handbook of modern sensors

44 3 Physical Principles of Sensing
difference between the two points. If we travel through the electric field along a
straight line and measure V as we go, the rate of change of V with distance l that we
observe is the components of E in that direction
E l =− dV
dl . (3.16)
The minus sign tells us that E points in the direction of decreasing V . Therefore, the
appropriate units for electric field is volts/meter (V/m).
3.2 Capacitance
Let us take two isolated conductive objects of arbitrary shape (plates) and connect
them to the opposite poles of a battery (Fig. 3.4A). The plates will receive equal
amounts of opposite charges; that is, a negatively charged plate will receive additional
electrons while there will be a deficiency of electrons in the positively charged plate.
Now, let us disconnect the battery. If the plates are totally isolated and exist in a
vacuum, they will remain charged theoretically infinitely long. A combination of
plates which can hold an electric charge is called a capacitor. If a small positive
electric test charge, q 0 , is positioned between the charged objects, it will be subjected
to an electric force from the positive plate to the negative. The positive plate will repel
the test charge and the negative plate will attract it, resulting in a combined push-pull
force. Depending on the position of the test charge between the oppositely charged
objects, the force will have a specific magnitude and direction, which is characterized
by vector f.
(A)
(B)
Fig. 3.4. Electric charge and voltage define the capacitance between two objects (A); a parallelplate
capacitor (B).