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Q.18.    Explain the following.

(e) Why are copper and aluminium wires usually employed for electricity transmission?

Copper and aluminium wires usually employed for electricity transmission because they have low resistivity and they are good conductors of electricity.

Q.18.    Explain the following.

(d) How does the resistance of a wire vary with its area of cross-section?

We know that                                             Thus,  the resistance of a wire is inversely proportional to its area of cross-section.

Q.18.    Explain the following.

(c) Why is the series arrangement not used for domestic circuits?

If any of the elements in the circuit get damaged the entire circuit will be affected. If an element brake there will not be any current flow through the circuit. So the series circuit is not preferred in the domestic circuit.

Q.18.    Explain the following.

(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?

The conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal because the resistivity of the alloy is more than a pure metal. So the resistance will be hight and heating effect will be high.

Q.18.    Explain the following.

(a) Why is the tungsten used almost exclusively for filament of electric lamps?

The tungsten used almost exclusively for filament of electric lamps because the melting point of tungsten is very high. The bulb glows at very high temperature. So tungsten filament does not melt when bulb glows.

Q.17.    An electric heater of resistance $8\Omega$  draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.

Given : R=8 Ohm ,I=15A  and t= 2hr Heat developed in heater is H. Rate at which heat is developed is given as

Q.16.    Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?

For TV set : Given :  Power = 250W  and  time = 1 hr = 3600 seconds Energy consumed = H=PI                             For toaster : Given :  Power = 1200W               and       time = 10 minutes = 600 seconds Energy consumed = H=PI                            Thus, TV set uses more energy than a toaster.

Q.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?

Given :  For lamp one:  Power = P1=100W    and        V = 220V             For lamp two:  Power = P2=60W    and        V = 220V             Thus, net current drawn from supply is 0.455+0.273=0.728A

Q.14.        Compare the power used in the $2\Omega$ resistor in each of the following circuits:

(i) a 6 V battery in series with $1\Omega$ and $2\Omega$  resistors

(ii) a 4 V battery in parallel with $12\Omega$ and $2\Omega$ resistors.

i) Given : V=6V          R=1+2=3Ohm In , series current is constant . So , power =P ii)Given : V=6V , R=2 Ohm In ,parallel  combination voltage in the circuit  is constant . So , power =P

Q.13.    A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of $24\Omega$ resistance, which may be used separately, in series, or in parallel. What are the currents in the three cases?

Given : V=220V and Resistance of each coil=R =24A When coil is used separately,current in coil is When two coils are connected in series, net resistance is              current in coil is I' When two coils are connected in parallel, net resistance is current in the coil is I''

Q.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?

Given: V=220V and P=10W Let x be the number of bulbs. I = 5A              and      V=220V For x bulbs of resistance 4680 Ohm, are connected in parallel to obtain 44 Ohm equivalent resistance. Thus,           Hence, 110 bulbs  of 4840 Ohm are connected in parallel to obtain 44 Ohm

Q.11.    Show how you would connect three resistors, each of resistance$6\Omega$ , so that the combination has a resistance of (i) $9\Omega$, (ii) $4\Omega$ .

(i)      R=Equivalent  resistance       (ii) .   R=Equivalent  resistance

Q.10.    How many $176\Omega$ resistors (in parallel) are required to carry 5 A on a 220 V line?

Given : V=220V and I =5A Let x number of resistors are connected in parallel to obtain 44 Ohm equivalent resistance. Thus,           Hence, 4 resistors  of 176 Ohm are connected in parallel to obtain 44 Ohm

Q. 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

The plot between voltage and current is as shown : The slope of the line gives resistance (R)

Q.9.    A battery of 9 V is connected in series with resistors of $0.2\Omega$ , $0.3\Omega$ ,$0.4\Omega$ , $0.5\Omega$ and $12\Omega$  respectively. How much current would flow through the $12\Omega$  resistor?

Total resistance  =R V = 9V Hence, All resistors are in series so 0.67A current would flow through the   resistor.

Q. 6.     A copper wire has diameter 0.5 mm and resistivity of $1.6\times 10^{-8}\; \Omega$ m. What will be the length of this wire to make its resistance $10\; \Omega$ ? How much does the resistance change if the diameter is doubled?

Given : diameter=d= 0.5 mm and resistivity = = m.,resistance =R=10 Area =A We know                                                                        If the diameter is doubled. d=1 mm Area =A' We know                                               Hence, new resitance is   of original resistance.

Q.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.

Given : V=12V  , I = 2.5 mA = 0.0025 A

Q. 5.     How is a voltmeter connected in the circuit to measure the potential difference between two points?

The Voltmeter should be connected in parallel to the points across which the Potential difference is to be measured. If it is connected in series due to high resistance of voltmeter the current will not flow in circuit.

DB..

Q. 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

If resistors are connected in parallel, the net resistance is given as                                                  If resistors are connected in series, the net resistance is given as                         Heat produced = H =  Thus, option C is correct.

Q. 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

Given : V=220V,P=100W          The resistance of the bulb If bulb is operated on 110 Vand resistance is the same , the power consumed will be P' Hence, option D is correct.
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