NCERT Class VIII Science Solutions: Chapter 14-Chemical Effects of Electric Current Part 6

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Question 21:

Observe the following circuit given in Fig.

As showing in fig is a magnetic compass in connected battery

Magnetic Compass in Connected Battery

As showing in fig is a magnetic compass in connected battery

Current does not flow in the circuit if there is a gap between the two wires. Does it indicate that air is a poor conductor of electricity? Does air never conduct electricity? Explain.

Answer:

Yes, air is a poor conductor of electricity. No, under certain conditions, such as during lightning, air may conduct electricity

Question 22:

Boojho made the circuit shown in Fig. He wanted to observe what happens when an electric current is passed through water. But he forgot to add a few drops of lemon juice to water. Will it make any difference to his observations? Explain.

As showing in fig is a carbon rod in connected battery

Carbon Rod in Connected Battery

As showing in fig is a carbon rod in connected battery

Answer:

If the water is distilled water and lemon juice is not added, no current will pass through the circuit. If the water taken is salty, then a feeble current will pass through the circuit and bubbles will be seen on the negative electrode.

Question 23:

Observing that the bulb does not glow in the circuit shown in Fig. A, Boojho changed the circuit as shown in Fig. B, He observed deflection in the magnetic compass.

Result for bulb does not glow in the circuit

Bulb Does Not Glow in the Circuit

Result for bulb does not glow in the circuit

  1. What does the deflection in magnetic compass indicate?

  2. Why did the bulb not glow in Fig. A?

  3. What would be the effect of increase in the number of turns in the coil wound around the magnetic compass in Fig. B?

  4. What will be observed if the numbers of cells are increased in the circuit shown in Fig. B?

Answer:

  1. It indicates the presence of current in the circuit.

  2. The bulb did not glow because the current was not sufficient to make it glow.

  3. Deflection in the magnetic compass will increase.

  4. Deflection in the compass will increase further.

Question 24:

You are provided with a magnetic compass, an empty match box, a battery of two cells and connecting wires. Using these objects how will you make a tester for testing an electric circuit? Draw the necessary circuit diagram and explain.

Answer:

As showing in fig is a current flows through the circuit the magnetic

Current Flows through the Circuit the Magnetic

As showing in fig is a current flows through the circuit the magnetic

Whenever current flows through the circuit the magnetic compass needle shows deflection due to magnetic effect of current.

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