Capacitors Flashcards
What is the function of a capacitor
To store electric charge
What does a capacitor consist of
2 metal plates separated by a layer of electrical insulator - a dielectric
Define Capacitance
The charge stored per unit potential difference across the plates
Formula for capacitance
C=Q/V
Describe the movement of electrons when a capacitor is charging
- The plates contain a large number of electrons moving freely within a lattice of positive metal ions. The number of free electrons is balanced by the positive charge of the metal ions
- Once attached electrons flow onto one metal plate giving it an excess of electrons making it negatively charged
- An equal number of electrons flow off the other plate to the positive cell making that plate positively charged
- As more electrons accumulate on the negative plate, they oppose further electrons flowing onto it
- Once the pd across the capacitor= EMF the flow of electrons stops and the capacitor is fully charged
What is the unit of capacitance
Farad
What is 1 Farad equal to
1CV^-1
What does the gradient of a Charge vs Pd graph represent
Capacitance
What does the area under a Charge vs Pd graph represent
Energy stored
What does the capacitance of a parallel plate capacitor depend on
The area of the plates, the separation of the plates and the type of dielectric
3 formulas for energy stored
- E= 1/2QV
- E= 1/2CV^2
- E=1/2Q^2/C
Describe the relationship between capacitance, area and plate separation
-Capacitance is directly proportional to the area of the overlap of plates,A
-Capacitance is inversely proportional to the plate separation,D
CαA/D
Why is energy stored only 1/2QV, not just QV
because 1/2QV is always wasted in the wires
What properties must dielectrics have and why
Good electrical insulator to prevent the flow of electrons between plates
Define relative permittivity
The factor by which the electric field between 2 charges is decreased by the presence of the dielectric, relative to a vacuum. Also equal to the factor by which capacitance is increased when using that dielectric compared to a capacitor with a vacuum between its plates
What is relative permittivity also known as
The dielectric Constant
What is a polar molecule
A molecule that has a partial positive charge in one part of the molecule and a partial negative in the other, although overall the molecule is neutral
Describe and Explain how the use of a dielectric increases capacitance
- The materials used as dielectrics contain polar molecules
- Before the capacitor is charged the molecules have random orientations, but on experiencing the electric field they rotate and align themselves with the field
- The polarisation of the dielectric creates an internal electric field, which partially cancels the electric field created by the charge on the plates, so reducing pd across capacitor
- To reurn pd to its original value requires the addition of more charge onto the plates
- Therefore the overll effect is to increase the amount of cahrge a capacitor can storefor a given pd, hence increasing its capacitance
What is present between the charged plates
A uniform electric field
When are capacitors particularly useful
When a high power output is required for a short time
How does the resistance effect the charging of a capacitor
The greater the resistance of the resistor, the more slowly the charging/discharging takes place
Why is the current always greatest just as the switch is closed
Because no charge has built up on the capacitor, therefore the pd across the resistor is equal to the EMF. This means the rate of flow of charge is at its greatest, as charge builds up on the capacitor electrostatic repulsion means the rate of flow of charge decrease
The graph of charging current verus time is a ………… curve
Exponential Decay (meaning the charging current reduces by the same factor in equal time intervals)
What is T1/2
0.69RC, This is the time taken for the charge/potential difference/current of a discharging capacitor to reach half of the value it was when it was fully charged
Define Time Constant
The time for the charging current to fall to 1/e of its original value, which is approximately 37%
What is the time constant equal to
It is the time taken for the charge/potential difference/current of a discharging capacitor to fall to 37% of its value when fully charged
Also, the time taken for the charge/potential difference of a charging capacitor to rise to 63% of its value when fully charged
How do you calculate time constant
Resistance x Capacitance
What is an estimate for the time it takes to fully charge/discahrge a capacitor
5RC
When using log graphs what does the gradient of the straight line represent
gradient=-1/Time Constant
If the charging voltage is doubled, how does this affect the total charge stored, the capacitance and the time taken to discharge
- The total charge stored doubles, as V is proportional to Q
- The capacitance wouldn’t change
- The time taken wouldn’t change
