Electrical Energy and Capacitance Physics - Chapter 18

Ch (Pages ) I. Electrical Energy & Electric Force Electrical potential energy - PE associated with an object due to position relative to an electric force. Electrical potential energy - PE associated with an object due to position relative to an electric force. Results from interactions of 2 objects’ charges Results from interactions of 2 objects’ charges

ΔPE electric  change in electric potential energy (final minus initial) ΔPE electric  change in electric potential energy (final minus initial) Factors determining PE electric Factors determining PE electric 1. Charge 2. Strength of electric field 3. Position in the direction of the field

PE electric = -q E d PE electric = -q E d d = displacement from ref. point q = charge q = charge E = electric field strength

PE electric = K c (q 1 q 2 ) PE electric = K c (q 1 q 2 ) r r = distance between charges in meters r = distance between charges in meters K c =8.99 x 10 9 N(m 2 /C 2 ) K c =8.99 x 10 9 N(m 2 /C 2 ) (pairs of charges) (pairs of charges) Reference point assumed to be infinity Reference point assumed to be infinity Homework : Page 669 #1-4

18-2 Potential difference (p ) Electric potential - electric potential energy of a charged particle in an electric field (divided by its charge) Electric potential - electric potential energy of a charged particle in an electric field (divided by its charge) Electric potential (V) = electric potential energy Electric potential (V) = electric potential energy charge of the particle charge of the particle V = PE electric q

V is measured in volts  1 volt = 1 J/C V is measured in volts  1 volt = 1 J/C Potential difference- change in electrical potential energy (  V) Potential difference- change in electrical potential energy (  V) *only changes in electric potential are significant and important ΔV = PE electric ΔV = PE electric q ΔV Measured in ΔV Measured in volts (V) = 1 J/coulomb volts (V) = 1 J/coulomb

ΔV measures the change in energy per unit of charge ΔV measures the change in energy per unit of charge A potential difference within a battery causes charge to move A potential difference within a battery causes charge to move ex.  12 Volt car battery ex.  12 Volt car battery -positive terminal has electric potential of 12 V -negative terminal has electric potential of 0 V positive terminal has a higher electric potential then the negative terminal positive terminal has a higher electric potential then the negative terminal

As 1 coulomb of charge moves from negative to the positive terminal the battery does work on the charge to increase the PE electric of the charge to 12 J of electric potential energy As 1 coulomb of charge moves from negative to the positive terminal the battery does work on the charge to increase the PE electric of the charge to 12 J of electric potential energy Each coulomb of charge that leaves the battery would have 12 J of energy Each coulomb of charge that leaves the battery would have 12 J of energy 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 battery from the battery

18-3 Capacitance Capacitor - a device used in electrical circuits to store charge Capacitor - a device used in electrical circuits to store charge A storehouse of charge A storehouse of charge Stores positive and negative charges separately Stores positive and negative charges separately

Parallel-plate-capacitor-two metal plates separated by a small distance Parallel-plate-capacitor-two metal plates separated by a small distance Connected to the 2 terminals of a battery (once charged, battery removed) Connected to the 2 terminals of a battery (once charged, battery removed) Charge is removed from one plate and accumulates on another Charge is removed from one plate and accumulates on another

Capacitance- the ability of a conductor to store energy in the form of electrically separated charges Capacitance- the ability of a conductor to store energy in the form of electrically separated charges  Ratio of net charge on  Ratio of net charge on each plate to each plate to potential potential difference difference

SI unit is the farad (F) = coulomb/volt SI unit is the farad (F) = coulomb/volt Capacitance depends on the size, shape, and materials of the capacitor Capacitance depends on the size, shape, and materials of the capacitor Larger plates = more storage of energy Larger plates = more storage of energy Materials between plates can change capacitance Materials between plates can change capacitance

Charged capacitors release energy rapidly Charged capacitors release energy rapidly  Ex. – camera flash, computer keyboard  Ex. – camera flash, computer keyboard