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 V2
16. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?
17. An electric heater of resistance 8 12 draws 15 A from the service mains 2 hours.
Calculate the rate at which heat is developed in the heater.
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 domestic 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?
Answers
15.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.Energy consumed by the TVset = 250*3600 =9*105J ∴ Energy consumed by a toaster = 1200 × 600 = 7.2× 105J ∴ the energy consumed by a 250 W TV set in 1 h is more than the energyconsumed by a toaster of power 1200 W in 10 minutes.
17.The resistance of the electric heater (R) = 8 Ω
Current drawn (I) = 15 A
Time = 2 hrs
T = 120 min x 60
Time (T) = 7200 secs
The rate of heat produced by the device in terms of power is expressed as
P= I2R
P = 15 x 15 x 8
P = 1800 watt
18a.Tungsten metal is used exclusively for making the filament of electric bulbs because it has a very high melting point. Due to its high melting point, the tungsten filament can be kept white hot without melting away.
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. ... So they are usually used for electricity transmission.
c.When appliances are connected in series, the damage of one of the appliances can disconnect the whole circuit, which means the circuit would stop conducting current.
d.the resistance of a wire varies inversely with the cross sectional area. Note: Since the resistance of a wire varies inversely with the cross sectional area, it means that a thicker wire has a smaller resistance and vice versa.
e.The resistivity of copper and aluminium are very low, so electric current flows easily through them. Hence copper and aluminum wires are usually used for electricity transmission.
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