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)

Applications of Capacitors:

Keyboards and Defibrillators

Charging… clear…

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

• EX: A 12 V battery produces a current of 0.2 amperes. What is the power in the circuit?

• 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.

• EX: Let’s use the previous example for electrical energy that we calculated. If we are charged $0.05/KHW, how much would it cost to run the toaster?