Flashcards for 21.6 Capacitance

Q: Power and capacitors
A:
 Capacitors are compact and can be charged easily to store energy
 Unlike chemical cells, capacitors cannot store a great deal of energy in a small volume
 However, capacitors can release the stored energy very quickly and thus generate a
high output of power
 This is exactly what happens in a camera flash
 You can store about a joule of electrical energy in a normal capacitor
o And when this is released in a time of 1ms, you get a power output of
about 1kW




Q: A rectifier circuit
A:




Q: Smoothing capacitors
A:
 Domestic electricity is supplied as AC
o The direction of the current changes rapidly and repeatedly as the
supply voltage cycles from positive to negative and back
 The simple rectifier circuit changes an alternating input voltage to a smooth direct
voltage
 The diode allows current in only one direction only
 Without the capacitor, the output voltage from the circuit would consist of positive
cycles only
 With the capacitor, the output voltage is smoothed out and becomes almost direct
voltage of constant value
 The ripple in the output voltage can be kept small by making the time constant of the
circuit much greater than the period of the alternating voltage