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Parallel-Plate Capacitor: Device used to store electrical potential energy and charge. It consists of two conducting sheets with equal and opposite charges. When the sheets are close to each other, the electric field inside is approximately uniform.
Isolated charged capacitor: Charge Q on plates is constant.
Suppose C (^) o is capacitance without dielectric ( K^ =1).
Q=Co Vo
Add dielectric K results in capacitance C= K^ C^ o
Q=CV So, V =
V
K
(^) and
E
E
K
For an isolated charged capacitor, the electric field and voltage difference decreases if a dielectric is placed between the plates. The charge on the plates remains unchanged.
For a capacitor connected to a battery, the voltage difference between the plates stays the same as a dielectric is placed between the plates, but the
charge increases: Q' Q K
Energy stored in a capacitor:
2 PE 1 Q V 1 C V^2 1 Q 2 2 2 C
Direct Current (DC): continuous flowing electric charges which are steady and in one direction
Alternating Current (AC): flowing charges which continuously reverse direction
Electric Current = elapsed time
charge flow t
Q
I
SI unit: coulomb per second ≡ Ampere (A)
Electric Resistance, Ohm’s Law
Resistance: Interaction between flowing charges and atoms in electric circuit which resists (slows) current flow
I
V
R SI units: volt per ampere ≡ ohm (Ω)
Ohm’s Law: Relationship between the potential difference across an electric device and the current that runs through it.
V IR
Ohmic resistor: resistance stays constant as voltage (potential difference) over circuit element varies.
Resistivity
Resistance= A
ρ L cross sectional area
resistivit ytimeslength
Resistivity (ρ) has units of ohm-meter [Ω • m]
Metals-low resistivity (good conductors) Insulators-high resistivity (poor conductors)
Resistivity of metals and semiconductors depends on temperature and impurities.
