Capacitors Flashcards
How does a capacitor work?
Two parallel metal plates connected to the terminals of a battery. The electrons are forced onto one of the plates whilst the electrons leave the other plate through positive terminal of the battery so each plate has an equal and opposite charge.
How to measure capacitance?
Put in series with a variable resistor and ammeter with a voltmeter in parallel across the capacitor. Measure value of potential difference as time changes or use a data logger. Plot graph of Q against V and the gradient is the capacitance.
What is 1 Farad?
1 coulomb stored with a potential difference of 1.
What is Capacitance?
The charge stored per unit volt.
2 examples of Capacitor uses:
Smoothing circuits (smooth out unwanted variation in p.d.) and in backup power supplies
How is energy stored on a capacitor?
Electrons are forced off one plate and onto the other which means electrical potential energy is stored
How was E = 1/2 QV derived?
work done = change in voltage * charge. Therefore if voltage is plotted against charge then the area under the graph is Energy. This can be rewritten as 1/2 CV^2 or 1/2 Q^2/C using C = Q/V
Why is only 50 percent of energy supplied by battery stored in the capacitor?
Half is wasted through work done against resistance in the circuit.
How can energy stored be measured?
Place joulemeter across a bulb and value is measured. After capacitor has discharged the value is measured again. The difference between these values is the total energy stored before.
Why does current gradually decrease to zero across a capacitor?
As the capacitor loses charge, the potential difference across the capacitor reduces slowly to zero, hence the current decreases to zero.
Why does it decrease exponentially?
When V is high lots of electrons discharge very quickly as there is high potential difference. As this happens, pd decreases over time so the electrons leave at a slower rate.
What is the time constant?
At time t = RC, the capacitor discharges to 0.37 of its initial value of Q.
How to investigate Capacitor discharge using oscilloscope?
Oscilloscope used because it has very high resistance so current passes only through fixed resistor. Capacitor is charged and then discharged through the oscilloscope and pd is measured at regular intervals. Plot V against t. The time taken for the voltage to decrease to 37 percent is time constant. Compare with measured value of RC.
Why does current decrease exponentially when charging?
The most electrons are flowing through the circuit at the start which then decreases as the charge on the plates are equal and opposite. At this point no charge is flowing in the circuit so current is zero. This is shown by I = I(0)e^-t/RC.
What is a dielectric and how does it work?
Electrically insulating materials that increase the ability of a capacitor to store charge. Works when it is placed between plates the molecules in the dielectric become polarised so electrons are pulled towards positive plate and away from negative. This increases charge as it causes greater force of attraction as the positive side of the dielectric attracts more electrons to this plate and vice versa.
