Introduction to SAT II Physics - FreeExamPapers

The negative value for the current across
means that the current actually flows in the
opposite direction of the arrow we drew. This makes perfect sense when we consider that
current should normally flow out of the positive terminal and into the negative terminal
of battery .
It doesn’t matter how you draw the current arrows on the diagram, because if you apply
Kirchhoff’s Rules correctly, you will come up with negative values for current wherever
your current arrows point in the opposite direction of the true current. Once you have
done all the math in accordance with Kirchhoff’s Rules, you will quickly be able to
determine the true direction of the current.
Capacitors
Capacitors rarely come up on SAT II Physics, but they do sometimes make an
appearance. Because capacitance is the most complicated thing you need to know about
DC circuits, questions on capacitors will usually reward you simply for knowing what’s
going on. So long as you understand the basic principles at work here, you’re likely to get
a right answer on a question most students will answer wrong.
A capacitor is a device for storing charge, made up of two parallel plates with a space
between them. The plates have an equal and opposite charge on them, creating a
potential difference between the plates. A capacitor can be made of conductors of any
shape, but the parallel-plate capacitor is the most common kind. In circuit diagrams,
a capacitor is represented by two equal parallel lines.
For any capacitor, the ratio of the charge to the potential difference is called the
capacitance, C:
For a parallel-plate capacitor, C is directly proportional to the area of the plates, A, and
inversely proportional to the distance between them, d. That is, if the area of the plates is
doubled, the capacitance is doubled, and if the distance between the plates is doubled, the
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