Q21 of 216 Page 486

(i) The electric power consumed by a device may be calculated by using either of the two expressions P = I2R or P = V2/R. The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. How can the seemingly different dependence of P on R in these expressions be explained?

ii. Explain the following:


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


(b) Why are copper and aluminium wires usually used for electricity transmission?


(i) The electric power consumed by a device may be calculated by using either of the two expressions P = I2R or P = V2/R. The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. The seemingly different dependence of P on R in these expressions be explained by the fact that the if in the first expression , if resistance increases the current decreases as per the ohm’s law and thus the power consumed if effectively given by P = IV. In the second expression ,if resistance increases the voltage increases as per the ohm’s law and again the effective power is given by the expression P = VI.

(ii) (a) Tungsten is used almost exclusively for the filament of electric lamps due to the following reasons:


• High melting point: The high melting point of tungsten ensures that it does not melt easily on high temperature.


• Tensile strength: The tungsten has the greatest tensile strength. Great tensile strength ensures that they does not break under tension.


• Vapour pressure: The tungsten has a lowest vapour pressure. The vapour pressure is the indication of the evaporation rate. Lower the vapour pressure, lower is the boiling point.


(b) Copper and aluminium wires are used for electricity transmission due to following reason:


• Copper and aluminium have very less resistivity.


• Lower the resistivity, easier the flow of electricity.


Hence, copper and aluminium wires are used for electricity transmission because of their low resistivity.


***note: resistivity is defined as power of a material to resist or oppose the flow of electricity through it.


More from this chapter

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19

i. Name the organ that produces sperms as well as secretes a hormone in human males. Name the hormone it secretes and write its functions.

ii. Name the parts of the human female reproductive system where fertilisation occurs.


iii. Explain how the developing embryo gets nourishment inside the mother’s body.

 20

Analyse the following observation table showing variation of image distance (v) with object distance (u) in case of a convex lens and answer the Question s that follow, without doing any calculations:


i. What is the focal length of the convex lens? Give reason in support of your answer.


ii. Write the serial number of the observation which is not correct. How did you arrive at this conclusion?


iii. Take an appropriate scale to draw ray diagram for the observation at S.No.4 and find the approximate value of magnification.


OR


i. To construct a ray diagram we use two rays which are so chosen that it is easy to know their directions after reflection from the mirror. List two such rays and state the path of these rays after reflection in case of concave mirrors. Use these two rays and draw ray diagram to locate the image of an object placed between pole and focus of a concave mirror.


ii. A concave mirror produces three times magnified image on a screen. If the object is place 20 cm in front of the mirror, how far is the screen from the object?


i. For observation no.3 clearly shows that the focal length is + 15 cm.

 22

What happens when ferrous sulphate crystals are heated?

 23

Iron filings were added to a solution of copper sulphate. After 10 minutes, it was observed that the blue colour of the solution changes and a layer gets deposited or iron filings. What is the colour of the solution and the layer deposited?