SAT Physics Subject Test
Chapter 9 Electric Potential and Capacitance
You may see a question on capacitors on the SAT Physics Subject Test, so let’s discuss them now. Consider two conductors, separated by some distance, that carry equal but opposite charges, +Q and –Q. This pair of conductors make up a system called a capacitor. Work must be done to create this separation of charge, and, as a result, potential energy is stored. Capacitors are basically storage devices for electrical potential energy.
The most common conductors are parallel metal plates or sheets, and these capacitors are called parallel-plate capacitors. We’ll assume that the distance d between the plates is small compared to the dimensions of the plates, and in this case, the electric field between the plates is uniform.
The ratio of Q to the voltage, Δ V, for any capacitor, is called its capacitance (C).
For a parallel-plate capacitor, we have
Capacitance is a measure of the capacity for holding charge. The greater the capacitance, the more charge can be stored on the plates at a given potential difference. The capacitance of any capacitor depends only on the size, shape, and separation of the conductors. From the definition C =Q/V, the units of C are coulombs per volt. One coulomb per volt is renamed one farad (abbreviated F): 1 C/V = 1 F .
A 10-nanofarad parallel-plate capacitor holds a charge of magnitude 50 C on each plate.
6. What is the potential difference between the plates?
7. If the plates are separated by a distance of 0.885 mm, what is the area of each plate?
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6. From the definition, C = , we find that
7. From the equation C = , we can calculate the area, A, of each plate.