Wednesday, December 9, 2009

Energy Labs: Electrical Fields and Magnetic Fields

As mentioned previously, the companion website to the PBS show Einstein's Big Idea includes activities related to the three parts of Einstein's equation E=mc2: energy, mass and velocity. We worked our way through them, substituting and adjusting where necessary. Not all of them were as successful or illuminating as we hoped, but they were quick and easy to do and got us started on physics labs. The instructions below include the adaptations we made to the instructions from the PBS teachers' material.

The energy lab consists of seven activities, which they suggest you set up at different stations. We did these over the course of a few days. On day one, we did the electrical and magnetic field activities. Here is what the teacher's guide calls its "learning objectives:"
Students will be able to:
  • explain what the E in E = mc2 represents.
  • name different kinds of energy.
  • show examples of how one kind of energy can be converted into another kind of energy.
  • describe how a field can exert a force and cause an object to move.
Electrical Field Lab

• one plastic spoon for each person
• 10 cm x 10 cm piece of wool or fur (we used a woolen scarf, and the hair on our heads!)
• pieces of plastic foam cup, crumbled into bits
• pieces of paper, about 0.5 cm by 1 cm each

1. Rub a plastic spoon with a piece of wool, some fur, or your hair. Place the spoon next to a small piece of paper. Can you make the piece of paper stand on edge and move back and forth?

2. Try to pick up several pieces of paper at the same time by touching the spoon to one edge of each.

3. Recharge the spoon by rubbing it again. Try to drop a small bit of plastic foam into the spoon from different heights above it.

Explanation: Students are examining the effects of an electric field produced by rubbing a plastic spoon on fur. Once the spoon is charged (negatively), it will attract an uncharged object like a piece of paper through electrostatic induction. The large negative charge on the spoon repels the electrons in the piece of paper and leaves the side of the paper near the spoon slightly positive. (Positive charges-in the nucleus of each atom within the paper-hardly move at all.) Then, the negative spoon attracts the now positive side of the paper. If students are careful in their approach to the paper, they should be able to make it "dance."

Plastic foam becomes instantly negatively charged when in contact with another negatively charged object. The bits of plastic foam acquire a negative charge when they touch the spoon and are repelled immediately. It is impossible to catch a piece of plastic foam, no matter how close to the spoon it is held. If students claim they can, have them recharge their spoons. Watch the pieces of foam cup go veering away from the spoon in this video:

Magnetic Field

• several types of magnets, including bar or horseshoe
• small shallow cardboard box
• piece of white paper (cut to fit box)
• small container of iron filings (I found one in an old chemistry set.)

1. If the box is not white inside, fit a white piece of paper into the bottom of the box.

2. Center one or more of the magnets under the box.

3. Sprinkle iron filings into the box over the magnet. Lines of magnetic force should begin to become visible. (See photo at top.)

Explanation: Students should realize that the field from the magnet is exerting a force on the iron particles. The filings will align with the north and south field lines. Watch as the lines form in the video below: