Capacitance Flashcards
What is a capacitor?
A component that stores charge and has two metallic plates separated by an insulator known as a dielectric material
How do capacitors work?
Electrons move from the negative terminal towards the plate and the electrons start to building up. The electrons move away from the other plate and toward the positive terminal so the plate become positive.
How do you know if the plates are fillly charged?
When the pd across the two plates is equal to the emf of the cell the current falls to zero.
What is the net charge on the plates?
Zero
What is the charge on each plate?
Equal magnitude but opposite charge
Define capacitance
The charge stored per unit pd across it.
What is the unit for capacitance?
Farads
What is the relationship between charge and pd across capacitors?
Pd is directly proportional to charge
If pd increases what happens to capacitance?
Capacitance is always constant
Calculating capacitance for parallel circuit?
Add them
What is the pd for parallel circuit?
The same
How do you find charge for capacitance parallel circuit?
Add them
How do you find capacitance for series circuit?
1/Ct =1/C1+1/C2
What is the charge for series circuit?
Charge stored is the same
How do you find pd for series circuit?
Add them
Electrons moving towards the negative plate?
As there are already electrons on the negative plate, an electron will experience repulsion so external work needs to be applied for it to go onto the plate. This external work is provided from the battery.
What is the time constant?
The time taken for charge/ current/ voltage to fall to 1/e of the original value
How does a capacitor discharge?
At first there is a big pd difference between the two plate so current flows fast and eventually it decreases.
The larger the time constant…
The larger the discharge
Formula for time constant=
T=RC
How long does it take a capacitor to discharge?d
About 5 time constant
What is time constant used for?
To find out how long the exponential decay will take
