Question

1. a machine works with an efficiency of 18%. the energy input is 500 joules/second. how much useful energy will the machine generate in 15 minutes? (1 point)

81 kj
135 kj
1.35 kj
25 kj

Explanation:

Step1: Convert time to seconds

Since \(1\) minute \( = 60\) seconds, then \(15\) minutes \(=15\times60 = 900\) seconds.

Step2: Calculate total energy input

Energy input per second is \(500\) joules. Total energy input \(E_{in}=500\times900=450000\) joules.

Step3: Calculate useful energy output

Efficiency \(\eta = 18\%=0.18\). Using the formula \(\eta=\frac{E_{out}}{E_{in}}\), we can find \(E_{out}=\eta\times E_{in}\). So \(E_{out}=0.18\times450000 = 81000\) joules.

Step4: Convert to kilojoules

Since \(1\) kilojoule \( = 1000\) joules, then \(E_{out}=\frac{81000}{1000}=81\) kJ.

Answer:

81 KJ