Thermal Physics

Step 1: Apply Stefan–Boltzmann law.
The power radiated is given by: Step 2: Radiation from black body.
For a black body, emissivity , hence: Step 3: Radiation from second body.
Step 4: Simplification.
Step 5: Final comparison.

Step 1: Magnification relation.
For a thin lens, linear magnification is

Step 2: Lens formula.

Step 3: Express in terms of .

Step 4: Substitute in lens formula.

Step 5: Solve for image distance.

Step 6: Conclusion.
The image distance is .

Step 1: Relation between weight and height.
The weight of a body at height above the earth’s surface is given by:
where is the radius of earth.
Step 2: Percentage decrease in weight.
The fractional decrease in weight is approximately:
when .
Step 3: Substituting values.

Step 4: Converting to percentage.

Step 5: Conclusion.
The percentage decrease in weight is 1%.

Step 1: Define emissive power.
Emissive power is defined as the radiant energy emitted per unit area of a surface per unit time.
Step 2: Write dimensional meaning.
Step 3: Identify SI unit.