Capacitance Flashcards
What is capacitance?
A capacitor is a device used to store charge, and capacitance is defined as the charges stored per unit volt, measure in Farads (F)
How do capacitor plates have opposite charges?
We can connect the two parallel metal plates (the capacitor) to a battery/ voltage source. From the capacitor plate that is connected to the positive terminal, electrons from the plate are attracted to the positive plate of the battery and this leaves that one capacitor plate with a net positive charge. The negative terminal pushes electrons onto the plate connected to it, leaving it with a net negative charge, and now we have 2 oppositely charges parallel plates.
How is a capacitor charged?
We connect the capacitor to a battery. Initially there is a high potential difference across the battery, and therefore this creates a high current and charge starts to flow. The electrons from the source are repelled from the negative terminal and build up on the capacitor, causing that plate to have a negative charge and simultaneously the positive plate loses electrons leaving it with a positive charge. As charge keeps building up, the current gradually starts to fall because there is less charge to push, and therefore the potential difference across the power supply falls, but the potential difference across the capacitor increases.
What happens when the capacitor is fully charged?
The capacitor voltage equals the initial potential difference of the power supply. At full charge, the current is 0 because there is no more charge to push around. Q max = CV
What is the process of discharging a capacitor?
To discharge the capacitor, we can connect it to a circuit, which typically consists of a resistor. The capacitor holds it maximum charge and once connected, the charge starts to flow around the circuit, and at this point, the current in the circuit is a maximum because the voltage across the capacitor is high. As the charge from the capacitor continues to move through the component, the current decreases, as the voltage across the capacitor decreases.
What is important about a capacitor when it is fully discharged?
When it is fully discharged, there is no more charge flowing, so the current is 0. Furthermore, the voltage across the capacitor is 0 whereas the voltage across the component is at its maximum and equal to the initial voltage across the capacitor which is equal to the initial Voltage across the power supply.
Why is there energy stored in a capacitor?
Electrons are forced onto one plate and taken off the other. The electrons experience a force of repulsion, and therefore work has to be done in moving and holding the electrons, and therefore as we do work, there is energy stored.
what is the equation for energy stored in a capacitor?
W = 1/2QV
W=1/2CV^2
W= 1/2Q^2/C
Explain the typical shape of a capacitor?
many capacitors are cylindrical shaped, rolled up with the plates parallel to each other and a dielectric placed in between. This is because this design enhances the surface area, and allows greater charge to be stored per unit volt.
What is a dielectric?
A dielectric is an electrically insulating material which becomes polarised by an electric field. A dielectric increases the charge stored on a capacitor when it is placed between the 2 parallel plates of the capacitor.
Describe the permittivity?
Permittivity is a measure of how much a material resists the formation of an electric field within it. It determines the ability of a material to polarize in response to an applied electric field and thus how much electrical energy can be stored.
Symbol: ε (epsilon).
Permittivity of free space (vacuum): ε₀ ≈ 8.85 × 10⁻¹² C²/N·m²
How is the dielectric constant / relative permittivity calculated?
ϵr = ϵ / ϵ0
ϵr = q /q0
ϵr = C/C0
All the symbols with nought (0) represent the permittivity in a vaccum.
Describe the process of polarisation of a dielectric that is already made up of polar molecules:
The dielectric is made out of polar molecules which means these molecules have a positive charge on one end and a negative charge on the other end. When this dielectric made out of polar molecules is placed in an electric field, the molecules become aligned. The external electric field exerts a torque on the dipoles, which causes them to rotate. This rotation aligns the dipoles along the field direction.This alignment occurs because the positive ends of the dipoles are attracted toward the negative plate of the capacitor , and the negative ends are attracted to the positive plate.
Desribe the process of polarisation of a dielectric that is made up of non polar molecules
Non polar molecules are those that are neutrally charged. When this dielectric made up of non polar molecules is placed in an electric field, the moelcules become polarised, meaning they gain a positice charge on the end facing the negative plate due to electron being attracted towards ths end, and a negative charge on the end facing the positive plate because electrons are pulled from this side.
What is the effect of a dielectric on the capacitance?
Inserting a dielectric between the plates of a capacitor allows it to store more charge. The polar molecules produce a counter electric field between the plates, which reduces the potential difference between the plates, but the charge stored is constant, so the capacitance increases.
