Electricity

kitefleaUrban and Civil

Nov 15, 2013 (3 years and 7 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’s Induction Law


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