the negative conductor. This means that less work is required to bring the charge near the positive conductor, which decreases the voltage.
3. Try to bring a positive charge near a positive insulated conductor with an earthed plate adjacent. The earthed plate neutralises the positive insulated conductor. This means less work is required to bring the charge near the positive conductor, which decreases the voltage.
From these three situations, it can be seen that changing the pd affects capacitance. As can be seen from the formula, the pd affects the ratio that defines capacitance:
Capacitance C = Q
__ V
Two main types of capacitors
Electrolytic capacitor An electrolytic capacitor is a Swiss-roll design of two plates separated by a dielectric material (such as mica, paper or ceramic; Fig. 22.2a). A dielectric is a non-conductive substance. The distance between the plates and dielectric material produces the desired capacitance, as we will see from the parallel plate capacitor formula later. The main advantage of an electrolytic capacitor is that its rolled design allows large capacitance in a very small size (Fig. 22.2b).
Parallel plate capacitor
A parallel plate capacitor is a simpler formation of two plates separated by a dielectric material (Fig. 22.3). Once a potential difference is applied, the plates of the capacitor charge up.
H
The factors affecting the desired capacitance of a parallel plate capacitor are:
• Distance between plates (metres): Increase distance → Decrease capacitance
• Dielectric material (permittivity (ε) measured in Farads/ metre (F m–1)):
Increase value of ε → Increase capacitance
• Common area of overlap between plates (metres squared): Increase area → Increase capacitance
If we take all of these proportionalities, we get the following: C í εA
___ d
248 FUSION Voltage VC
FIG. 22.3 A parallel capacitor consists of two parallel plates with a dielectric separating them
dielectric Symbol
paper and electrolyte
a aluminium foil
paper oxide film
b
FIG. 22.2 The dielectric is the paper separating the two metal films
