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Q. Why the bulb glows when the electric current passes through it? Ans. Due to the heating effect of current, the filament of the bulb gets heated to a high temperature and it starts glowing. Q. Write a short note on:
Ans. LED- Light Emitting Diodes are used for lighting.
There are two wires(called leads) attached to a LED. One lead is longer than the other. Longer lead
– connected to positive terminal of battery
Shorter lead- connected to negative terminal of battery
Advantages over CFL- LEDs consumes less electricity
•LEDs glow even in weak current
•CFL contain mercury which is toxic and hence CFLs require safe disposal
Disadvantages over CFL- LEDs are expensive
Q. Ionic compounds do not conduct electricity in solid form but their aqueous solution conduct electricity. Why? Or Common salt does not conduct electricity but its aqueous solution conduct electricity. Why? Ans. Ionic compounds (e.g.- Common salt) have strong electrostatic forces of attraction between their constituent ions.
Thus ions are strongly held and cannot move. Therefore, cannot conduct electricity. Aqueous solution means solution prepared by dissolving in water. Water has a high dielectric constant(ability to screen charges or ability to separate opposite ions in ionic compounds) and thus diminish the attraction of the two opposite charges. Ions are now free to move and thus conduct electricity.
Q. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason? Ans.
The magnetic or compass needle shows deflection due to magnetic effect of electric current. This proves that the given solution is a conducting solution as circuit is complete and current is flowing through the wire wrapped around magnetic compass resulting in deflection of magnetic needle.
Q. Name three liquids, which when tested in the manner shown in Fig. 14.9, may cause the magnetic needle to deflect. Ans. Conducting liquids like lemon juice(acid), caustic soda solution(base) and brine(salt solution) allow current to pass through them. Thus, circuit gets complete and current flows through the wire wrapped around magnetic compass resulting in deflection of magnetic needle.
Q. Does pure water conduct electricity? If not, what can we do to make it conducting? Ans. No, Pure or distilled water does not conduct electricity. Reason- As pure or distilled water does not contain dissolved impurities like salts. Addition of acid, base or salt make water conducting as these ionic compounds ionise or dissociate into ions due to high dielectric constant of water. Flow of ions conducts electricity.
Q. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this. Ans. Water used by fireman is not pure or distilled. Such water or tap water can conduct electricity as it contains dissolved impurities like salts. If the electrical supply for the area is not shut off and water by mistake falls over electrical appliances, then current will pass through water harming the firemen. That is why, in case of a fire, the firemen shut off the main electrical supply for the area before they use the water hoses.
Q. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason? Ans. Both drinking water and sea water contains dissolved salts or ionisable impurities. But the amount is more in the sea water which makes it more conducting than the drinking water. Therefore, the compass needle deflects more in the case of seawater.
Q. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain. Ans. No, it is not safe to repair electrical appliances outdoors during heavy downpour. As we know rain water is acidic. Hence, rain water can conduct electricity. The electrician may get electrical shocks while working outdoors during rain or heavy downpour.
Q. Paheli had heard that rain water is as good as distilled water. So, she collected some rain water in a clean glass tumbler and tested it using a tester. To her surprise, she found that the compass needle showed deflection. What could be the reasons? Ans. Rain water when precipitate dissolves impurities from atmosphere. It contains dissolved impurities like acids which makes it a conducting solution. There are no dissolved acids or impurities present in the distilled water. Hence, rain water can allow electric current to pass through it while distilled water cannot. Passage of current completes circuit and hence compass needle showed deflection due to magnetic effect of electric current.
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