Discovering the Electric Force

Learning Objectives

The goal of this activity is for students to understand the basic properties of the electric force. They should understand that it acts at a distance on objects that carry a charge.

Materials

• Paper clip
• Ping pong balls
• Styrofoam cups
• Plastic comb
• Wool
• Playdough
• Ruler
• Stopwatch / regular watch / wall clock (if none of these are available, have the kids race against each other, rather than timing each maze run)
• Basin

Motivation

The electric force is one of the most fundamental interactions in our universe. It acts between objects which are charged under a few simple rules. 1) Two objects that are charged alike (either positively or negatively) will repel. 2) Two objects that are oppositely charged will attract. 3) The closer the objects are, the stronger the force between them will be.

In Part 1, when we rub the comb in the wool we are giving it a net positive charge. When we then bring it close to the paper clip, the negative charges in the clip are attracted to the positive charge on the comb, and that is what draws the paper clip to the comb.

Part 2 is an exploration of the idea that since the electric force is a force, it can affect the motion of objects. Ping pong balls are very light, and therefore easy to accelerate. When you charge the ping pong ball and the comb, then bring them near to each other, the like charges repel, causing the ping pong ball to roll away from the comb.

Water is a charged molecule, and this is hopefully the conclusion that students will reach (either on their own or with assistance) as you work through Part 3. Again, rubbing the comb in wool gives it a net positive charge, which attracts the negative ends of the water molecules, which is why the stream of water bends.

Procedure

Part 1

Tie a paper clip to a string and dangle it from something (The edge of a table works well). Charge the comb by rubbing through some wool fabric and bring the comb close to the paper clip. Use a ruler to estimate how close the comb needed to be before the paper clip starts to move.

Part 2

Before this part of the experiment, have the kids build a small maze out of Playdough. Charge the comb and the ping pong ball with the wool fabric and estimate with a ruler how close the comb needs to get before the ping pong ball starts to move. Try to get the ping pong ball through the maze you constructed without touching it. Time how long it takes.

Part 3

Have one person charge the comb, while another person pokes a small hole in the bottom of a Styrofoam cup. Fill the cup with water. Make sure there is a basin or bin below to catch the falling water. Now bring the comb close to the water and estimate with the ruler how close it needs to be for the water to react.

Investigating Questions

• Is the electric force an impact force, or a force at a distance?
• Why does the paper clip move?
• Why does the ping pong ball move?
• Why does the water bend?
• Is water charged?
• What is one way we can give objects a charge?