7.6 - Capacitance Flashcards
What are capacitors?
Electrical devices used to store energy in electronic circuits.
What are capacitors commonly used as?
A backup release of energy if the power fails in an electronic circuit (since as they store energy).
What are capacitors made of? (what do they look like internally)
Made of 2 conductive parallel plates with often a dielectric in between them, connected to a voltage supply.
These are often rolled up to reduce the overall size of the device.
What is a dielectric?
A substance that is a poor conductor of electricity, but are good insulators and can store electric energy in an electric field.
Why are dielectrics used in the formation of capacitors?
To ensure charge does not freely flow between the plates. They also enhance the performance of a capacitor by increasing its capacitance.
What is the capacitor circuit symbol?
⎯⎯⎯⎯⏐ ⏐ ⎯⎯⎯⎯
What is the definition of capacitance?
The charge stored per unit potential difference between the plates. (basically the capacity of energy stored in a capacitor).
What is the equation for capacitance?
C = Q/V
C = capacitance (F)
Q = charge (C)
V = potential difference (V)
What is the definition of potential difference?
The energy transferred per unit of charge flowing from one point to another V = E/Q
What is capacitance measured in?
Farads (F)
If a capacitor is made of parallel plates (as usual) what is Q and V in this context?
Q - the charge stored on the plates
V - the potential difference of the capacitor
What are polar molecules and where are found in a capacitor?
Polar molecules are molecules that have a positive and negative end (poles). These are found in the dielectric of a capactitor.
In what direction are the polar molecules aligned when there is no charge stored in the capacitor?
The polar molecules are pointing in a bunch of random directions since there is no electric field (due to no charge).
When charged is applied to a capacitor, what happens inside a capacitor, specifically the polar molecules in the dielectric?
The parallel plates becomes charged (one + one - ).
A uniform electric field is generated between the plates.
The polar molecules align (+ end of molecule faces - plate)
The molecules oppose the electric field applied by the capacitor
When the polar molecules align in the dielectric of a capacitor, what happens to the properties of the capacitor?
The polar molecules aligning reduces the overall electric field (created by the capacitor), which reduces the potential difference needed to charge the capacitor- SO the capacitance increases.
What is relativity permittivity (εr) for a given material defined as? (εr also known as dielectric constant)
The ratio of the permittivity of a material to the permittivity of free space. εr = ε/ε0
what is the relationship between the electric field created by the polar molecules in the dielectric and the permittivity of it?
When the opposing electric field is large, the permittivity of the material is also larger.
(so permittivity is how well the polar molecules in a dielectric align with an applied electric field).
What is the equation for the capacitance of a capacitor in terms of the relative permittivity?
C = Aε0εr/d
A = cross-sectional area of the plates (m^2)
d = separation of the plates (m)
εr = relative permittivity of dielectric
ε0 = permittivity of free space (F/m)
How does electrical energy become stored on the plates of a capacitor?
When charging a capacitor, the power supply pushes electrons from the positive to negative plate.
It therefore does work on the electrons and electrical energy becomes stored on the plates.
What is the equation for energy stored in a capacitor? (different options)
E = 1/2 QV E = 1/2 CV^2 E = Q^2/2C
E = work done or energy stored
Q = charge
V = potential difference
What is the property of the area under the graph of a potential difference, charge graph?
The electric potential energy stored.
How do you investigate what happens when you charge a capacitor?
set up a circuit.
D.C power supply, high resistance resistor, switch and capacitor (all in series). connect to data logger.
turn on D.C supply and let data logger record voltage and current over time.
How do you know when a capacitor is fully charged in a circuit?
Current is 0.
