Capacitance Flashcards
Describe a capacitor
2 conducting plates separated by a dialectric
What is a dielectric?
An insulating material
Define capacitance
Amount of charge per unit p.d. stored
How much is a pico?
10^-12
Why do we use μF?
Because the farad is a huge unit
What is permittivity?
How hard it is to generate an electric field in a certain material
How do you calculate relative permittivity?
ε/ε₀
What is another phrase for relative permittivity?
Dielectric constant
What is another phrase for dielectric constant?
Relative permittivity
What happens to a polar molecule in the presence of an electric field?
It rotates so that the negative end is attracted to the positively-charged plate
What is a dielectric made up of?
Lots of polar molecules
How do you introduce an electric field to polar molecules?
Apply a charge to the dielectric
What is the area under the graph of charge-pd
Energy stored
What is the equation for energy stored in a capacitor?
½QV
Describe the shape of a charge-pd graph?
Straight line through the origin
Why is the shape of a charge-pd graph a straight line?
pd is directly proportional to the charge stored
How does current change as you charge a capacitor?
Decreases (negative curve)
When a capacitor is fully charged, what is the current in the circuit?
0
What is the potential difference between the plates of a fully-charge capacitor?
Large
When current is flowing in a circuit with a capacitor, which plate do the electrons flow onto?
The one connected to the battery’s negative terminal
making the terminal negatively-charged
How does the build-up of negative charge on a plate move electrons in the circuit?
Negative charge repels electrons off the plate connected to the positive terminal
How do you discharge a capacitor?
Remove the power supply and complete the circuit
How does current change as you discharge a capacitor?
Decreases (negative curve)
What are the two factors for time taken to charge/discharge a capacitor?
Capacitance, resistance (of circuit)
For a capacitor, which formula in the formula book do you substitute V for Q?
“capacitor charging”
the first one
For a capacitor, which formula in the formula book do you substitute I for Q?
“decay of charge”
the second one
Define the “time to halve”?
Time taken for the charge/current/pd of a discharging capacitor to halve
How do you calculate time to halve?
ln2 × time constant
