Energy conservation: total input minus light output equals energy to other forms

Steps:

• Convert time to seconds: 2.0 hours × 3600 s/hour = 7200 s.
• Calculate total energy input: power × time = 1.0 W × 7200 s = 7200 J.
• Identify light energy output: 7.0 × 10³ J = 7000 J.
• Subtract to find energy to other forms: 7200 J - 7000 J = 200 J.

Why B is correct:

• 200 J equals total electrical input energy minus light output, following the law of conservation of energy.

Why the others are wrong:

• A. 120 J: underestimates total input by using incorrect time conversion (e.g., 2 hours as 120 minutes without seconds).
• C. 340 J: results from erroneous subtraction, like 7200 J - 6860 J.
• D. 7200 J: represents full input energy, neglecting to subtract light output.

Final answer: B