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.
Capacitor discharge through fixed resistor:
When a capacitor discharges through a fixed resistor, the discharge current…
because…
so current through resistor…
decreases gradually to sero. This is bc the pd across the capacitor decreases as it loses charge. Bc resistor is connected directly to capacitor, resistor current (=pd/resistance) decreases as the pd decreases.
Describe the graph of current, charge and p.d. against time during discharge.
Current, charge and p.d. decrease exponentially - exponential decay rgaph.
Exponential decay:
If Q0 decreased to 0.9Q0 in time t1, what will the charge be after nt1?
If Q0 decreased to 0.9Q0 in time t1, the charge will be 0.9^nQo after nt1.
Graphical eq : Q = Qoe^-t/RC
What’s the time constant?
RC = time constant.
At t=RC after start of discharge, the charge falls to 0.37 (=e^-1) of its initial value.
Charging of capacitor through a fixed resistor:
When a capacitor is charging by connecting it to a source of constant p.d., the charging current…
When fully charged, p.d. is.. and current…
decreases as the capacitor charges.
When fully charged, its p.d. is equal to the source p.d., and the current is zero bc no more charge flows in the circuit.
What does the Q-t graph look like for a charging capacitor?
The curve of the graph is inverted exponential decay, flat at Q0 (=CVo).
V=Q/C ∴ same graph for V
What’s the time constant for this circuit?
The time taken for charge to reach 63% of final charge (37% more charge needed to be fully charged).
Graph for I-t when charging?
Capacitor current I decreases exponentially to zero from I0. Current is rate of change of charge ∴ is gradient of Q-t graph ∴ it decreases exponentially.
What’s a dielectric. What does it do? Give two examples.
Dielectrics are electrically insulating materials that increase the ability of a parallel-plate capacitor to store charge when placed between the plates.
Eg polythene, waxed paper.
How does it do this? ( literal simple terms)
Each molecule of the dielectric becomes polarised, or polar molecules rotate and align w/ the field.
As a result more charge is stored on the plates because? (2 v.v)
-the positive side of the dielectric attracts more e-s from the battery to the -ve plate.
-the negative side of the dielectric pushes e-s back to the battery from the +ve plate.
Dielectrics increase charge stored for any given p.d. across a capacitor i.e…
i.e. it increases the capacitance of the capacitor.
What is relative permittivity, εr, of the dielectric substance?
(also called dielectric constant)
εr = Q/Qo = charge stored by capacitor when space between parallel plates filled with dielectric / charge in capacitor when empty space.
What is relative permittivity, εr, of the dielectric substance?
(also called dielectric constant)
εr = Q/Qo = charge stored by capacitor when space between parallel plates filled with dielectric / charge in capacitor when empty space.
How can we find εr?
εr = I/Io = Q/Qo
current/charge w/ over current charge w/o
Constant p.d. needed (use potential divider)
Polarisation mechanisms of dielectric (3)
Orientation polarisation
Ionic polarisation
Electronic polarisation
1 - Orientation polarisation
Unequal sharing of e-s in covalent bond ∴ dipoles between the atoms.
2 - Ionic polarisation
Ions of each bond form a dipole.
3 - Electronic polarisation
Electron distribution and nucleus form a dipole.
In an alternating electric field, polar diploes rotate and non-polar dipoles oscillate one way then the opposite way as the field strength increases and decreases. At low frequencies…
the 3 polarisation mechanisms alternate in phase with the field.
As the frequency increases…
each mechanisms ceased to work due the inertia of the particles involved and the resistive forces that oppose the motion of the diploes.
Therefore, as frequency increases, walk me through the order that the mechanisms decrease by:
Orientation decreases
Then ionic decreases
Then electronic decreases.
The half life:
0.69 x RC = t1/2
Resistance of voltmeter really high in these circuits because?
Capacitor mustn’t lose charge through the meter.
