Electric Fields and Potential

Energy Stored in a Capacitor

What it shows:

The electrical energy stored in a capacitor is converted to mechanical work, driving a motor and raising a weight.

How it works:

A motor 1 is mounted atop a 2.5m length of 2×4. As it turns, it raises a 1 lb mass on a string from the ground by wrapping the string around a spindle (figure 1). The motor is driven by the discharge of a 12800µF, 75V capacitor previously charged by a DC power supply. 2 A double throw switch allows a clean change-over from one circuit to the other.
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Capacitance of Human Body

What it shows:

Determine the capacitance of the human body as follows. Charge a person of unkown capacitance to 1000 volts. The person is subsequently connected (in parallel) to an external capacitor of known capacitance. The voltage measured across the capacitor combination allows one to determine the unknown capacitance of the person (typically between 180 — 200 pF).

How it works:

A 1000 volt power supply (output is in the microamp range) is used to put charge on a person. We assume that the amount of charge transferred to the...

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Giant Capacitor

What it shows:

The basic principles of the parallel plate capacitor made large.

How it works:

The capacitance C of a simple parallel plate capacitor is given by


the ratio of the magnitude of the charge Q on either conductor to the potential difference between the...

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Walk-In Faraday Cage

What it shows:

A lecturer's faith in the principle that an electric field cannot exist inside a charged conductor is put to the test using a Faraday cage that is large enough to sit in.

How it works:

The lecturer (or some volunteer) climbs the three steps and sits upon a plain wooden chair. Their assistant pulls the mesh door closed and fastens it. A Van de Graaff, whose dome is in contact with the cage, begins to charge itself and the cage up to a high voltage. The person inside is oblivious to the large amount of charge now...

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Benjamin Franklin's Thunder House

A replica of Franklin's Thunder House demonstating the efficacy of his invention—the lightning rod. The class will get a charge out of this one.

thunder house

Electric Force on Neutral Object

A neutral conductor (or dielectric) experiences a torque, but no net force, when placed in a uniform electric field. It does experience a net force in a non-uniform field.

What it shows:

When an electrically neutral object is suspended in a uniform electric field, it becomes polarized. The electric force on the separated charges produces a torque about the suspension point and the object rotates. There is no translational motion—the object simply aligns itself with the electric field.

When an electrically neutral object is...

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