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

Question 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?

Answer:

(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.

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

Question 16:

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

Answer:

For T.V set

Power (P) = 250w, Time (t) = 1hr=60×60=3600s

For Toaster

Power (P) = 1200w, Time (t) = 10min=10×60=600s

We know that Energy (E) = P×t

∴ Energy consumed by a TV set = 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 energy

consumed by a toaster of power 1200 W in 10 minutes.

An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours

Question 17:

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

Answer:

Here Given that Resistance of the electric heater (R) = 8 Ω

Current drawn (I)= 15 A

We know that P = I²R

                         = 15×15×8

                          = 1800w or J/s

∴ Heat is produced by the heater at the rate of 1800 J/s.

Two lamps, one rated 100 W at 220 V and the other 60 W at 220 V

Question 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?

Solution:

Both the bulbs are connected in parallel. Therefore, potential difference across each of them will be same that is 220 V, Current drawn by the bulb of rating 100 W is given by

I=P/V

 =100/220=0.4545...A

Similarly, current drawn by the bulb of rating 60 W is given by

I=P/V

 =60/220=0.2727...A

∴ the total current drawn from the line =0.4545+0.2727=0.727A

Compare the power used in the 2 Ω resistor in each of the following circuits

Question 14:

Compare the power used in the 2 Ω resistor in each of the following circuits: (i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.

Answer:

 (i) Potential difference (V) = 6 V

 Here 1 Ω and 2 Ω resistors are connected in series.

 ∴ equivalent resistance of the circuit (R) = 1 + 2 = 3 Ω

According to Ohm’s law

V = IR

Or I=V/R=6/3=2A

the current through the circuit (I)=2A

Now the power used in the 2 Ω resistor

we know that Power = I²R

                                 = 2×2×2=8W

(ii) Potential difference, V = 4 V

12 Ω and 2 Ω resistors are connected in parallel. The voltage across each component of a parallel circuit remains the same. Hence, the voltage across 2 Ω resistor will be 4 V. Power consumed by 2 Ω resistor is given by

∴ the power used by 2 Ω resistor is 8 W.