High School Physics : Understanding Capacitance

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Capacitors

Which of these is the correct SI unit for capacitance?

Possible Answers:

Ohms

Farad-Joules

Newtons

Farads

Volts

Correct answer:

Farads

Explanation:

The correct unit for capacitance is the Farad, named after the English physicist Michael Faraday.

Newtons are used to measure force. Ohms measure resistance and volts measure electrical energy or potential. Joules are a unit of energy, but Farad-Joules are not a recognizable unit.

Example Question #2 : Capacitors

By using a dielectric, it is possible to store more energy in a capacitor. Why is this? 

Possible Answers:

The dielectric replaces air, which breaks down when the electric fields get strong enough

The dielectric changes the distance between the two plates of the capacitor

The dielectric provides a direct energy source to the capacitor

The dielectric can store excess energy

Correct answer:

The dielectric replaces air, which breaks down when the electric fields get strong enough

Explanation:

Dielectrics are insulating materials that can be polarized, and can be used to increase the separability of the two plates of a capacitor. When a dielectric is absent, air is present between the capacitor plates. At high voltages, when the electric field between the plates is incredibly strong, air begins to break down and becomes unable to prevent the flow of electricity between the capacitor plates. When this occurs, the capacitor cannot store energy, nor can it properly function. The presence of a dielectric eliminates this problem, allowing the capacitor to store much more energy.

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