Energy Practice Problems

  1. A drop of water falls from the top of a water fall that is 50 meters high above a lake. Assume the gravitational acceleration g = 10 m s-2, and the air resistence to be zero. Further more, assume that the water drop does not evaporate any watermolecule during its decent.

    1. What is the speed of the water drop just before it hits the lake surface?

    2. What is the increase in temperature if all the kinetic energy of the water drop is used to heat up the water drop?

    3. Assume the efficiency of converting hydro-energy to electric energy to be 100%. How much water from the fall (of 50 m height) is required to be diverted in order to generate one-kilowatt-hour of electric energy?

    4. Assume the efficiency of converting hydro-energy to electric energy to be 50%. How much water from the fall (of 50 m height) is required to be diverted in order to generate one kilowatt-hour of electric energy?

    5. When 2.00 grams of H2 is oxidized, 242 kJ of energy is released. How much water is produced in order to generate one kilowatt-hour (3.6e6 J) of energy?

    6. The efficiency of converting chemical energy into electric energy is 30%. How much hydrogen gas is required in order to generate one kilowatt-hour of energy?

  2. A photon in the X-ray region has a wavelength of 1.0e-10 m.

    1. What is the frequency?

    2. What is the energy of this photon?

    3. A particle with a mass of 1.67e-30 g has a kinetic energy of the photon, what is the velocity of this particle?

    4. If all the energy of this photon is converted to two particles of equal mass, what is the rest mass of each particle?

Hint for these practice problems