Chapter 21 - Capacitors Flashcards
What is a capacitor?
Two metallic plates separated by an insulator, where charge is stored.
Define capacitance.
The charge stored per unit P.D. across a capacitor.
Define the farad.
Capacitance is one farad in a capacitor when 1C is stored across a P.D. of 1V .
What happens in terms of electrons when charging a capacitor?
Electrons build up on one plate (making it negative), and none are left on the other plate (making it positive).
What happens in terms of electrons when discharging a capacitor?
Electrons move to the positive plate until they are evenly distributed (P.D. = 0).
How do you determine total capacitance in a series circuit?
⅟Ctotal = ⅟C₁ + ⅟C₂ + …
How do you determine total capacitance in a parallel circuit?
Ctotal = C₁ + C₂ + …
Explain how work is done to charge a capacitor.
Work is done to overcome electrostatic forces, moving electrons away from the positive plate and towards the negative plate.
How is energy stored in a capacitor, and how can it be released?
Energy is stored when electrons are forced to sit next to each other on the same plate. Energy is released when the P.D. across the capacitor is removed.
How are potential difference-charge graphs used to determine the work done on a capacitor?
Work done = area under graph.
Explain what happens in terms of charge carriers when a capacitor is discharged through a resistor.
-Electrons move away from the negative plate and towards the positive.
-The rate at which this happens decreases until electrons are evenly distributed throughout the circuit.
When a capacitor is discharged through a resistor, what happens to its:
-Potential difference
-Current
-Charge stored
All decrease exponentially.
What is the time constant of a capacitor-resistor circuit?
The product of its capacitance and resistance. It is equal to the time it takes for the P.D. of a capacitor to decrease to ⅟e of its initial value.
Outline an experiment you could use to investigate the charge/discharge of a capacitor.
-Connect a capacitor and resistor in parallel.
-Connect/disconnect a cell.
-Record P.D./current across capacitor every 10 seconds.
-Plot a graph of P.D./current against time.
What is meant by the constant-ratio property of decay graphs?
The factor by which the P.D./current/charge decreases is always the same over the same Δt.
Give 2 uses of capacitors.
-A camera flash.
-Back-up power supplies in case of a short outage.
-Smoothing out p.d. when converting a.c. to d.c.
