List of practice Questions

A pump on the ground floor of a building can pump up water to fill a tank of volume 30 \(m^3\) in 15 min. If the tank is 40 \(m\) above the ground, and the efficiency of the pump is 30%, how much electric power is consumed by the pump ?

The blades of a windmill sweep out a circle of area A. (a) If the wind flows at a velocity v perpendicular to the circle, what is the mass of the air passing through it in time t ? (b) What is the kinetic energy of the air ? (c) Assume that the windmill converts 25% of the wind’s energy into electrical energy, and that A = 30 \(m^2\) , v = 36 km/h and the density of air is 1.2 kg \(m^{-3}\). What is the electrical power produced ?

A family uses 8 kW of power.
(a) Direct solar energy is incident on the horizontal surface at an average rate of 200 W per square meter. If 20% of this energy can be converted to useful electrical energy, how large an area is needed to supply 8 kW?
(b) Compare this area to that of the roof of a typical house.