Chapter - Electricity
21. Why should the resistance of:
(a) an ammeter be very small?
(b) a voltmeter be very large?
Ans: (a) We know that resistance opposes the flow of current, so the resistance of ammeter should be very small so that it should not change the value of the current flowing in the circuit and the perfect reading can be made by it.
(b) We know that resistance opposes the floe of current, so the resistance of voltmeter should be very large so that it takes a negligible current from the circuit and having the large resistance the current flowing through the circuit will be less and we can get the perfect reading of voltmeter.
22. Draw circuit symbols for (a) fixed resistance (b) variable resistance (c) a cell (d) a battery of three cells (e) an open switch (f) a closed switch.
23. If the charge on an electron is 1.6 x 10^-19 coulombs, how many electrons should pass through a conductor in 1 second to constitute 1 ampere current?
Ans: Charge on electron, Q = 1.6 x 10^-19 C
Current, I = 1 ampere
Time, t = 1 sec
Q = ne ........ (i)
Q = It = (1 x 1) C
Now putting Q = 1 C in equation (i)
1 = n x 1.6 x10^-19
n = 1/1.6 x10^-19
= 6.25 x 10^18 electrons -Ans
24. In 10 s, a charge of 25 C leaves a battery, and 200 J of energy are delivered to an outside circuit as a result.
(a) What is the p.d across the battery?
(b) What current flows from the battery?
Ans: t = 10 s, Q = 25 C and W = 200J
(a) p.d = W/Q
= 200/25
= 8 volt - Ans
(b) I = Q/t
= 25/10
= 2.5 A - Ans
25. An electric heater is connected to the 230 V mains supply. A current of 8 A flows through the heater.
(a) How many charge flows around the circuit each second?
(b) How many energy is transferred to the heater each second?
Ans: V = 230 volt, I = 8 A
(a) Q = It = (8 x 1) C
= 8 C - Ans
(b) W = VQ = 230 x 8 = 1840 J - Ans
26. What is the general name of the substances having infinitely high electrical resistances?
Ans: The general name of the substances having infinitely high electrical resistances is "insulators".
27. Which has less electrical resistances: a thin wire or a thick wire (of the same length and same material)?
Ans: Thick wire has less electrical resistances as resistivity is inversely proportional to the area of cross section.
28. Keeping the potential difference constant, the resistance of a circuit is halved. By how much does the current change?
Ans: V = constant , R = halved
Since V = IR
I = V/R
Now when resistance is halved, i.e R/2
New v, V = I(R/2)
I' = 2V/R
I' = 2(V/R)
I' = 2I
Now current is twice the previous current.
29. What is ohm's law? Explain how it is used to define the unit of resistance.
Ans: According to Ohm's law the current flowing through a current is directly proportional to the potential difference across its ends, at constant temperature.
Here we get that when 1 ampere of current passes through the conductor at potential difference of 1 volt, then resistance of the circuit is 1 ohm.
Thus Ohm's law is used to define the unit of resistance.
30. (a) Why do electricians wear rubber hand gloves while working with electricity?
(b) What p.d is needed to send a current of 6 A through an electrical appliance having a resistance of 40 Ohm?
Ans: (a) Rubber is an insulator which does not allow electric current to pass through it, hence electricians wear rubber hand gloves while working with electricity.
(b) I = 6A ; R = 40ohm
V = IR
= 6 x 40
= 240 volt
Reference : "Science for Tenth Class Part - 1 Physics" by Lakhmir Singh and Manjeet Kaur
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