Energy resources and energy transfer：練習問題

A motor with an efficiency of 72% is used to lift a load of 600 N through a vertical height of 4.0 m. How much total electrical energy must be supplied to the motor?

An object is dropped from a height of 15 m. Assuming air resistance is negligible and the gravitational field strength is \(10 \text{ N/kg}\), what is the speed of the object just before it hits the ground?

In an experiment to investigate thermal energy transfer, a student places a shiny silver block and a dull black block in direct sunlight. After 10 minutes, the black block is significantly hotter. Which statement explains this result?

An electric pump is used to lift 150 kg of water to a height of 12 m every minute. The power input to the pump is 600 W. What is the efficiency of the pump? (Take \(g = 10 \text{ m/s}^2\))

A Sankey diagram represents the energy transfers in a light bulb. The width of the input arrow (electrical) is 20 mm. The width of the light energy arrow is 3 mm and the thermal energy arrow is 17 mm. What is the percentage efficiency of the light bulb?

A Sankey diagram for a power station shows that for every \(2000\text{ MJ}\) of chemical energy input from fuel, \(700\text{ MJ}\) is converted into electrical energy. Calculate the efficiency of the station and identify the main form of wasted energy.

An electric motor is used to lift a crate of mass \(80\text{ kg}\) to a height of \(15\text{ m}\). The motor takes \(20\text{ seconds}\) to complete the task and has an efficiency of \(65\%\). Use \(g = 10\text{ N/kg}\).

(a) Calculate the useful work done by the motor in lifting the crate.
(b) Calculate the total electrical energy input required by the motor.
(c) Calculate the power rating of the motor (the total electrical power input).