Energy：練習題

Which of the following is a renewable source of energy?

A heater is used to warm 2 kg of water. If the heater provides \( 42,000 \) J of energy and all of it is absorbed by the water, what is the temperature rise of the water? (Given: Specific heat capacity of water = \( 4,200 \text{ J kg}^{-1} \text{ }^\circ\text{C}^{-1} \))

A metal block of mass \( m \) is dropped from a height \( h \) above the ground. Just before it hits the ground, its kinetic energy is measured. If the same block is dropped from a height of \( 4h \), how many times greater will its velocity be just before impact compared to the first case?

When a ball is thrown vertically upwards into the air, which of the following statements correctly describes the energy changes occurring as it rises, ignoring air resistance?

The diagram shows a Sankey diagram for an electric motor. The total input energy is \( 500 \) J. If \( 150 \) J is lost as heat and \( 50 \) J is lost as sound, what is the efficiency of the motor?

A student holds a heavy book at a constant height above the ground for two minutes. According to the scientific definition of work done, explain whether the student has performed any work on the book during this time.

Consider the concept of Energy and its conservation.

(a) A battery-powered flashlight is turned on. Describe the energy transformations that take place, starting from the battery to the light emitted. (2 marks)
(b) State the Law of Conservation of Energy. (1 mark)
(c) Explain why the total energy output in the form of useful light is always less than the total energy input from the battery. (2 marks)