Solar radiation, intensity and the solar constant
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Question
Define intensity.
Answer
The radiation **power received per unit area** (perpendicular to the rays). Unit: **W m⁻²**.
Question
Formula for intensity?
Answer
$I = \dfrac{P}{A}$ — power ÷ area. **Given** in the data booklet.
Question
What is the unit of intensity?
Answer
**W m⁻²** (watts per square metre).
Question
Intensity a distance d from a source radiating equally in all directions?
Answer
$I = \dfrac{P}{4\pi d^{2}}$ — the power spread over a sphere of radius d (so I ∝ 1/d²).
Question
State what is meant by the solar constant.
Answer
The **intensity of the Sun's radiation arriving at Earth's distance** (just above the atmosphere): **S = 1.36 × 10³ W m⁻²**.
Question
Value of the solar constant?
Answer
**1.36 × 10³ W m⁻²** — given in the data booklet.
Question
Why does intensity fall with distance?
Answer
A fixed power spreads over an ever-larger **sphere** (A = 4πd²); same power ÷ bigger area = smaller intensity.
Question
Double the distance from a source — what happens to the intensity?
Answer
It drops to a **quarter** (× 1/4), because I ∝ 1/d² (inverse-square law).
Question
How do you find a source's total power from the intensity at distance d?
Answer
Multiply by the whole sphere area: **P = I × 4πd²**.
Question
Useful power output of a solar panel?
Answer
Incident **intensity × panel area × efficiency** (efficiency as a decimal).
Question
Whose power is the solar constant — the Sun's total, or per m²?
Answer
**Per m²** — it is an intensity (W m⁻²) at Earth's distance, not the Sun's total power (W).
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