Capacitors Flashcards
What is a capacitor and how does it work?
A device designed to store charge. It consists of two parallel metal plates placed near but insulated from each other. When they are connected to a battery, electrons are forced from its negative terminal onto one plate and an equal number of electrons leave the other plate to return to the battery via its positive terminal so each plate gains an equal and opposite charge.
What is the charge stored in a capacitor when one plate stores +Q and the other stores -Q?
Q
Define capacitance
The charge stored per unit pd of a capacitor. Units F means 1 C/V
What type of energy is stored in a capacitor?
Electric potential energy
What is the area under a pd against charge graph?
The energy stored by a charged capacitor.
Why does the current in the resistor decrease as a capacitor discharges?
The pd across the capacitor decreases as it loses charge. The resistor is directly connected to the capacitor so the current in it is pd/R. So current decreases as pd increases.
What can be used in the exponential decay of charge formula?
Charge, current or voltage
What is the time constant
The time taken for the charge to decrease to 1/e of its initial value. It equals the resistance times capacitance.
Explain what happens to charge, current and pd as a capacitor is charged through a fixed resistor.
The charge and pd increase until the capacitor is fully charged and then the pd equals the source pd. The current decreases to 0 as eventually no more charge can flow.
What is a dielectric?
A material that increases the capacity of a parallel-plate capacitor to store charge when placed between the plates of the capacitor.
How does a dielectric work?
When it is between the two oppositely charged plates, each molecule becomes polarised (electrons pulled slightly towards the positive plate). Some dielectrics have polar molecules that are in random directions. These turn when placed between the plates. The positive side of the dielectric attracts more electrons from the battery onto the negative plate and negative side of the dielectric pushes electrons back to the battery from the positive plate. Therefore the charge stored increases and the capacitance increases.
Relative permittivity (dielectric constant)
The ratio of the charge stored by a parallel plate capacitor with a dielectric filling the space between its place to the charge stored without the dielectric for the same pd.
How can capacitance be increased?
Making the surface area of each plate larger. Making the distance between the plates smaller. Filling the space between the plates with a dielectric with a larger relative permittivity.
How are most capacitors arranged?
Two strips of aluminium foil separated by layer of dielectric. All rolled up. Makes surface area as large as possible and distance between plates as small as possible.
Graph of relative permittivity against frequency of alternating electric field.
Horizontal. Diagonal down. Horizontal. Up a bit, diagonal down, up a bit. Horizontal. Up a bit, diagonal down, up a bit. Horizontal. The diagonal downs decreases in length.
