NCERT Solution of Class 10 Science Ch-11 Electricity
Page No.172
Question 1:
What does an electric circuit mean?
Answer
A continuous closed path made of electric components through which an electric current flows is known as an electric circuit.
Question 2:
Define the unit of current.
Answer
The unit of electric current is the ampere.
One ampere is defined as the flow of one coulomb of electric charge passing through a conductor in one second. It is commonly abbreviated as A.
Question 3:
Calculate the number of electrons constituting one coulomb of charge.
Answer
Charge on one electron, e = 1.6 x 10-19 C
Total charge, Q = 1 C
Number of electrons, n = Q/e
Page No. 174
Question 1:
Name a device that helps to maintain a potential difference across a conductor.
Answer
A battery.
Question 2:
What is meant by saying that the potential difference between two points is 1 V?
Answer
It means that 1 J of work is done in moving 1 C charge from one point to the other.
Question 3:
How much energy is given to each coulomb of charge passing through a 6 V battery?
Answer
Energy given by battery = charge x potential difference
or W = QV = 1C X 6V = 6J.
Page No. 181
Question 1:
On what factors does the resistance of a conductor depend?
Answer
The resistance of a conductor depends
(i) on its length (ii) on its area of cross-section and (iii) on the nature of its material.
Question 2:
Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?
Answer
The relation between resistance and the area of cross section can be given as:
Resistance is inversely proportional to the area of cross-section of the wire.
As the resistance decreases, the current increases.
Thicker the wire, less current will pass through it whereas thinner the wire, more current will pass.
Question 3:
Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
Answer
When potential difference is halved, the current through the component also decreases to half of its initial value. This is according to ohm’s law i.e., V ∝ I.
Question 4:
Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
Answer
The coils of electric toasters, electric irons and other heating devices are made of an alloy rather than a pure metal because the resistivity of an alloy is much higher than that of a pure metal.
Question 5:
Use the data in Table 11.2 to answer the following –
(a) Which among iron and mercury is a better conductor?
(b) Which material is the best conductor?
Answer
(i) Resistivity of iron = 10.0 x 10-8 Ω m
Resistivity of mercury = 94.0 x 10-8 Ω m.
Thus iron is a better conductor because it has lower resistivity than mercury.
(ii) Because silver has the lowest resistivity (= 1.60 x 10-8 Ω m), therefore silver is the best conductor.
Page No. 185
Question 1:
Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 W resistor, an 8 W resistor, and a 12 W resistor, and a plug key, all connected in series.
Question 2:
Redraw the circuit of Question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 W resistor. What would be the readings in the ammeter and the voltmeter?
Page No. 188
Question 1:
Judge the equivalent resistance when the following are connected in parallel –
(a) 1 W and 106 W, (b) 1 W and 103 W, and 106 W.
Question 2:
An electric lamp of 100 W, a toaster of resistance 50 W, and a water filter of resistance 500 W are connected in parallel to a 220 V source.
What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?
Question 3:
What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?
Question 4:
How can three resistors of resistances 2 Ω, 3 Ω, and Ω W be connected to give a total resistance of (a) 4 Ω, (b) 1 Ω?
Question 5:
What is (a) the highest, (b) the lowest total resistance that can be secured by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?
Page No. 190
Question 1:
Why does the cord of an electric heater not glow while the heating element does?
Question 2:
Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.
Question 3:
An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.
Page No. 192
Question 1:
What determines the rate at which energy is delivered by a current?
Question 2:
An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.
Page No. 193
Exercise
Question 1:
A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R¢, then the ratio R/R¢ is –
(a) 1/25 (b) 1/5 (c) 5 (d) 25
Question 2:
Which of the following terms does not represent electrical power in a circuit?
(a) I2R (b) IR2 (c) VI (d) V2/R
Question 3:
An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be –
(a) 100 W (b) 75 W (c) 50 W (d) 25 W
Question 4:
Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be –
(a) 1:2 (b) 2:1 (c) 1:4 (d) 4:1
Question 5:
How is a voltmeter connected in the circuit to measure the potential difference between two points?
Question 6:
A copper wire has diameter 0.5 mm and resistivity of 1.6 × 10–8 W m. What will be the length of this wire to make its resistance 10 W? How much does the resistance change if the diameter is doubled?
Question 7:
The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –
I (amperes) 0.5 1.0 2.0 3.0 4.0
V (volts) 1.6 3.4 6.7 10.2 13.2
Plot a graph between V and I and calculate the resistance of that resistor.
Question 8:
When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.
Question 9:
A battery of 9 V is connected in series with resistors of 0.2 W, 0.3 W, 0.4 W , 0.5 W and 12 W, respectively. How much current would flow through the 12 W resistor?
Question 10:
How many 176 W resistors (in parallel) are required to carry 5 A on a 220 V line?
Question 11:
Show how you would connect three resistors, each of resistance 6 W, so that the combination has a resistance of (i) 9 W, (ii) 4 W.
Question 12:
Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?
Question 13:
A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 W resistance, which may be used separately, in series, or in parallel. What are the currents in the three cases?
Question 14:
Compare the power used in the 2 W resistor in each of the following circuits:
(i) a 6 V battery in series with 1 W and 2 W resistors, and (ii) a 4 V battery in parallel
with 12 W and 2 W resistors.
