: When (+) terminal of a cell is joined to (+) terminal and (-) terminal to (-) terminal of other cell, the arrangement is called parallel.

Question 8.
Why shouldn’t the two terminals of an electric cell be joined directly?
Answer:
Never join the two terminals of the electric cell without connecting them through a switch and a device like a bulb: Because if you do so, the chemicals in the electric cell get used up very fast and the cell stops working.

A Bulb Connected to an Electric Cell

Question 1.
How do we connect wires to the electric cell?
Answer:
Two separate wires are connected to the two terminals of the cell as shown below:

Question 2.
How is the bulb connected to the wires?
Answer:
We can stick the wires to the bulb at the two terminals by the tape or using solding machine as shown in the figure.

Question 3.
How is an electric bulb connected to the cell? Show by a diagram.
Answer:

Question 4.
What are the advantages of dry cells?
Answer:
Advantages of dry cells are:

• They are light in weight and small in size.
• They can be transported from one place to another easily.
• There is no fear of leakage/spillage in dry cells.

Activity 2
Take four lengths of electric wire with differently coloured plastic coverings. Remove a little of the plastic covering from each length of wire at the ends. This would expose the metal wires at the ends of each length. Fix the exposed parts of two wires to the cell and the other two of the bulb as shown in Fig. (A) and Fig. (B).

You can stick the wires to the bulb with the tape used by electricians. Use rubber bands or tape to fix the wires to the cell.

Now, connect the wires fixed to the bulb with those attached to the cell in six different ways as have been shown in Fig. C [(a) to (f)]. For each arrangement, find out whether the bulb glows or not. Write “Yes’ or ‘No’ for each arrangement in your notebook.

Now, carefully look at the arrangements in which the bulb glows. Compare these with those in which the bulb does not glow.

Can you find the reason for the difference?

An Electric Circuit

Question 1.
What is electric circuit?
Answer:
Path from one terminal of the electric cell to the other terminal of the electric cell through wires passing to and from the electric bulb is called electric circuit.

Question 2.
What is the direction of the current in the electric circuit?
Answer:
In an electric circuit, the direction of current is taken to be from the positive terminal to the negative terminal of the electric cell.

Question 3.
Sometimes an electric bulb does not glow even if it is connected to the cell. What could be the reason?
Answer:
This may happen if the bulb has fused.

Question 4.
What is a filament?
Answer:
A filament is a thin wire fixed in the middle of a bulb connected to the two supporting wires.

Question 5.
Why does a fused bulb not light up?
Answer:
A fused bulb means a break in its filament which results in a break in the path of the current between the terminals of the electric cell. Thus a fused bulb does not light up as no current passes through its filament.

Electric Switch

Question 1.
What is the purpose of using an electric switch? Name some electrical gadgets that have switches built into them.
Answer:
A switch is a simple device that either breaks the circuit or completes it. Hence we use electric switches in order to either

• stop an electrical gadget in function when required, or
• start an electrical gadget when needed.

Some electrical gadgets that have switches built into them are—electric iron, table fan, television set, radio, etc.

Question 2.
How are cells arranged in a torch—in parallel or in series?
Answer:
In torch, cells are connected in series.

Question 3.
If in a circuit you have arranged cells in series. Will the bulb continue to shine even if the connection of one of the cells is reversed?
Answer:
If cells are connected in a series in a circuit, change of ends of any one of the cells will stop the flow of electricity and the bulb might not illuminate as circuit will not be completed.

Question 4.
Give the symbol of following:

1. Cell
2. Battery
3. Lamp
4. Switch (OFF)
5. Switch (ON)

Answer:
1. cell

2. Battery

3. Lamp

4. Switch (OFF)

5. Switch (ON)

Electric Conductors and Insulators

Question 1.
What is a conductor? Give examples.
Answer:
A conductor is a material that allows an electric current to pass through it. For example, copper, aluminium, zinc, iron, etc.

Question 2.
What is an insulator? Give few examples.
Answer:
Insulator is the material that does not allow an electric current to pass through it. For example, wood, mica, asbestos, rubber, etc.

Question 3.
Why does one remove the plastic coating of connecting wires before making circuits?
Answer:
Plastics is a bad conductor or insulator of electricity. It prevents us from electric shocks. To complete the circuit, we have to remove the plastic coating to connect wires so that the current may flow through the circuit.

Question 4.
Why do you have to clean the ends of wires used for making a circuit with sand paper to make the bulbs shine?
Answer:
A layer of bad conductor polish is coated on the wire. The current will flow properly after removing this coat. Therefore, we have to remove the coating with sand paper.

Question 5.
Is air a good or a bad conductor of electricity?
Answer:
Air is a bad conductor of electricity. If air were the good conductor of electricity, electricity could reach to the bulb without wiring and the bulb could light without wires. Moreover, the electricity running through wire could scatter all around the wire and then it might be dangerous.

Question 6.
What would happen if air were a good conductor of electricity?
Answer:
If air were a good conductor of electricity, the electricity could scatter all around in atmosphere nearby the power stations. The men at work could die with electric shocks. The electricity could not be transmitted to homes, factories from power stations.

Question 7.
Out of aluminium foil, paper, dry cloth and rubber sheet, which is the good conductor? Tell by using a torch.
Answer:
Aluminium is a good conductor of heat and electricity. For this explanation, take a torch with cells. 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.

Question 8.
Give some uses of conductors and insulators.
Answer:
Conductors and insulators are equally important for us. Switches, electrical plugs and sockets are made of conductors. On the other hand, rubber and plastic are used for covering the electrical wires, plug tops, switches and other parts of electrical appliances, which people might touch.

Question 9.
What is “conduction tester”?
Answer:
It is a simple device to test whether a given object is a conductor or an insulator its set up is shown in the Fig. 12.7.
To test whether a given object is a conductor or an insulator, we connect the two ends of that object to A and B. If after connecting the object between AB, the bulb lights up, the object is a conductor and if bulb does not light up, the object is an insulator.

Question 10.
Why is the handle of electrician’s screw driver made of plastic?
Answer:
A screw driver is made of steel. It is a good conductor of electricity. So electricity can easily flow through it. Plastic is a bad conductor and does not allow electricity to pass through it. So, the electrician has a plastic handle to protect him from any shock.