Chapter - Electricity
31. The potential difference between the terminals of an electric iron is 240 V and the current is 5.0 A. What is the resistance of the electric iron?
Ans: V = 240 Volt
I = 5 A
R = V/I
= 240/5
= 48 ohm - Ans
32. A p.d of 10 V is needed to make a current of 0.02 A flow through a wire. What p.d is needed to make a current of 250 mA flow through the same wire?
Ans: V = 10 V
I = 0.02 A
R = V/I
= 10/0.02
= 500 ohm
Now, I' = 250 mA = 0.25 A
V' = IR
= 0.25 x 500
= 125 V - Ans
33. If the length of a wire is doubled by taking more of wire, what happens to its resistance?
Ans: If the length of a wire is doubled by taking more of wire, its resistance gets doubled.
34. Give two reasons why nichrome alloy is used for making the heating elements of electrical appliances?
Ans: Nichrome alloy is used for making the heating elements of electrical aplliances beacuse:
(a) Nichrome has very high resistivity,
(b) Nichrome do not burn easily even at high temperature.
35. (a) How does the resistance of a pure metal change if its temperature decreases?
(b) How does the presence of impurities in a metal effect its resistance?
Ans: (a) The resistance of all pure metal decreases if the temperature decreases.
(b) The presence of impurities in a metal increases its resistance.
36. What do you understand by the "resistivity" of a substance?
Ans: Resistivity depends on the nature of the substance and temperature, which is numerically equal to the resistance of a conductor which is 1 metre long and 1 square metre in cross section.
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