Transcript What is a field?

What is a field?
 In
physics, a field is a
physical phenomena that has
a value everywhere in space.
 Loudness
has a value
everywhere around a stereo.
 This
means you can describe
the loudness with a field.
 All
interactions between
matter and energy occur by
way of fields.
Fields and energy
 Any
field is a form of
energy that is distributed
through space.
A
magnetic field has
energy because it can
exert force over distance,
or do work, on another
magnet.
Adding fields
 Fields
of the same kind
can be added or
subtracted.
 The
field from an
electromagnet can either
cancel the field from a
permanent magnet or
add to it.
The inverse square law
 The
inverse square law
states that a field
decreases as the square
of the distance from the
source of the field
increases.
 While
sound, light, gravity
and electricity follow this
trend, magnetism does
not.
Intensity

All the sound energy you hear must pass
through 1 cm2 at the opening to the ear.

The intensity—the number of watts per unit
area—reaching your ear is 0.01 watts per
square centimeter.
— Close to the stereo, 0.01 watt of power is
captured by the small opening of your
ear.
— Because the power spreads out, far away
from the stereo the intensity drops so that
the same square centimeter of your ear
captures less than 0.01 watt of power
Light intensity and distance
 At
a radius of 1 meter, 8 watts of light fall on a 1meter-square area, so the light intensity is 8 W/m2.
 The
intensity at 2 meters is one-fourth the intensity at
1 meter or 2 W/m2.
The inverse square law
 Magnetism
is an
exception to the inverse
square law because all
magnets have two
opposite poles, not just
one.
 The
magnetic field
decreases much faster
than an inverse of the
square of distance.
 The
north and south
poles cancel each other
out as you move farther
from the magnet.
The speed of a field
 The
magnetic field exerts a force
of one magnet on another at the
speed of light.
 The
speed of light is 300 million
m/s, so it takes only a tiny
fraction of a second for the force
to be exerted by one magnet on
another when the distance is a
few meters.
The speed of
light
 All
interactions are carried by
fields, and the fastest that any
field can spread is the speed
of light.
 Information
like your cell
phone number and the number
you are calling is coded in
pulses of energy.
 The
information spreads as an
electromagnetic field that
expands at the speed of light.
Gravity
 The
 All
gravitational field is created by mass.
mass creates a gravitational field.
 Gravity
is a relatively weak force, so it takes a planetsized mass to create a field strong enough to exert a
significant force.
Gravitational field of Earth
 The
gravitational field is a
force field because it
creates a force on masses
at all points in space.
 The
force (Fw) on an
imaginary mass (m) is
equal to the mass
multiplied by the
gravitational field (g)
Earth and Moon
Gravitational force acts in
two steps.
1. Earth creates a
gravitational field.
2. The Moon feels a
force from the
gravitational field
that causes it to
orbit Earth.
Gravitational fields
 The
gravitational field is a
vector field because a
gravitational force has a
direction at all points in space.
 Like
the magnetic field, you
can draw field lines to show
the direction of the
gravitational field.
Law of gravitation
 The
formula for Newton’s law
of gravitation can be
rearranged.
 The
strength of the
gravitational field (g) is given
by the quantity Gm2/r2.
 If
we know the mass and radius of a planet, we
can use this quantity to calculate the strength of
gravity on that planet.
Calculating gravitational force
The planet Mars has a mass of 6.4 × 1023 kg and a radius of 3.4
million m. Calculate the value of g on the surface of Mars.
1.
Looking for: …the value of g in N/kg for Mars
2.
Given: …the mass (6.4 x1023 kg) and radius 3.4 x106 m) of
Mars
3.
Relationships: Use g = Gm2 ÷ r2 and G= 6.67 x10-11
N•m2/kg2
Solution: g = (6.67 ×10−11 N•m2/kg2)(6.4 ×1023 kg)
3.4 ×106 m
= 3.7 N/kg on Mars compared to 9.8 N/kg on Earth
4.