Question

The emissive power of a sphere of area $0.04\text{ m}^2$ is $0.7\text{ kcal}\cdot\text{s}^{-1}\cdot\text{m}^{-2}$. The amount of heat radiated in 20 seconds is

• A. $2.8\text{ kcal}$
• B. $0.28\text{ kcal}$
• C. $5.6\text{ kcal}$
• D. $0.56\text{ kcal}$

Hint:

When multiplying decimals like $0.7 \times 0.04 \times 20$, combine the constants first: $0.04 \times 20 = 0.8$. Then, simply calculate $7 \times 8 = 56$ and place the decimal point two positions over to get $0.56$ without any scratching!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
A sphere with area A = 0.04 m² and emissive power E = 0.7 kcal·s⁻¹·m⁻² radiates for t = 20 s; find total heat radiated Q.

Step 2: Key Formula or Approach:
Emissive power E = Q/(A·t) → Q = E·A·t.

Step 3: Detailed Explanation:
Q = 0.7 × 0.04 × 20 = 0.7 × 0.8 = 0.56 kcal.

Step 4: Final Answer:
Heat radiated is 0.56 kcal, matching option (D).