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CLASS - 10 SCIENCE IMPORTANT QUESTIONS OF MARK 5   FOR BOARD EXAMINATION

QUESTION NO.9

(a) List four characteristics of the image formed by plane mirrors.
(b) A 50 cm tall object is placed at a distance of 20 cm from a concave mirror of focal length 30 cm. Use mirror formula to determine position and size of the image formed.

ANSWER  NO.9

(a) 
  1. Image is virtual and erect.
  2. The size of the image is equal to that of the object.
  3. The image formed is as far behind the mirror as the object is in front of it.
  4. The image is laterally inverted.
(b) Height of object = 5 cm
       u = - 20 cm
       f = - 30 cm
  Using mirror formula
      1/f = 1/v + 1/u
     1/- 30 = 1/v + 1/- 20
     1/-30 - 1/-20 = 1/v
     1/-30 + 1/20 = 1/v
      -2+3/60 = 1/v
     1/60 = 1/v
     v= 60 cm
   The image of formed 60 cm on the back side of the mirror.
     The height of the image is
       m= h'/h = - v/u
       h'/5 = -60/-20 => 3
       h' = 15 cm
  Hence the image is virtual and height of the image is 15 cm.

QUESTION NO.10

Bulb is rated 41W, 220 volt. Find the current drawn by it, when it is connected to a 220 volt supply. Also find its resistance. If the giving bulb is replaced by a bulb of ratings 25 Watt, 220 volt then there be any change in the value of current and resistance? Justify your answer and determine the change.

ANSWER NO.10

 CASE I
   Power of bulb P = 40 W
   Voltage supplied across the bulb V = 220 V
   P = vi
   i = P/V = 40/220 = 0.18 A
    Resistance of bulb
    P = v.v/R = R =v.v/P = 220X220/40
      = 1210 ohm

 CASE II
 Power of bulb  P = 25 walt
                       v = 220 volt
          P = vi
           i = P/v = 25/220
             = 0.1136 
    Resistance of bulb
    P = v.v/R' 
    R' = v.v/P = 220X220/25
       = 1936 ohm

 When bulb will change, the current and resistance will be changed.
Changed in current = 0.18 - 0.1136
                                      = 0.0664 A
( current decrease when we use 25 watt bulb in place of 40 watt bulb.)
Changed in resistance  1936 ohm - 1210 ohm = 726 ohm
( Resistance increase when we use 25 watt bulb in place of 40 watt)

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