Capacitors (A2) Flashcards
what is capcitance?
the amount of charge that an object is able to store per unit PD across it
What is capacitance measured in?
Farads (1 farad =1CV^-1)
What is a capacitor?
An electrical component that can store charge. They are 2 conducting plates separated by a dielectric
what happens when a capacitor is connected to DC?
Charge builds up on plates (one positive, one negative). The insulator prevents charge moving between them, so pd builds up between the plates
What are capacitors used to do?
used to provide power for a small time (they can store charge until needed and then discharge quickly)
This can be used in camera flashes, back up power supplies and to smooth out variations in DC supplies (absorb peaks and fill troughs)
What happens as the plates charge? and what does this mean?
Like charges are forced together (requires energy), this energy is called electric potential energy and it is held until charge is released.
LEARN DERIVATION FOR ENERGY STORED EQUATIONS
LEARN
What does a change in capacitor properties mean?
A change in how much charge can be stored at a given voltage
How can you change capacitance and why does this change it?
Change the dielectric material. This is because different materials have different relative permittivities?
What is meant by relative permittivity?
its a measure of how difficult it is to generate an electric field in a given medium (higher = more Q needed to generate an electric field)
learn relative permittivity equation
Er = E1/E0
What is a dielectric made up of?
polar molecules (no charge stored = no field = molecules randomly aligned)
What happens to a dielectric when a charge is applied?
A field is generated, negative end of polar molecules attracted to + and vice versa (molecules aligned perpendicular to electric field)
What does each polar molecule have? What does this mean?
Its own electric field which opposes the electric field of the capacitor (larger permittivity = larger opposing field - reduces overall field = lower pd to transfer a given Q to capacitor, resulting in capacitance increasing)
What also decreases capacitance?
increasing distance between plates
LEARN DIELECTRIC EQUATIONS
LEARN
Describe the charging of a capacitor
1) currents flows until capacitor is fully charged (then stops)
2) electrons flow from negative terminal onto connected plate (negative plate)
3) At the same time, electrons on other plate flow to positive terminal (repelled by negative plate) - same number repelled as built up
4) Causes an equal but opposite charge on plates (therefore pd between plates
Why doesn’t charge flow between plates?
because they’re separated by a dielectric
Why does current decrease over time?
As charge builds up, electrostatic repulsion makes it harder for electrons to build up on negative plate
What happens when pd across capacitor = pd across supply?
- no current
- capacitor fully charged
Describe charging through a fixed resistor
- when pd applied, total voltage = resistor voltage
- pd across capacitor = 0 at first
- pd of battery causes high current at start
- BUT, as capacitor charges, pd across capacitor increases and pd across resistor decreases (so current drops)
LEARN CHARGING GRAPHS AND EQUATIONS
LEARN
What is charge on capacitor during charging proportional to?
the potential difference across it
What happens when a charged capacitor is connected across a resistor?
potential difference drives current through the circuit (flows opposite from the charging current)
When is a capacitor fully discharged?
pd difference across plates and current in circuit = 0
LEARN GRAPHS FOR DISCHARGING AND EQUATIONS
LEARN
What does time to charge/discharge depend on?
- Capacitance (affects amount of charge transferred at a given voltage)
- Resistance (affects current)
What is RC?
the time constant
What is the time constant?
time for Q on discharging capacitor to fall by 37% of the initial charge or time for charging capacitor to rise by 63% of the initial charge
What does a larger R in RC cause?
longer time to charge/discharge
What is the average time constant?
5RC
What can you use to find time constant?
RC or graphs
What is the time to halve?
the time for Q, I or pd of a discharging capacitor to half its initial value
How do you find the time to halve?
- time to halve = 0.69RC (learn derivation)
- use Q-t, pd-t or I-t graphs (where the values are half of the initial)
