Key concepts
Potential energy
Kinetic energy
Hydrostatic pressure
Hydroelectricity
Gravity

Introduction
You may not think much about using water for drinking, cooking, washing or swimming. But did you know that water can also be used to make electricity? Try this fun activity to learn about hydroelectric power.

Background
Thanks to gravity, all objects and materials have potential energy when they are raised above the ground. When these objects fall back toward the ground, the potential energy is converted to kinetic, or motion, energy. In the case of falling water it can be used to turn something called a turbine, which looks like a propeller. If the spinning turbine is connected to an electric generator, it can make electricity. This type of electricity generated by falling water is called hydroelectric power ("hydro" means water).

How do we harness the energy of falling water? To generate a lot of electricity you need a lot of water falling very fast in one place. That means rain is not a good way to generate hydroelectricity, because it is spread out over a very large area and occurs at unpredictable times. Waterfalls have lots of water falling in one place, but because of their natural beauty many are considered natural landmarks and tourist destinations, so people would not be happy if we built ugly power plants on top of them. One solution to this problem is to build dams, or large structures that hold back a stream or river. This causes a deep lake, aka a reservoir, to form, and some of that water can be used to generate electricity when it flows through the dam (whereas the rest continues to flow over the falls). In this activity you will investigate where a turbine should be placed to generate the most electricity.

Note that although hydroelectric power is considered a type of renewable energy, it does have some environmental impacts: the large lakes formed by dams can destroy existing animal habitats; blocking off rivers can mean many organisms such as fish can no longer move from one area to another; and the catastrophic failure of a dam can cause severe flooding and loss of life. If you are interested, you can do your own research on the costs and benefits of hydroelectric power.

Materials

  • Milk jug or two-liter soda bottle
  • Small nail
  • Tap water
  • Duct tape
  • Chair, stool or stepladder
  • Bathtub, large sink or outdoor area where you can spill water
  • Tape measure (optional)

Preparation

  • Gather your materials in an area where you can spill water on the ground, such as outdoors or in a bathtub.

Procedure

  • Use the nail to carefully punch three small holes in one side of the milk jug: one near the top, one in the middle and one near the bottom. Make sure to offset the holes from one another horizontally by a few centimeters. (The holes should not be directly above one another in a straight line.)
  • Use pieces of duct tape to tightly seal over the holes.
  • Fill the jug with water. Make sure you fill it past the top hole. Look carefully to see if the duct tape seal is holding in the water before you start your test.
  • Set the jug on an elevated surface such as a stool or stepladder in an area where you can pour water on the ground.
  • Get ready to quickly remove the duct tape. What do you think will happen when you remove the tape? How will water flow out of the holes? Will the water coming out of the three holes behave the same or differently?
  • Quickly peel off all three pieces of duct tape. Watch closely. What happens? Where do the streams of water hit the ground?
  • Continue watching as the water drains out of the jug. Do the streams of water continue to hit the ground in the same place or does it change as the jug drains?
  • Pretend that your milk jug is a dam and the water inside it is the lake it has created. Where would you place a turbine to generate the most electricity from the water?
  • Extra: Use a tape measure on the ground to record how far the water gushes out. How does this distance change if you raise or lower[OK?] the height of the jug off the ground? How does it vary among the holes?
  • Extra: Try the activity with different shapes of containers, with holes punched at the same heights from the bottom. What happens if you compare a milk jug and a two-liter soda bottle? Does the shape of the container affect how far the water streams out?
  • Extra: Use different size nails to punch holes (or have an adult help you use a power drill to make holes). Does the size of the hole affect how far the water flows or how fast the jug drains?

Observations and results
You should observe that the water from the hole at the bottom of the jug gushes out the farthest, followed by the middle hole then the top one. This is because the hydrostatic pressure (the pressure applied by the water at a certain depth) at the bottom of the jug is the highest, causing the water to rush out at a higher velocity. This high-velocity water has more kinetic energy to turn a turbine and produce electricity, so the bottom location would be the best place to put your turbine.

Depending on where you poked it (and how much you filled the jug), water from the top hole might just trickle down the side of the jug and not actually spurt out horizontally. As the water drains, it will not spew as far from the holes. This happens because hydrostatic pressure is determined by the differences in height between each of the holes' locations and the water’s surface—and the surface gets lower as the water drains.

More to explore
Hydropower, from National Geographic
Locks and Dams, from Twin Cities Public Television
Leaky Clues to Dam Design: How Reservoir Height Affects Hydroelectric Power Production, from Science Buddies
Science Activities for All Ages, from Science Buddies

 

This activity brought to you in partnership with Science Buddies

Science Buddies