physics chapter 23.1+23.2 capacitance

a capacitor

a device designed to store charge

how does a capacitor gain charge when the plates of a capacitor are connected to a battery

electrons move through the battery, + are forced from its negative terminal of the battery onto one of the plates - an equal number of electrons leave the other plate to return to the battery via its positive terminal = each plate gains an equal and opposite charge

why does one conductor of a capacitor store charge +Q and the other conductor store charge -Q

because when the capacitor is connected to a battery, one of the 2 conductors gains electrons from the battery, whilst the other conductor loses electrons to the battery

the capacitance of a capacitor, C

the charge stored per unit potential difference

equation for capacitance, relating charge and potential difference

C = Q/V

examples of uses of capacitors

smoothing circuits, back-up power supplies, timing circuits, pulse-producing circuits, tuning circuits, filter circuits

why is energy stored in a capacitor when it is charged, + what is energy stored as

because electrons are forced onto one of its plates and taken off the other plate - the energy is stored in the capacitor as electric potential energy

how will a charged capacitor release its energy when discharged across a torch bulb

will release its energy in a brief flash of light from the bulb, as long as the capacitor has first been charged to the operating pd of the bulb

what is the energy stored in the capacitor equal to

the work done to force the extra charge on to the plates - given by change in E = v x change in q

equations for energy stored by a capacitor

E = 1/2QV = 1/2CV^2 = 1/2Q^2/C

how much energy is transferred from a circuit vs how much energy supplied by the battery is stored in the capacitor, + what is the remaining energy transferred as

battery transfer energy QV to the circuit; 50% of energy is stored in the capacitor = 1/2Qv; other 50% is wasted due to resistance in the circuit + transferred to the surroundings

why exists because a thundercloud is charged

a strong electric field exists between the thundercloud and the ground

why does the energy stored in a thundercloud increase if the thundercloud is forced by winds to rise higher, + what does the thundercloud have to overcome to move away from the ground

because work is done by the force of the wind - the thundercloud has to overcome the electrical attraction between the thundercloud and the ground to make the charged thundercloud move away from the ground

over what electric field strength does the insulating property of air break down

subjected to an electric field stronger than about 300kVm^-1

how does current change when a capacitor discharges through a fixed resistor, + why does this happen

the discharge current decreases gradually to zero = because the pd across the capacitor decreases as it loses charge

what would happen to the resistor current as pd in the circuit decreases, in a circuit in which a capacitor is discharged through a resistor

the resistor current would decrease as the pd decreases, because the resistor is connected directly to the capacitor

shape of a graph of how current and charge change against time when a capacitor discharges

an exponential decrease = any of these quantities decrease by the same factor in equal intervals of times

relationship between current and charge, + relationship between charge and pd

current is proportional to charge, + charge is proportional to pd

does the charge on the plates of a capacitor every reach zero, + why

charge never becomes zero, because charge decreases exponentially

when doe exponential changes occur

whenever the rate of change of a quantity is proportional to the quantity itself