Capacitors
Define "Capacitance" & what are its units
The charge per unit potential difference across a capacitor
Units: (F)
What is a capacitor made up of?
Two metal plates separated by a dielectric(Insulator)
How do capacitors build up energy
The plates store electric charge, which creates a build up in electric potential energy
Describe the process of capacitors being charged
- The power supply's positive terminal removes negative electrons from one plate causing it to become +ve charged and transfers it to the opposite plate making it -ve charged. The more e-'s that build up, the less e-'s are pushed on the plate due to electrostatic repulsion. The pd becomes the same as the power supply
Describe the process of capacitors being discharged
e-'s leave the negative plate and are transferred to the +ve plate, this occurs until each plate has a neutral charge and the pd between them is zero
When a capacitor is being discharged, what other component is needed in the circuit and why?
- A protective resistor.
-Prevents too quick discharging which can damage the circuit
What happens when a capacitor is discharged quickly through a low resistance circuit
It produces a short burst of a large current
What happens when a capacitor is discharged slowly through a high resistance circuit.
It produces a low current over a long period of time
Give an example of where a quick discharging capacitor is used
A flash camera
Give an example of where a slow discharging capacitor is used
In a temporary power supply e.g. power to the internal memory of a device while its batteries are charged
How does area affect capacitance
Greater area of overlap between plates results in a greater amount of e-s that can be stored on the plates, so a greater capacitance
How does distance affect capacitance
Greater distance between plates = weaker the electrostatic field is between the plates resulting in a lower capacitance
How does permittivity affect capacitance
The higher the permittivity of the dielectric, the weaker the electric field between the plates are due to the dielectric's field opposing the field of the capacitor, so a higher capacitance
Give the formulas for capacitance
C = Q/V C = εA/d
C- Capacitance
Q - Charge
V = Voltage
ε = Material permittivity
d = separation distance
How do you find capacitance in a parallel circuit
C(total) = C1 + C2 + C3
EACH CAPACITOR HAS THE SAME VOLTAGE
How do you find capacitance in series
1/C(total) = 1/C1 + 1/C2 + 1/C3
EACH CAPACITOR HAS THE SAME CHARGE
Give the formulas to get EPE stored in a capacitor
W = 1/2QV
W = 1/2CV²
W = 1/2(Q²/C)
Finish the sentence "Greater capacitance results in..."
Greater number of e-s can be stored on each plate, this increases the no of e-s that need to be discharged from the capacitor which increases the time taken for a capactor to discharge completely
Finish the sentence "Greater resistance results in..."
The more difficult it is for charge to flow, increasing time taken for e-s to leave the capacitor resulting in the time taken for the capacitor to discharge
Finish the sentence, At t=time constant....
The charge, pd and current drop to 37% of its initial value
Give another use of Capacitors
Converting AC to DC
Describe what an AC Voltage signal looks like
A sinusoidal wave
Describe what an DC Voltage signal looks like
Bumps
Describe what an AC Voltage signal (with a capacitor) looks like
Up like a sine wave when charging and down like a straight line graph when discharging
How do you find permittivity of a non vacccum insulator
ε = ε0 × εr
Why will the positive plate and negative plate have the same magnitude of charge?
Due to kirchoffs first law (Conservation of charge)