# Electricity

Urban and Civil

Nov 15, 2013 (4 years and 5 months ago)

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Electricity

Charge

Defn
: Property of matter that determines how it will interact

with other matter via the electrostatic force; opposite charges

attract, like repel

All atoms are composed of subatomic

particles that contain charge; equal

amounts of + and
-

results in neutrality

Designation of positive and negative is

completely arbitrary

If like charges repel and unlike attract, what holds protons and

neutrons together?

Electrostatic Force

The force of repulsion depends

upon the size of the charges

and the distance between them

Force is linear in charge

Force is proportional to the inverse square of
the distance

k Q
1

Q
2

r
2

F =

Metals

For current, need charged particles that are
readily given up by atom

Protons are tightly bound in nucleus

Metals are atoms

that have outer

electrons that

are not tightly

bound

Current

Current

time rate of flow of

net charge particles past a

given point

In order to have a current, need

a) source of charged particles

b) a conducting path

Electric Potential

An electric field is created by any separation of
charges

Needed to accelerate/move electrons

Electrical potential energy related to electric
field

E.P.E. = Q E d (Note: G.P.E. =
mgh
)

Electric potential is V = E
d

E.P.E. = V d

Potential Sources

Batteries

chemical storage unit; metals in
acid or base

Fuel Cell

similar to battery; chemicals react
with no metals necessary

Generators

electromagnetic induction;
kinetic energy to electricity

Photovoltaic Cells

direct creation of
electricity from sunlight (later chapter)

Battery

Two dissimilar metals in an acid or base

Negative and positive ions

are able to migrate in solution

With a completed circuit,

negative ion goes to negative

terminal, releasing election

At positive terminal, electron combines with
positive ion

Fuel Cell

Similar to battery in that chemicals are
reacting to produce current

Reactants are fed on demand

into fuel cell

Exchange of ions across

semi
-
permeable membrane

allows electrons to complete

circuit

Weight is still major issue

Conducting Path

For current, need path that will conduct
electricity; metals best for this in most cases

Except for superconductors, some resistance
to current will occur

Amount of resistance depends on a) length,

b) cross sectional area, and c) resistivity

r

L

A

R =

Ohm’s Law

The amount of current flowing through a
device depends on the potential and
resistance

V = I R

As resistance increases,

the amount of current

decreases, and vice versa

Circuits

Two ways to wire a circuit if

more than one element

Parallel

each element directly wired to
source; more current needed as add new
elements; 1/R = 1/R
1

+ 1/R
2

+ …

Series

same current flows through each
element; current drops as add new ones, as
resistance increases; R = R
1

+ R
2

+ …

Moving Charges

Oersted

(1820) discovered that a current

creates a magnetic field

This means a current has a force on a

magnet

Newton’s Third Law means that a

magnet has a force on a moving charge

Direction of force is perpendicular to

velocity and magnetic field

Faraday (1831) noticed that electric potential
could by induced by changing magnetic field

If potential induced in closed loop, get a
current

Size of potential depends upon rate at which
magnetic field changes

Generator

Magnet

Electric generator: rotate coils of wire perpendicular

to magnetic field

Layers of wire coils

Front view

How do you make electricity?

Need energy source to do the work of turning the
magnet, causing a flow of current (electricity) in
the wire coils

Convert the potential energy in
something to kinetic energy to turn
magnet

Water behind a dam

Options

Chemical energy in biomass or
fossil fuels

How do you make electricity?

Most electricity formed by steam turbine electric
generators

Electrical Power

P = I V

We know that P =
D
E/
D
t

from the definition of power

The amount of energy delivered by a current is equal to the

amount of charge delivered times the voltage,

D
E =
D
q

V

P = (
D
q

V )/
D
t

But

I

=
D
q
/
D
t

Power Lost

Resistance of wire between source and
appliance causes loss

Since the voltage in a resistor is given by
Ohm’s Law (V = IR), the power lost is given by

P
lost

= I V = I (IR) =

I
2

R

This loss accounts for about 10% of all
electricity in the U.S.

Transformers

Energy loss in lines is decreased by sending
electricity at high voltage/low current over
long distances

Near homes, the voltage

must be stepped down to

prevent lethal accidents

Transformer use electromagnetic

induction to do this