Capacitors Flashcards
What do capacitors do?
They store electric charge, they store energy and release charge
What is the formula so find capacitance?
Charge/Potential difference
C = Q/V
What is the unit for capacitance
Farads or F
What is the structure of a capacitor
two parallel plates with a dielectric in between
On a Q against V graph what does the gradient represent
Capacitance
In a Q against V graph what does the area indicate
The energy stored
What is permitivity
How difficult it is to generate an electric field
What are inside dielectrics
Polarised molecules
How does a capacitor generate an electric field
Electrons stack up on a side of the parallel plates unable to cross over, this makes the plate negatively charged, the electrons on the other plate are repelled so it becomes positively charged
How is a second electric field generated in the dielectric
Polar molecules are realigned by the parallel plates charge, which creates an electric field in the opposite direction
What does tao stand for
The time constant which is = RC
Define what is meant by 370F
370C of charge stored by the capacitor per unit of voltage
Why would you want to decrease the time base setting of an oscilloscope
This would increase the wave horizontally
By a factor of how much
this increases the resolution of the x-axis
since we are measuring a larger distance, the % uncertainty is reduced
In what direction does the electric field made by the polarised molecules go?
Opposite to the electric field made by the capacitor, as a result this reduces the p.d, whilst charge stays the same which increases capacitance
While charging a capacitor, what does a graph of current, P.D and charge against time look like
Current - exponential decay
Charge - exponential growth
Voltage - exponential growth
When discharging a capacitor, what does a graph of current. P.D and charge against time looks like
Current - exponential decay
Charge - exponential decay
Voltage - exponential decay
Why does current fall when charging the capacitor
P.D decreases over time so current decreases
pd = Vr + Vc
As Vr falls, Ir would fall
The time constant is the time taken for what?
for a capactior to fall to 37% of its original value
What are the most reactive material from a reactor
The daughter nuclei produced by fission
What are uranium 238 used for?
They do not conduct nuclear fission how do help control the rate of fission by absorbing neutrons
What is normally the % of uranium-235 to uranium-238
5% 235
95% 238
How can you work out the time difference to charge from 2V to 4V
Find the difference of time constants relative to max emf
How do you find the charge of a capacitor on a current against time graph
Find the area of the graph
When discharging a capacitor what resistors must you consider
Total resistance of entire circuit (discharge and charging stages)
