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P. 30
Physics for Secondary Schools
capacitance can be varied is termed a
variable capacitor. On the other hand,
the capacitor whose capacitance
cannot be changed is a fixed capacitor.
Charging a capacitor
FOR ONLINE READING ONLY
Consider the circuit in Figure 1.32 for
charging a capacitor.
Figure 1.30: Electrolytic capacitor A B
e - e -
4. Air-filled capacitors Electrons are V Electrons
accumulate
drawn off the
on the
In an air-filled capacitor, air forms the positive plate negative
plate
insulating material (Figure 1.31). The
capacitance of such devices is altered A I
by changing the overlapping area
between the plates.
S R
Figure 1.32: Charging a capacitor
An uncharged capacitor has no potential
difference between its plates. It does
not prevent the current from arriving at
one plate or leaving the other. This is
because the initial current in the circuit is
determined only by the resistance of the
connecting wires. Due to the presence of
insulation between the capacitor plates,
electrons tend to accumulate on the plate
Figure 1.31: Air-filled capacitor connected to the negative terminal of the
charging cell. This is partly due to the
An air-filled capacitor is a good attraction of the positive side of the cell
example of a variable capacitor. The
semicircular plates are separated by or repulsion from the nearby negative
air. Variable capacitors are mainly charge. Current flows until the p.d across
used for tuning radio sets. One set the capacitor is equal to the voltage of the
of plates is fixed, while the other charging cells. Charging of the capacitor
one can be rotated using a knob. then stops. Figure 1.33 shows a graph
Rotation changes the area of the of charge against time for a charging
plates. Generally, the capacitor whose capacitor.
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Student’s Book Form Two
Physics Form 2 Final.indd 24 25/10/2025 10:25
