Summary OCR A level Physics A 6.1 Capacitors

6.1 Capacitors

, Capacitors
Electrical component in which
charge is stored A .


capacitor consists of
two charged plates separated from each other
equally by an insulator
(dielectric) such as air ,
paper ,
ceramic or mica .




+
_




> >
direction of
^ v flow of
+ Q -
Q
electrons
-


+
-


+
' <
± =
A B



•
when the capacitor is connected to the cell , electrons flow from the cell for a short period
of time .




•
Electrons cannot travel between the plates because of the insulation The current .
causes
electrons to be removed from plate A.
making it
positively charged . Plate B gains
electrons , aqui ring a negative charge .




•
The two plates have
charge that is equal in
magnitude but opposite in charge as

the circuit 's current must be equal in all places and charge must be conserved .




Therefore there is potential difference across the plates
•
a .




•
When the p d.
-


across the plates is equal to the e.m.fi of the cell the current , falls
to zero



the capacitance of a capacitor is defined as the charge stored per unit potential
difference across the capacitor :




C = Q Q = VC




capacitance C , ,
in farads ,
F

charge stored ,
Q in coulombs C
, ,




potential difference V. ,
in volts V ,




charge on the capacitor a
potential difference across the capacitor