Capacitors - uses, time constant and application of capacitors Flashcards
What is the circuit sign for a capacitor and how does it differ from a cell?
- Same length of lines
- Electrons do not cross the gap between the plates
Can the electrons cross between the plates in a capacitor? Why?
No, not in an ideal world.
Because there is an insulator between the plates.
Capacitance is defined as …
… the ratio of the charge on the plates to the potential difference difference between the plates.
Unit for capacitance
F (farad)
Capacitance is directly proportional to … . Why?
… surface area of the plates.
Because the larger the surface area;
- the more space there is on each plate so
- each plate can hold more electrons so
- more charge each can hold
- so there is a higher capacitance (for a larger S.A.)
Capacitance is inversely proportional to … in a uniform field. Why?
… distance between the plates.
Because the smaller the distance is between the plates;
- the opposite charges will attract one another due to their electric fields
- so the electric potential energy (V) is smaller because
- less work is being done to keep the plates from each other when they are closer so
- there is less electric potential energy
- so there is a higher capacitance (for a smaller distance)
What would a potential difference - charge graph look like for a cell? (2 things)
- Voltage remains constant as charge increases
- Flat line at the p.d. level that the battery provides
What would a potential difference - charge graph look like for a capacitor? (2 things)
- Voltage increases as the charge increases
- Directly proportional straight line
Which W.d equation would you use for charge moving through a potential difference (eg. cell)?
W = QV
Which W.d equation would you use for charge moving onto capacitor plates of a potential difference?
W = 1/2 QV
Why do we use the different W.d. equations when working with cells and capacitors?
- Cell has a potential difference from the start so work is done to the charges to move them across the p.d. (as the p.d. on the cell is constant so the W.d. is too)
- A capacitor starts with no p.d. but as the charge flows, the potential difference goes up with the amount of charge at each plate (as the charge is directly proportional to the p.d.). SO in a graph this creates a triangle so it is 1/2.
What will happen to the charge and the p.d. if a capacitor is in series with a cell and is charged up, then disconnected and attached onto a series circuit with an uncharged capacitor? Why (for each point)?
- The charge that is currently on the negative plate of the capacitor, flows through the wires to one of the capacitors’ plates. This is due to the electrons on the negatively charged plate, being repelled by their own charge. This happens until the charge is HALVED between the capacitors.
- Therefore, the total p.d. on the first capacitor is then HALVED due to the charge difference on each plate being halved, due to the charge on each plate being halved. Therefore, p.d. will be HALVED between each capacitor.
What does Q0 stand for?
= Initial charge on a charged capacitor
What does V0 stand for?
= Initial p.d. of the charged capacitor/max. capacitor of cell
What does I0 stand for?
= Initial current
