Introduction to SAT II Physics - FreeExamPapers

The unit of current is the coulomb per second, which is called an ampere (A): 1 A =
1 C/s.
Direction of Current
Although the electrons are the charge carriers and move from the negative terminal to the
positive terminal of the battery, the current flows in the opposite direction, from the
positive terminal to the negative terminal. This may seem odd, but we can draw an
analogous example from everyday life. Suppose you arrange 12 chairs in a circle, and get
11 people to sit down, leaving one chair empty. If each person in turn were to shift over in
the clockwise direction to fill the vacant spot, the vacant spot would appear to move in the
counterclockwise direction. If we think of the electrons in a circuit as the people, then the
current moves in the direction of the vacant spot.
Resistance
Some materials conduct current better than others. If we had a copper wire and a glass
wire with the same length and cross section, and put the same potential difference across
them, the current in the copper wire would be much larger than the current in the glass
wire. The structure of copper, a conductor, is such that it permits electrons to move about
more freely than glass, an insulator. We say that the glass wire has a higher resistance,
R, than the copper wire.
We can express resistance in terms of the potential difference, , and the current, I:
Generally, the
is omitted. For a given voltage, the larger the current, the smaller the
resistance. The unit of resistance is the ohm (
ampere: 1
= 1 V/A.
). One ohm is equal to one volt per
Ohm’s Law
Ohm’s Law relates the three important quantities of current, voltage, and resistance:
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