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The National MagLab is funded by the National Science Foundation and the State of Florida.


A capacitor is similar to a battery, but a few key differences make them crucial additions to many machines.

Like batteries, capacitors store energy. They have positive and negative ends, called terminals, that provide a voltage between them. If batteries or capacitors are part of a closed circuit, electrical current flows. Unlike batteries, however, capacitors do not free up electrons. They only store them.

The tutorial below demonstrates a capacitor functioning in a direct current circuit that powers an electric motor used to lift a small weight.


  1. Click the battery button to close the circuit and allow electrons to flow from the battery to the capacitor.
  2. Click the motor button to connect the motor and release the electrons stored in the capacitor.
  3. Watch how the electrons quickly flow to the motor and raise the weight.
  4. Repeat the process to continue to lift the weight.
  5. Click Reset to start from the beginning.

The battery alone could not have caused the weight to move much, because batteries discharge slowly and gradually. Capacitors, on the other hand, discharge quickly, providing stronger current over a shorter period of time.

Inside the capacitor, the positive and negative terminals connect to two metal plates separated by an insulating substance referred to as the dielectric. The dielectric is made from a material that is highly resistant to electric current, such as ceramic or glass, that keeps the plates from touching each other and allows them to hold opposite charges. Those opposite charges allow the capacitor to maintain an electric field across the gap between the plates.

When the knife switch closes the circuit to connect the motor, the electrons on the capacitor’s negative side are drawn through the circuit to the capacitor’s positive side. They rush along the now available path straight through the motor. The electricity powers the motor to lift the weight until the capacitor charge dissipates. The surge of electrical energy that the capacitor provides makes it perfect for applications like flash bulbs and the starter in your car.