electric fields and capacitance Flashcards
where do electric field lines emanate from
the surface of the charge
what is couloms law
2 charges separated by a distance will experience a force where F ∝ 1/r^2
what is the definition of electric fields strength
what is definition of gravitational field strength
magnitude of the force per unit charge
magnitude of force per unit mass
why can electric potential be positive or negative
if a positive charge is near a negative charge electric potential is negative
if a positive charge is near a positive charge the electric potential is positive
1 similarities and some differences between electric and gravitational fields
- electric fields can be both attractive and repulsive whereas gravitational fields are only attractive
- gravitational potential is always whereas electric potential is positive and negative
- The electric field strength of radial fields in both are inverse square
- field lines are directed toward center of mass for a gravitational field however in an electric field they are directed in the direction of which a unit positive charge would travell
if gravitational potential is work done per unit mass what is electric potential
work done per unit positive charge
how can you tell when you have escaped an electric field
electric potential = 0
what is the definition of electric potential
work done bringing a unit positive charge form infinity to a point
how do you find electric potential cause by multiple charges
add the individual potentials together
what is definition of capacitance
charge stored per unit potential difference
what is dielectric constant
insulating medium between plates that prevents charge flow between plates
how does dielectric constant increase capacitance
- dielectric polar molecules will align with the electric field and the electric field created by the polar molecules cancels the electric field of the capacitor and because E=V/D the potential difference will reduce across capacitor therefore meaning capacitance is increased
what is the relative permittivity
measure of how easy it is to produce an electric field in a material
what does it mean if the relative permittivity of the dielectric is large
it creates a larger opposing electric field within the capacitor therefore increasing the capacitance more
what does the graph of voltage across a capacitor vs charge on positive plate show
gradient is capacitance as it is Q/V
energy stored = 1/2QV
therefore shows capacitors are inefficient
when capacitor is charging what do the
current/time
voltage/time
charge/time
graphs look like
current time - t=0 is maximum on y intercept then decays exponentially
voltage time/charge time - t=0 is minimum at (0,0) then increases exponentially to a maximum value
what is the same about capacitor discharge graphs
the are all exponential decay curve starting with maximum at y intercept
what is the time constant
time taken for the capacitor to discharge to 1/e of its original value
time taken for capacitor to charge to 1-1/e of start value
what is the half-life of capacitor
0.69C
