physics

The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

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Answer explanation

R=2f

f=10cm

The refractive indices of glass and water with respect to air are 3/2 and 4/3 respectively. If speed of light in glass is 2 × 10⁸ m/s, find the speed of light in water.

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Answer explanation

2.25*10^8 m/s

In the following given circuits, the heat produced in the resistor or combination of resistors connected to a 12 V battery will be

• a. Same in all the cases

• b. Minimum in case (i)

• c. Maximum in case (ii)

• d. Maximum in case (iii)

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Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be:

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Answer explanation

1:4

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be

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Answer explanation

25W

The refractive index of a medium V with respect to a medium ‘y’ is 2/3 and the refractive index of medium ‘y’ with respect to medium ‘z’ is 4/3. Find the refractive index of medium ‘z with respect to medium V. If the speed of light in medium ‘x’ is 3 × 10⁸ m s^-1, calculate the speed of light in medium ‘y’.

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Answer explanation

Given, refractive index of medium x with respect to y,
^yµx = 2/3
Refractive index of medium y with respect to z,
^zµy = 4/3
∴ Refractive index of medium x with respect to z,
^zµx = ^yµx . ^zµy = 2/3 × 4/3 = 8/9
∴ Refractive index of medium z with respect to x,
^xµy = 1𝑧µ𝑥 = 9/8
v_y = 23 × 3 × 10⁸ = 2 × 10⁸ m/s

When a 4 V battery is connected across an unknown resistor there is a current of 100 mA in the circuit. The value of the resistance of the resistor is:

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Answer explanation

40 ohm

A boy records that 4000 joules of work is required to transfer 10 coulombs of charge between two points of a resistor of 50 Ω. The current passing through it is _____.

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Answer explanation

8A

A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination.

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Answer explanation

Power of convex lens of focal length 25 cm is
p₁ = 100/25(𝑖𝑛𝑚) = 4 D
Power of concave lens of focal length 10 cm is 100
p₂ = 100/−10(𝑖𝑛𝑚) = -10 D
∴ Power of the combination = P = P₁ + P₂
∴ P = 4 – 10 = -6D

An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained ? Find the size and the nature of the image.

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Answer explanation

v=-54cm

hi=-14cm