capacitors

• a capacitor is an electrical component that temporarily stores electric charge. a capacitor consists of two parallel metal plates separated by an insulating layer
• the capacitance, C, of a capacitor is defined as the charge stored per unit p.d across the capacitor
• capacitance is measured in farads
• the capacitance can be found from the gradient of a graph that plots the variation of charge stored with applied potential difference
• when a capacitor is connected to a battery (energy source) there is zero voltage across the capacitor since it has no charge on it initially. storing charge on the capacitor involves the battery doing work to transport charge between the battery and the plates of the capacitor. the potential difference across the plates increases in proportion to the charge stored. the capacitor is charged and will store this charge even when the energy source is disconnected.
• when a capacitor discharges through a fixed resistor, the discharge current decreases exponentially
• electrolytic capacitors are polarised. it is essential that the electrolytic capacitors are connected into the circuit with correct polarity, as indicated in the capacitor body
• a component with polarity is asymmetric meaning that it can only be connected in one direction (with respect to the terminals of the source pd)
• the quantity RC is called the time constant for the circuit in seconds. it is a measure of how quickly the capacitor charges/discharges
• the time constant RC is the time taken for the capacitor to discharge to 37% of its initial charge
• as a capacitor discharges the charge, current and pd all decrease exponentially
• the time constant RC is the time taken for the capacitor to charge to 63% of the maximum charge
• when placed in an electric field, polar molecules will rotate in order to align themselves with the field direction. in other words, the substance becomes polarised
• dielectric materials are a category of insulating materials which, when in the region of an electric field, undergo polarisation
• the charge stored on the plates of a capacitor can be increased by inserting a dielectric between them
• the permittivity, ε, of a capacitor insulating material is a measure of how easily an electric field can store charge. it is measured relative to the permittivity of free space (vacuum), ε, using a relative permittivity εr.

  $εr=$$ε/ ε0$
• the relative permittivity εr of a dielectric substance can also be defined as the ratio of charge stored with the dielectric to the charge stored without the dielectric

  $εr=$$Q/Q0$
• the capacitance of a parallel-plate capacitor depends on:

  the area of the plates

  their distance apart

  the ability of the insulating material between the plates to separate the charge, a property known as permittivity

  $C=$$A ε0 εr/d$