7.4 Capacitance Flashcards
What is a capacitor?
An electric component that stores charge.
Define capacitance.
A capacitor that stores Q amount of charge per volt of potential difference across its plates.
Define permittivity.
Is a measure of how difficult it is to generate an electric field in a medium.
(or the measure of the electric polarisability of a medium)
What is a dielectric?
An electrically insulating material that increases a capacitors ability to store charge.
How does a dielectric work?
- Polarised molecules each have a electric field
- This electric field opposes the electric field of the capacitor plates
- Hence the capacitor can store more charge for the same pd
What is relative permittivity?
(aka dielectric constant)
The permittivity of a dielectric material relative to the permittivity of the vacuum.
What equation for capacitance involves the dimensions of a parallel plate capacitor?
What do the letters represent in this equation?
What is the area of a charge-pd graph?
Energy stored in the capacitor
What are the equations for energy stored in a capacitor?
What do the letters represent in this equation?
How is energy stored in a capacitor?
By doing work removing electrons from the positive plate and adding them to the negative plate.
Why does energy stored increase when the distance between the plates is increased?
Work is done to force the plates apart, overcoming the electrical attraction of the plates.
In what proportionality does current, charge and pd of a capacitor decrease?
Exponentially.
What are the equations for charge, pd, and current for a discharging capacitor?
What do the letters represent in this equation?
What are the equations for charge, pd, and current for a charging capacitor?
What do the letters represent in this equation?
What is the gradient of a charge-time graph?
Current.
What is the area of a current-time graph?
Charge.
Draw are the graphs for charge, pd, and current for a discharging capacitor.
Draw the graphs for charge, pd, and current for a charging capacitor.
What two factors control the time taken for a capacitor to charge/discharge?
- Capacitance
- Resistance of circuit
What is the equation for the time constant?
What do the letters represent in this equation?
What is the time constant in terms of charge, voltage, and current?
Time taken for a discharging capacitor to reach 37% of original charge, voltage, and current.
(Or for a charging capacitor to reach 63% charge and voltage)
What is the time to half in terms of charge, voltage, and current?
Time taken for a charging/discharging capacitor to reach 50% of original charge, voltage, and current.
What is the equation for the time to half?
