Electrical Energy and Electrical Energy and Capacitance Capacitance

Physics - Chapter 18Physics - Chapter 18

Ch. 18-1 (Pages 666-669) Ch. 18-1 (Pages 666-669) I. Electrical Energy & Electric I. Electrical Energy & Electric

ForceForce Electrical potential energy - PE Electrical potential energy - PE

associated with an object due to associated with an object due to position relative to an electric force.position relative to an electric force.

Results from interactions of 2 Results from interactions of 2 objects’ chargesobjects’ charges

ΔPE ΔPE electricelectric change in electric potential change in electric potential energy (final minus initial)energy (final minus initial)

Factors determining PE electricFactors determining PE electric1. Charge1. Charge2. Strength of electric field2. Strength of electric field3. Position in the direction of the field3. Position in the direction of the field

PE PE electricelectric = -q E d = -q E d

d = displacement from ref. point d = displacement from ref. point

q = chargeq = charge

E = electric field strengthE = electric field strength

PE PE electricelectric= K= Kcc ( (qq11qq22) )

r r

r = distance between charges in r = distance between charges in metersmeters

KKcc=8.99 x 10=8.99 x 1099 N(m N(m22/C/C22))

(pairs of charges)(pairs of charges)

Reference point assumed to be infinityReference point assumed to be infinity

Homework : Page 669 #1-4Homework : Page 669 #1-4

18-2 Potential difference 18-2 Potential difference (p.670-675)(p.670-675)

Electric potential - electric potential energy of Electric potential - electric potential energy of a charged particle in an electric field (divided a charged particle in an electric field (divided by its charge)by its charge)

Electric potential (V) = Electric potential (V) = electric potential electric potential energy energy

charge of the particlecharge of the particle

V = V = PE electricPE electric

qq

V is measured in volts V is measured in volts 1 volt = 1 1 volt = 1 J/CJ/C

Potential difference- change in Potential difference- change in electrical potential energy (electrical potential energy (V)V)

*only changes in electric potential *only changes in electric potential are significant and importantare significant and important

ΔV = ΔV = PE electricPE electric

qq ΔV Measured in ΔV Measured in

volts (V) = 1 J/coulombvolts (V) = 1 J/coulomb

ΔV measures the change in energy per ΔV measures the change in energy per unit of chargeunit of charge

A potential difference within a battery A potential difference within a battery causes charge to movecauses charge to move

ex. ex. 12 Volt car battery 12 Volt car battery-positive terminal has electric potential -positive terminal has electric potential

of 12 Vof 12 V-negative terminal has electric potential -negative terminal has electric potential

of 0 Vof 0 V positive terminal has a higher electric positive terminal has a higher electric

potential then the negative terminalpotential then the negative terminal

As 1 coulomb of charge moves from As 1 coulomb of charge moves from negative to the positive terminal the negative to the positive terminal the battery does work on the charge to battery does work on the charge to increase the PE electric of the charge increase the PE electric of the charge to 12 J of electric potential energyto 12 J of electric potential energy

Each coulomb of charge that leaves Each coulomb of charge that leaves the battery would have 12 J of the battery would have 12 J of energyenergy

This energy is given up to the This energy is given up to the

parts of the device which is run parts of the device which is run

from the batteryfrom the battery

18-3 Capacitance18-3 Capacitance Capacitor - a device used in electrical Capacitor - a device used in electrical

circuits to store chargecircuits to store charge A storehouse of chargeA storehouse of charge Stores positive and negative charges Stores positive and negative charges

separatelyseparately

Parallel-plate-capacitor-two metal Parallel-plate-capacitor-two metal plates separated by a small distanceplates separated by a small distance

Connected to the 2 terminals of a Connected to the 2 terminals of a battery (once charged, battery battery (once charged, battery removed)removed)

Charge is removed from one plate Charge is removed from one plate and accumulates on anotherand accumulates on another

Capacitance- the ability of a Capacitance- the ability of a conductor to store energy in the form conductor to store energy in the form of electrically separated of electrically separated chargescharges

Ratio of net charge on Ratio of net charge on

each plate to each plate to

potential potential

differencedifference

SI unit is the farad (F) = coulomb/voltSI unit is the farad (F) = coulomb/volt Capacitance depends on the size, Capacitance depends on the size,

shape, and materials of the capacitorshape, and materials of the capacitor Larger plates = more storage of Larger plates = more storage of

energyenergy Materials between plates can change Materials between plates can change

capacitance capacitance

Charged capacitors release energy Charged capacitors release energy rapidlyrapidly

Ex. – camera flash, computer Ex. – camera flash, computer keyboardkeyboard