Q29 of 30 Page 1

(a) For the combination of resistors shown in the following figure, find the equivalent resistance between M & N.


(b) State Joule's law of heating.


(c) Why we need a 5 A fuse for an electric iron which consumes 1 kW power at 220 V?


(d) Why is it impracticable to connect an electric bulb and an electric heater in series?


(a) The resistance R3 and R4 are in parallel combination with each other and the combination is in series resistance R2 and R1.

The effective value of R3 and R4 is,



Equivalent set between M and N



(b) Joule’s law of heating states that when a current ‘I’ passes through a conductor of resistance ‘R’ for time ‘t’ then the heat developed in the conductor is equal to the product of the square of the current, the resistance and time.


H = I2Rt


(c) In the given case the maximum current drawn by the electric iron is


P = V I


I = P/V



This value is less than the current rating of the fuse. Hence, a 5 A rated fuse can be used.


(d) When a bulb and a heater are connected in series the current remains the same and voltage is divided according to the resistance of the appliances. Now, the reduction in the voltage ultimately reduces the power delivered to the devices.



Where V is voltage and R is resistance


if V reduces one fold, power will reduce by 2 times.


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(i) Pollen Grain


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(a) A security mirror used in a big showroom has radius of curvature 5 m. If a customer is standing at a distance of 20, from the cash counter, find the position, nature and size of the image formed in the security mirror.

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OR


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(i) State the nature of the lens and reasons for its use.


(ii) Where should the palmist place/hold the lens so as to have a real and magnified image of an object?


(iii) If the focal length of this lens is 10 cm, the lens is held at a distance of 5 cm from the palm, use lens formula to fine the position and size of the image.