Capacitors (17.7-17.9)Capacitors (17.7-17.9)

Capacitors are elements used in Capacitors are elements used in circuits to store electric charge (and circuits to store electric charge (and

energy) and to block surges of energy) and to block surges of electricity, protecting circuitselectricity, protecting circuits

Capacitors: what are they?Capacitors: what are they?

• Caps typically consists of 2 conducting plates placed near each other but not touching.

• Sometimes they are wrapped into a cylinder like a burrito or one of those “healthy wraps”

CapacitorsCapacitors

• When placed in a circuit and voltage source, charges flow onto the plates.

• Q = CV = total charge on the capacitor • C is called the Capacitance and its

measured in Farads.• C depends on the geometry of the

Capacitor…C = ЄA/d where Є = a constant , A is the area of the plate, and d = distance between the plates.

Capacitors and DielectricsCapacitors and Dielectrics

• A dielectric is a material which is placed between the plates in a capacitor.

• A dielectric does not allow electric charge to jump across the gap as easily as air, so higher voltages can be used.

• Dielectrics also increase the Capacitance of the capacitor.

Dielectrics (17.8)Dielectrics (17.8)

• Dielectrics increase the capacitance by a factor of K.

• K = dielectric constant = material property• C = KЄA/dMtl KVacuum 1.0Paper 3.7Rubber 6.7Waffle 9.3

Energy in a capacitorEnergy in a capacitor

• Caps store energy• E = ½ CV2 (V = Voltage, not velocity…duh)

Capacitors in circuits (19.5)Capacitors in circuits (19.5)

• Capacitors are commonly used in circuits to store energy and influence how quickly the current changes.

• Caps in Parallel have the same voltage and you find the total charge Q on each plate using Q = CV

• Caps in Series have the same charge on them.

Capacitors in series and parallelCapacitors in series and parallel

• The rules for “combining capacitors” in a circuit is just the opposite as the resistor rules…..

Circuit Voltage Charge Cequiv

Parallel Same AddsCeq=C1+C2

Series Adds Same I/Ceq= 1/C1 + 1/C2