Capacitors Flashcards
What is a capacitor?
A capacitor is a device that stores charge.
What is the setup of a capacitor?
Two parallel metal plates placed near each other form a capacitor. When the plates are connected to a battery, they gain equal and opposite charges.
Charging a capacitor at constant current: describe setup of circuit and why each component is used.
What’s the graph drawn?
battery connected in series to a capacitor, variable resistor and microammeter, with a data logger connected in parallel to the capacitor. Stopwatch and data logger to measure p.d. at diff times. Variable resistor is continuously adjusted to keep current constant. Hence Q =It
Plot charge stored against p.d. to get straight line through the origin.
Define capacitance.
Units?
Charge stored per unit p.d.
C = Q/V
Unit is Farad (F).
Therefore graph above, gradient is capacitance.
Capacitor uses: what types of circuits? (6)
-smoothing circuits
-back-up power supplier
-timing circuits
-pulse-producing circuits
-tuning circuits
-filter circuits
-smoothing circuits
ie circuits that smooth out unwanted variations in voltage
eg supplying current if mains supply interrupted
-back-up power supplier
ie circuits that take over when the mains supply is interrupted
-timing circuits
ie circuits that switch on or off automatically after a present delay
eg say an alarm only goes off if the p.d. falls to a certain value. if we use capacitor, we can slow down the decrease in p.d. across the capacitor by using a higher capacitance capacitor, hence loner delay before alarm sounds.
-pulse-producing circuits
ie circuits that switch on and off repeatedly
-tuning circuits
ie circuits that are used to select radio stations and TV channels
- filter circuits
ie circuits that remove unwanted frequencies
When a capacitor is charged, energy is stored in it as — because—
stored as electric potential energy bc e-s are forced onto one of its plates and taken off the other plate.
Energy stored by capacitor, E = 1/2QV (area of Q-V graph). In the charging process, the battery forces charge Q through p.d. V and the circuit therefore transfers energy QV to the circuit. What does this tell us?
Thus 50% of the energy supplied by battery (=1/2QV) is stored in the capacitor. The other 50% is wasted due to resistance in he circuit as it is transferred to the surroundings when the charge flows in the circuit.
What apparatus can we use to measure energy stored in a charged capacitor?
A joulemeter (and voltmeter).
Energy stored in a thundercloud:
For a thundercloud carrying a constant charge, Q, the energy stored = 1/2 QV = 1/2 QE/d.
Effect of increasing d?
(d is distance between Earth and thundercloud)
If thundercloud is forced by winds (wind does work) to rise up to a new height d’, then E = 1/2 QE/d’.
Bc electric field strength is unchanged ( bc depends on charge per unit area) then the increase in energy stored E= 1/2 QE/Δd.
