11/4-10 Capacitance

Like a battery, a capacitor

does something different by stores energy inside an electric field

Created by creating a charge difference between _ conductors

2

Simplest capacitors are

2 parallel plates

Charge is equal but

opposite on each plate

Change in V across the 2 plates is equal to the voltage source if directly 

connected 

• 9V battery would cause 9V difference between plates

• If inside a circuit, then it will depend on the voltage drop

For any given capacitor, its capacitance is

constant

k_d =

dielectric constant, based on the material between the plates

A =

area of the plate

d = 

distance between the plates 

“k” is a

ratio of capacitances

K_air =

1

K_v =

1

The ration between the charge of the plate and the potential difference between the plates is known as

Capacitance

Capacitance is measure in 

Fards(F)

Dielectrics in capacitors

must be an insulator

Insulators allow E→ to pass through,

unlike conductors

Bound charges are those that are

neutralized by the dielectric

Polarization is only partial, 

unlike conductors 

Capacitors (1)

protect sensitive electronics 

Capacitors (2)

help keep voltage constant, once full

Capacitors (3)

can power electronics, but in reverse

Any 2 objects that have different charges can

form a capacitor

Your touchscreens measure

capacitance changes to register touches

Capacitors in circuits due to Kirchhoff’s Loop rule, they must have the same

voltage drop 

Capacitors in series (and nothing in the circuit) must see the same

current. Thus they must have the same magnitude of charge on their plates

I = change in Q over change in T;

since current and time must be the same. Since current flows through capacitors in series for the same amount of time, the magnitude of Q on all of the plates must be the same

Charging a Capacitor (1)

Potential is applied (circuit turns on)

Charging a Capacitor (2)

An electron is pulled/pushed to the other side

Charging a Capacitor (3)

Number of electrons per second equals the initial current 

Charging a Capacitor (4)

The capacitor acts like a wire

Unlike only capacitors, VIRP is now

time-dependent

When does current flow through a capacitor?

When the capacitor is not fully charged

When is current maximum?

When the circuit is closed and the capacitor is “empty” of energy

What happens as time increases?

Current decreases and approaches zero

At t=0 s,

capacitors act like wires

time constant (= RC) determines

how quickly a capacitor charges and discharges

More resistance (R),

the slower the current to charge

More capacitance (C),

more capacitance to charge