Electricity TheoryVIR PIV and Capacitors!!!

Energy When an object is at

some height in a gravitational field it is said to have gravitational potential energy, PEg

PEg

Energy Like gravitational fields causing masses to

have potential energy, Electric Fields cause charges to have electric potential energy, PEE

PEE is a type of mechanical energy

MEtotal = KE + PEg + PEs + PEE

Energy To give something PE you must do work

(apply force over a distance) on the something (raising up in g-field)

For PEE to occur a FE must be applied by either

a. An E-Field (uniform)

b. A pair of charges

EnergyUniform E-field

AB

Line Color

Red: E-Field

Black: Equipotential lines

Blue: charge displacement

E

W PE Fd

F Eq

PE qEd

Energy Pair of Charges

1 22

1 2

c

E c

W PE Fd

q qF k

rq q

PE kr

Electric Potential

Any point in an electric field is said to have Electric Potential, V. However, only a Difference in PE is measurable (remember zero point) so we talk of electric potential difference AKA potential difference, ΔV.

EPEV

q

PEV

q

unit Volt, V

J1V=1

C

Potential Difference

Potential Difference

Potential Difference Back to the zero point

A convenient zero point to chose in a circuit or any electric system is the “ground”

Battery (cells) A battery produces

electricity by transforming chemical energy into electrical energy

BatteryCarbon Electrode

Zinc Electrode

Sulfuric Acid

+

Capacitor A capacitor is a storehouse of charge and energy that

can be reclaimed when needed for a specific application

A capacitor will only charge to the potential difference between the terminals of the battery

Capacitance Capacitance, C: The ability of a conductor to

store energy in the form of electrically separated charges

Capacitance is the ratio of charge to potential difference

QC

V

unit Farad, F

C1F=1

V

Capacitance Capacitance depends on size and shape

0

AC

d

2-12

0 2permittivity of free space, 8.85x10

Area of one plate

d distance between plates

C

NmA

Capacitor Energy stored in a

capacitor

21 1

2 2U energy QV CV

Electric Current Movement of electric charge Rate of charge movement

QI

t unit Ampere, A

C1A=1

s

Charge Movement

Charge Movement

Circuit Analogy

Types of Current AC Alternating current charges

continuously change direction forward and back at 60 Hz

Example: outlets (approx 120 V)

DC Direct current charges move in one direction

Example: batteries

AC-DC Debate births the Electric Chair

Resistance Resistance is the impedance of the motion of

charge through a conductor The ratio of potential difference across a

conductor to the current it carries

VR

I

2

unit ohm,

V Js1 1 1

A C

Ohm’s Law

V IR

Resistance Depends on: Length, cross sectional area,

material, and temperature

LR

A

2

resistivity, m

L length, m

A cross sectional area, m

Resistance and Temp

Resistance and Thickness

Resistor An electronic element

that provides a specified resistance.

A current or voltage REGULATOR

Power (it’s Electric!) Power: Rate at which work is done. OR Rate

at which energy is transformed Electric Power: The rate at which charge

carriers convert PEE into non-mechanical energy

P IVunit watt, W

J1 W = 1

s

Reading and Homework Read Chapter 17

pp 593 - 625

HW due on test day:p 599 1-3; p 601 2, 3, 5-9;

p 607 1 – 4 (B); p609 1 – 5

p 615 1 – 6; p 616 2-4, 7,9

p 621 1 – 5

Extra Practicep 626 – 628 11, 20 – 54