3.7.4.1 - Capacitance Flashcards
What is capacitance?
The charge required to cause unit potential difference in a conductor.
What is capacitor made up of?
Two conducting plates separated by a gap or a dielectric.
What is created in a capacitor and how?
A uniform electric field due to positive and negative charge building up on opposite plates.
What is permittivity?
A measure of how difficult it is to generate an electric field in a medium.
What does a higher permittivity indicate?
The more charge is needed to generate an electric field of a given size.
What is a dielectric?
Material made up of polar molecules, which have a positive and negative end.
When no charge is applied what happens in the dielectric material?
No charge is being stored so no electric field is being generated. The molecules align randomly.
What happens in a dielectric material when charge is applied?
An electric field is generated between the plates. The negative ends of the molecules are attracted to positively charged plates and vice versa. This causes the molecules to rotate and align.
What happens to the electric field in a capacitor when charging with a dielectric material and why?
The overall electric field strength reduces as each of the polar molecules have their own electric field which opposes the applied electric field of the capacitor.
What does a larger permittivity result in for the electric field strength?
The larger the permittivity, the larger the opposing field is. This reduces the overall electric field.
What does larger permittivity overall result in?
Reduced electric field, therefore reduced potential difference but higher capacitance.
From what can you calculate the energy stored by a capacitor?
Underneath a Q V graph.
What happens during charging?
As soon as the switch is closed, current starts to flow. The electrons on the plate connected to the negative terminal causes negative charge to build up. The build up of negative cage repels electrons off the plate connected to the positive terminal, making that plate positive. An equal but opposite charge builds up on each plate, causing a potential difference between the plates.
What is the IT graph for charging?
Exponentially decreasing.
Graph for charging of QT
Exponentially increasing
Graph of VT for charging?
Exponentially increasing
How can you find the time constant for an exponentially increasing graph?
Read off at 63% of original y axis value.
How can you find the time constant for exponentially decreasing graph?
Read off at 37% of original y axis value.
What happens during discharging?
Current flows in the opposite direction from the charging current. As the potential difference decreases, the current decreases as well. When a capacitor is discharging the amount of charge on and potential difference between the plates fall exponentially with time. That means it always takes the same length of time for the charge or potential difference to halve, no matter what value it starts at.
What are all the graphs for discharging?
Exponentially decreasing.
What is the time to half?
The time taken for the charge, current or potential difference of a discharging capacitor to reach half the value it was when fully charged.
