Energy and Fields - Introduction to Geophysics - Lecture Slides, Slides of Geology

Download Energy and Fields - Introduction to Geophysics - Lecture Slides and more Slides Geology in PDF only on Docsity!

Energy

and

Fields

Gravity Electrical

Magnetic

Common

misconceptions

Energy

is

NOT

a

force.

A

force

applied

over

a

distance

has

units

of

energy

(work).

Energy,

Work,

Force,

and

Momentum

are

all

different

things.

A

mass

moving

at

a

non

‐zero

speed

has

energy

of

motion

(kinetic)

that

can

be

transformed

into

heat

or

do

work.

Throw

a

ball

into

a

spring

and

lock

the

spring

when

the

ball

stops…where

does

the

energy

go

I^

do

work

by

raising

a

mass

upwards

in

the

earth’s

gravity

field.

Where

did

the

energy

go?

Release

the

ball,

the

gravitational

energy

causes

acceleration

of

mass

to

make

KE.

It

feels

like

I

am

doing

work

while

pushing

on

a

wall,

but

there

is

no

displacement,

so

no

work

is

being

done.

Wrong…your

muscle

cells

are

contracting/expanding

hence

doing

work.

Energy

never

disappears

it

just

degrades

into

more

dispered

forms

as

measured

by

the

entropy

which

always

increase

in

a

closed

system.

Fields

cannot

exist

if

I

cannot

see

them.

Not

true.

Try

to

push

the

North

Pole

of

two

magnetic

together,

do

you

feel

the

force

field

Energy:

Power

is

just

energy

per

second

Energy

is

conserved

because

the

laws

of

physics

have

never

been

found

to

change

over

time.

Repeat:

no

one

has

EVER done

an

experiment

where

energy

is

not

conserved

(constant).

In

addition,

to

violate

conservation

of

energy

you

would

have

to

disprove

a

mathematical

theorem

(Noether’s)

which

would

gut

the

foundations

of

mathematics.

Energy

is

neither

made

nor

destroyed:

it

is

conserved

both

local

and

on

the

scale

of

the

entire

universe.

Energy

can

be

exchanged

into

different

forms:

Kinetic,

potential

gravity,

heat,

electromagnetic

and

elastic

waves.

This

is

the

power

of

viewing

physics

from

an

energy

viewpoint;

one

can

track

the

change

in

time

of

the

energy

into

different

forms.

No

one

EVER

REALLY

USES

energy

because

that

implies

the

energy

is

somehow

destroyed!

What

we

mean

is

we

DEGRADE energy

(electrical

or

chemical)

into

a

more

dispersed

form,

usually

heat!

The

energy

from

burning

gasoline

in

an

engine

ends

up

in

two

new

places:

doing

work

in

pushing

the

car

in

the

gravity

field

up

and

down

hills;

combustion

makes

heat

which

comes

out

the

exhaust

pipe

and

the

radiator.

Energy:

name

the

energy

change?

What

is

a

(force)

field?

In

physics,

a

field

is

a

physical

quantity associated

to

each

point

of

spacetime.

[1]

A

field

can

be

classified

as

a

scalar

field,

a

vector

field,

or

a

tensor

field,

according

to

whether

the

value

of

the

field

at

each

point

is

a

scalar,

a

vector,

or,

more

generally,

a

tensor,

respectively.

A

field

cannot

be

seen,

but

nonetheless

it

is

real

and

can

do

work

(by

applying

forces)

on

objects

and

exchange

energy.

It

took

scientist

along

time

to

accept

field

(1890’s).

Scalar

field

(temperature)

Dipole

magnetic

field

Fluid

velocity

field

(vortex)

Fields

emanate

from

their

charges

or

dipole

electrical

gravity

magnetic

Electrical. Vector.

Two

charges:

plus

(proton)

minus(electron)

Field

falls

off

from

charge

as

1/r*

and

field

can

be

attractive

or

repulsive

depending

on

sign

of

charges.

If

you

accelerate

the

charge

you’ll

add

a

magnetic

field

to

the

electric

field.

Gravity.

Vector.

One

charge

(mass).

Field

falls

off

from

mass

charge

as

1/r*

and

field

is

always

attractive

(no

such

thing

as

negative

mass).

If

you

accelerate

the

mass

you’ll

make

a

ripple

in

the

space

time

continuum. Magnetic.

Vector.

No

charges!! Always

a

dipole

with

a

‘N’

and

‘S’

end

to

make

a

magnetic

dipole.

Field

falls

off

from

dipole

as

1/r*3.

If

you

accelerate

a

magnetic

field

near

a

conductor,

an

electrical

current

will

be

inducted

(a

generator!).

Summary

When

energy

is

used,

we

mean

that

we

degrade

it

into

other

forms

(e.g.,

heat),

but

energy

is

always

conserved.

Many

different

forms

of

energy:

Kinetic,

gravitational,

electrical,

magnetic,

heat,

strain,

nuclear,

electro

magnetic.

Electromagnetic

waves

transmit

energy

at

the

speed

of

light.

Light

is

an

E

M

wave

that

our

eyes

are

sensitive

too.

Other

EM

waves

types:

radio,

microwave,

infared.

Fields

that

transmit

force

and

can

do

work

on

objects

exist

even

though

you

cannot

directly

see

them.

Two

main

kinds

of

fields:

the

scalar

field

which

is

just

a

number

(temperature,

pressure)

a

vector

field

(velocity,

gravity,

magnetic,

electrical).

The

strength

of

a

field

is

proportional

to

the

amount

of

charge

(coulombs

or

kg).

Field

lines

for

a

certain

force

never

cross

themselves,

but

when

magnetic

field

line

touch

they

reform.