1 joule is equal to how many ergs? MCQ with Answer and Explanation

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1 joule is equal to how many ergs?

A. 10⁵

B. 10⁷

C. 10⁸

D. 10⁶

Answer: Option B

Solution (By JKSSB Mock Tests)

1 J = 1 kg m²/s². In CGS, 1 kg = 10³ g, 1 m = 10² cm => 1 J = (10³ g) × (10² cm)² / s² = 10⁷ g cm²/s² = 10⁷ erg. Key conversion.

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The velocity of light in vacuum is 3×10⁸ m/s. In a medium of refractive index 1.5, speed is

A. 4.5×10⁸ m/s

B. 3×10⁸ m/s

C. 1.5×10⁸ m/s

D. 2×10⁸ m/s

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Correct Answer: Option D

Explanation:
v = c/n = 3×10⁸/1.5 = 2×10⁸ m/s.

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Question #2

The value of g at the centre of Earth is

A. Infinite

B. Zero

C. 9.8 m/s²

D. 4.9 m/s²

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Correct Answer: Option B

Explanation:
g decreases with depth: g' = g(1 - d/R). At centre d=R, g'=0. Weightless at centre. At poles g=9.83, equator 9.78. At surface it's maximum.

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Question #3

In a step-up transformer, the primary coil has

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B. More turns and thinner wire

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D. More turns and thicker wire

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Correct Answer: Option C

Explanation:
Step-up: Ns > Np. Primary fewer turns, thicker wire because current higher on primary side (for step-up, Vp < Vs, Ip > Is).

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