How to prove experimentally that an electric field created by two oppositely charged plates has energy?

Place two metal plates parallel to each other on insulating stands. Let us inform the plates of charges of the same modulus, but different in sign. Introduce a negatively charged foil sleeve suspended on a silk thread into a uniform electric field. The sleeve under the action of the field will move to the positively charged plate. The force acting from the side of the electric field does the work of moving the charge. Hence, the electric field has energy.

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