NCERT solution of class 10 chapter electricity
Electricity
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?
Ans:
Electricity
page no.200
1. What does an electric circuit mean?
Ans: A continuous and closed path of an electric current is called an electric circuit consists of electric device,source of electricity and wires that are connected with the help of a switch.
2. Define the unit of current.
Ans: The unit of electric current is ampere(A). 1 A is defined as the flow of 1 C of charge through a wire in 1 s.
3. Calculate the number of electrons constituting one coulomb of charge.
Ans:
page no. 202
1. Name a device that helps to maintain a potential difference across a conductor.
Ans: Any source of electricity like battery, cell, power supply, etc. helps to maintain a potential difference across a conductor.
2. What is meant by saying that the potential difference between two points is 1 V?
Ans: If 1 J of work is required to move a charge of amount 1 C from one point to another, then is is said that the potential difference between the two points is 1V.
3. How much energy is given to each coulomb of charge passing through a 6 V battery?
Ans: The energy given to each coulomb of charge is equal to the amount of work which is done in moving it.
Now we know that,
Potential difference = Work done/ charge
Work done = Potential difference x charge
Where, charge = 1 C
Potential difference = 6V
Work done = 6 x 1 = 6 Joule
Page no. 209
1. On what factors does the resistance of a conductor depend?
Ans: The resistance of a conductor depends upon the following factors:
- Length of the conductor
- Cross-sectional area of the conductor
- Material of the conductor
- Temperature of the conductor
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?
Ans: The current will flow more easily through thick wire. It is because the resistance of a conductor is inversely proportional to its area of cross-section. If thicker the wire, less is resistance and hence more easily the current flows.
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?
Ans: According to Ohm's law
V= IR
I=V/R
Now potential difference is decreased to half
New Potential difference V'=V/2
Resistance remains constant
So the new current l'= V'/R
=(V/2)/R
=(1/2)(V/R)
=(1/2)l
=l/2
Therefore, the amount of current flowing through the electrical component is reduced by half.
4. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
Ans: The resistivity of an alloy is higher than the pure metal. Moreover, at high temperatures, the alloys do not melt readly. Hence, the coils of heating appliances such as electric toasters and electric irons are made of an alloy rather than a pure metal.
Page No: 213
1. Draw a schematic diagram of a circuit consisting of a battery of three cells of 2V each, a 5 ohm resistor, an 8 ohm resistor, and a 12 ohm resistor, and a plug key, all connected in series.
Ans:
2. Redraw the circuit of question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure potential difference across the 12 ohm resistor. What would be the readings in the ammeter and the voltmeter?
Ans: An ammeter should be connected in the ciruit in series with the resisitors. To measure the potential difference across the resistor it should be connected in parallel, as shown in the following figure.
The resistance are connected in series.
Ohm's law can be used to obtain the readings of ammeter and voltmeter.
According to Ohm's law
V=IR
Where Potential difference V = 6V
Current flowing through the circuit = l
Resistance of the circuit, R= 5 + 8 + 12 = 25 ohm
l = V/R
= 6/25
= 0.24 A
Potential difference across 12 ohm
Resistor = V1
Potential difference across 12 ohm resistor, l = 0.24 A
Therefore, using Ohm's law, we obtain
V1 = IR
=0.24 x 12
= 2.88 V
Therefore, the reading of the ammeter will be 0.24 A
The reading of the voltmeter will be 2.88 V.
Page no: 216
1. Judge the equivalent resistance when the following are connected in parallel
Ans:
2. An electrical lamp of 100 ohm, a toaster of resistance 50 ohm, and a water filter of resistance 500 ohm 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?
Ans:
3. What are the advantage of connecting electrical devices in parallel with the battery instead of connecting them in series?
Ans: There is no division of voltage among the appliances when connected in parallel. The potential difference across each appliance is equal to the supplied voltage.
The total effective resistance of the circuit can be reduced by connecting electrical appliances in parallel.
4. How can three resistance of resistance 2 ohm, 3 ohm and 6 ohm be connected to give a total resistance of (a) 4 ohm (b) 1 ohm ?
