Final answer:
The efficiency of the kettle is 75%, calculated by dividing the useful energy transferred to the water (750J), by the total energy input (1000J) and then multiplying by 100%.
Step-by-step explanation:
The efficiency of a kettle is calculated based on the ratio of the energy output, useful for its primary function, to the energy input. The kettle receives 1000J of electrical energy input. We know the following energy transfers: 100J as sound, 50J as light energy, and 100J to the surroundings as thermal energy. Therefore, efficient energy used to heat the water, which is the useful energy, is calculated by subtracting all other forms of energy transfers from the total input:
Efficient thermal energy = Total energy input - (Sound energy + Light energy + Thermal energy to surroundings)
Efficient thermal energy = 1000J - (100J + 50J + 100J)
Efficient thermal energy = 1000J - 250J
Efficient thermal energy = 750J
Now, we can calculate the efficiency of the kettle using the formula:
Efficiency = (Useful energy output / Total energy input) × 100%
Efficiency = (750J / 1000J) × 100%
Efficiency = 0.75 × 100%
Efficiency = 75%
The efficiency of the kettle, therefore, is 75%.