Demos: 5A-11 Electrostatic Shielding (Faraday Cage)

A charged sphere is brought close to a previously grounded electroscope. It responds in the usual way. The electroscope is again grounded and a cage constructed of metal screen wire is place over the electroscope, completely surrounding it. The charged sphere is again brought near the electroscope but it does not respond, illustrating the shielding effect of metallic enclosures.

Directions: First insure that the electroscope is grounded. Then charge an insulated spherical conductor and bring it near the electroscope. How near you bring it depends on the response of the electroscope. Remove the sphere from the vicinity of the electroscope and momentarily ground the latter. Then place the wire cage over the electroscope. Once again bring the charged sphere near the cage.

Suggestions for Presentation: Ask the class what will happen if a charged sphere is brought near an electroscope. (This is a good chance to review the charging process.) After illustrating the phenomenon, ask what would happen if the electroscope were completely screened by a wire cage. Suggest that since the cage is an electric conductor, it might “spread the charge around” and make it even more effective in its interaction with the electroscope. When nothing happens, begin a discussion of what happened and why.

If you wish to make this demo even more dramatic, have the lecture-demo staff set up the giant cage made of copper screen wire. Either you or the student can get inside the cage. An electrostatic generator is attached to the copper screen and the screen is charged up. (This can be demonstrated in a variety of ways, including drawing sparks to your knuckles.) Have the student (or yourself) touch the inside of the screen to illustrate that no charge resides there.

Want to add some humor while illustrating another effect? The cage is also a shield to electromagnetic waves. Stand outside the cage and tune a portable radio to a loud station. Then while the radio is playing, step inside the cage. The sound will be virtually unheard. Now for the humor part: Tell the class that you want to review what happened. Say: “While I’m standing outside the cage, you can hear my voice just fine [now begin stepping into the cage], but when I’m in the cage [now just move your lips but don’t say anything!!] ...... And you might emphasize that you’re kidding, just in case!

Applications: Remind the students that it is often suggested that the safest place in a lightning storm is in your car. Especially emphasize that is ISN'T the rubber tires that make it safe (a common explanation). Also, they probably have noticed that when their cars travel under a highway bridge, the radio station fades slightly. Although the bridge isn’t an enclosed metal cage, there is enough metal to make it a partial shield.

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