Dec 26, 2015
CAPACITORS (look on PSE page 212)
A capacitor is a device that can store electric charge and consists of two conducting objects placed near one another but not touching. A typical capacitor consists of a pair of parallel plates of area A separated by a distance d. Often the two plates are rolled into the form of a cylinder with paper or other insulator.
The capacitance of a parallel-plate capacitor can be found by using this equation:
CA
do
εo = 8.85x10-12 C2/Nm2
To increase capacitance…
-increase area of plates-decrease gap between
If a voltage is applied to a capacitor it becomes charged. The amount of charge acquired by each plate is proportional to the voltage.
CQ
V
C is the capacitance in farads (F), Q is the charge in (C) V is the voltage (V)
I like to remember as Q=VC
Michael Faraday (1791-1867)
SAMPLE A capacitor whose plates are 12 cm x 12 cm separated 1 mm by an air gap. Find the charge on each plate if the capacitor is connected to a 12 V battery.
= (0.12 x.12)/0.001 = 1.27x10-10 F
Q = CV = 1.27x10-10 (12) = 1.53x10-9 C
CA
do
Example: A 127 μF capacitor is connected to a 12 V battery, find the charge on each plate.C
Q
V Q = CV
= 127 x 10-6 (12) = 1.52 x 10-3 C
practice problems
• BASIC VOCAB:
• is the measure of electron flow. It is measured in amperes or amps. 1 amp = 1 COULOMB/second.
I = Δq Δ t I = V/R
• is the amount of potential energy that each unit of electrical charge has AKA potential difference. It is measured in volts or V.
V = W/q V = Ed V = IR
• (AKA LOAD) is a force or device that slows down or uses electron energy. It is measured in ohms or = Volt/amp or V/A.
R = V/I• The resistance produced by an object depends on:
– Material– Length (longer = more)– Width (larger = less)– Temperature (higher = more)
I = V/R I = V/R or or V = IRV = IR
• R = R = resistanceresistance– Units: Units: OhmsOhms
• V = V = voltagevoltage– units :units :VoltsVolts
• I = currentcurrent– unitsunits : Amps (Amperes)Amps (Amperes)
• the amount of work done in a given amount of time.
• is VOLTAGE times CURRENT• Power has the units of WATTS, W or J/s. • ** Remember voltage has the units of Volts or V and current is Amperes,
Amps or A.
• Formula for electrical power is on formula chart. • P = VI P = W/Δt P = ΔqV/ Δ t
• AKA electrical work.• is POWER times TIME.• Its units are KWH, so have to convert your
power if it is in WATTS. • Its formula is on the formula chart. • E = P x t
• EX: A toaster operates on 2 amperes of current on a 110V circuit for 3 hours. Calculate the electrical energy used by the toaster.
– 1. Calculate power.
– 2. Convert
– 3. Calculate Electrical Energy
• You use electrical energy (electrical work) and multiply it by how much $/KWH the utility company charges. There is not necessarily a formula but it is something you should be able to calculate.