Ans
5. What is (a) The highest, (b) the lowest total resistance that can be secured by combinations of four coils of resistance 4 ohm, 8 ohm, 12 ohm, 24 ohm?
Ans: There are four coils of resistances 4 ohm, 8 ohm, 12 ohm, 24 ohm respectively.
(a) If these coils are connected in series, then the equivalent resistance will be the highest,given by the sum 4 + 8 + 12 + 24 = 48 ohm.
(b) If these coils are connected in parallel, then the equivalent resistance will be the lowest, given by
Ans: Therefore, 2 ohm is the lowest total resistance.
page No. 218
1. Why does the curd of an electric heater not glow while the heating element does?
Ans: The heating element of the heater is made up of alloy which has very high resistance so when current flows through the heating element, it becomes too hot and glows red. But the resistance of cord which is usually of copper or aluminium is very law so it does not glow.
2. Compute the heat generated while transfering 96000 coulomb of charge is one hour through a potential difference of 50 V.
Ans
3. An electric iron of resistance 20 ohm takes a current of 5 A. Calculate the heat developed in 30 s.
Ans: The amount of heat (H) produced is given by the joule's law of heating as
page no. 220
1. What determine the rate at which energy is delivered by a current?
Ans: The rate of consumption of electric energy in an electric appliance is called electric power. Hence, the rate at which energy is delivered by a current is the power of the appliance.
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.
Ans:
page no. 221
Excercise
1. A piece of wire of resistance R is cut into five equal parts. There 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
Ans: (d) 25
2. Which of the following terms does not represent electrical power in a circuit?
3. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be -
(a) 100W
(b) 75 W
(c) 50 W
(d) 25 W
Ans: (d) 25 W
4. Two conducting wire 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
Ans: (c) 1:4
5. How is a voltmeter connected in the circuit to measure the potential difference between two points?
Ans: To measure the potential difference between two points, a voltmeter should be connected in parallel in the points.
Therefore, the length of the wire is 122.7 m and the new resistance is 2.5 ohm.
Ans:
Therefore, the total resistance is 4 ohm.
Ans:
Number of electric bulbs connected in parallel are 110.
13. A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 ohm resistances, which may be used separately, in series, or in parallel. what are the currents in the three cases?
Ans: Supply voltage, V= 220 V
Resistance of one coil, R = 24 ohm
14. Compare the power used in the 2 ohm resistor in each of the following circuits:(i) a 6 V battery in series with 1 ohm and 2 ohm resistors, and (ii) a 4 V battery in parallel with 12 ohm and 2 ohm resistors.
Ans:
15. Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?
Ans: Both the bulbs are connected in parallel. Therefore, potential difference across each of them will be 220 V, because no division of voltage occurs in a parallel circuit.
Current drawn by the bulb of rating 100 W is given by,
Power = Voltage x Current
Current = Power/voltage
= 60/220 A
Hence, current drawn from the line = 100/220 + 60/220 = 0.727 A.
16. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?
Ans:
18. Explain the following.
(a) Why is the tungsten used almost exclusively for filament of electric lamps?
(b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?
(c) Why is the series arrangement not used for demestic circuits?
(d) How does the resistance of a wire vary with its area of cross-section?
(e) Why are copper and aluminium wires usually employed for electricity transmission?
Ans:
(a) The melting point and of tungsten is an alloy which has very high melting point and very high resistivity so does not burn easily at a high temperature.
(b) The conductors of electric heating devices such as bread toasters and electric irons are made of alloy because resistivity of an alloy is more than that of metals which produces large amount of heat.
(c) In series circuits voltage is divided. each component of a series circuit receives a small voltage so the amount of current decreases and the device becomes hot and does not work properly. Hence, series arrangement is not used in domestic circuits.
(d) Resistance (R) of a wire is inversely proportional to its area of cross-section (A), i.e when area of cross section increases the resistance decreases or vice versa.
(e) Copper and aluminium are good conductors of electricity also they have low resistivity. So they are usually used for electricity transmission.
