# Hydroelectric Dam: Energy Transformations

1. Jul 8, 2010

### cj

I'm a teacher. I need to teach this subject to a group of smart 9th grade students that I am prepping to begin a "Green Physics" project.

What are the energy transformations associated with a hydroelectic dam? I think I have it up to a certain point, and then I'm less than sure. Here's what I have so far:

1) Gravitational potential energy of water is transformed into kinetic energy of water.

2) Kinetic energy of water is transformed into rotational kinetic energy of a turbine.

3) Rotational kinetic energy of a turbine is transformed into magnetic field energy in an AC generator.

4) Magnetic field energy in an AC generator is transformed into electic field energy in an AC generator.

5) Electric field energy in an AC generator causes electrons to flow.

Where am I incorrect or incomplete? Thank you.

Last edited: Jul 8, 2010
2. Jul 8, 2010

### N-Gin

These two are OK.

Now, for the rest of it, I'm afraid it's not so simple. The basic principle of generator is a coil rotating in magnetic field. So, magnetic field already exists. Now, when the coil rotates, magnetic flux in it changes, and it creates electromotive force (electric tension). During that process, coil resists that change and becomes a magnet itself, causing the coil to stop. Otherwise the coil would be rotating at enormous speeds. Part of the energy "stored" in rotation of turbine is "wasted" as a work that keeps the coil rotating, and the other part is transformed into electric energy (energy which is used to create current).

Fundamentally, there is no distinction between energies of magnetic and electric field. You could say that magnetic field is a form of electric field (it's caused by moving charges due to relativistic effects). So, we say electromagnetic field and electromagnetic energy.

In this case, I think it's best to say that the part of rotational energy of a coil is directly transformed into electric energy with a "help" of magnets, and the other part is transformed into work to keep the coil moving. That's why we have energy losses.

I hope I was not too confusing. I tried to be as much simple as I can, so I hope that fellow physicists won't mind.

3. Jul 8, 2010

### turbo

A little clarity is in order around ~3 or so. The turbine causes the rotor of the generator to spin, but before you can induce a flow of current in the windings of the rotor, you have to flash the stator to produce a magnetic field that the rotor can move in. For this to happen, large generators are supplied with massive banks of batteries, so they can "bootstrap" in the case that there is no external electric power available. Once the generator is running, it can provide enough power to keep the stator field active without the need for the batteries.

If you can tour a hydro-dam, you can see that they use LOTS of large lead-acid batteries to flash the stator on start-up.