Electricity
Electricity and Magnetism
Unit 5: Electricity and Magnetism
Chapter 17: Fields and Forces
17.1 Fields and Forces
17.2 Gravity
17.3 The Electric Field
17.1 Investigation:
The Magnetic Field
Key Question:
Does magnetic force spread out in the area
around a permanent magnet?
Objectives:
Use a compass to map a magnetic field.
Map out gravitational and electric fields.
Compare and contrast magnetic, gravitational, and electric
fields.
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 cm
2
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 1

meter

square area, so the light intensity is 8 W/m
2
.
The intensity at 2 meters is one

fourth the intensity at
1 meter or 2 W/m
2
.
The inverse square law
The north and south
poles cancel each other
out as you move farther
from the magnet.
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 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.
Unit 5: Electricity and Magnetism
Chapter 17: Fields and Forces
17.1 Fields and Forces
17.2 Gravity
17.3 The Electric Field
17.2 Investigation:
Using Fields
Key Question:
What does a field tell you about
what created it?
Objectives:
Use a compass to map the magnetic field of a hidden magnet
and to find its location and orientation.
Gravity
The
gravitational field
is created by mass.
All mass creates a gravitational field.
Gravity is a relatively weak force, so it takes a planet

sized 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 (F
w
) 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
Gm
2
/
r
2
.
If we know the mass and radius of a planet, we
can use this quantity to calculate the strength of
gravity on that planet.
The planet Mars has a mass of 6.4
×
10
23
kg and a radius of 3.4
million m. Calculate the value of g on the surface of Mars.
Calculating gravitational force
1.
Looking for:
…
the value of
g
in N/kg for Mars
2.
Given:
…
the mass (6.4 x10
23
kg) and radius 3.4 x10
6
m) of
Mars
3.
Relationships:
Use
g
=
Gm
2
÷
r
2
and G= 6.67 x10

11
N•m
2
/kg
2
4.
Solution:
g
=
(6.67
×
10
−
11
N
•
m
2
/kg
2
)(6.4
×
10
23
kg)
3.4
×
10
6
m
= 3.7 N/kg on Mars compared to 9.8 N/kg on Earth
Unit 5: Electricity and Magnetism
Chapter 17: Fields and Forces
17.1 Fields and Forces
17.2 Gravity
17.3 The Electric Field
17.3 Investigation:
Electric Fields and Forces
Key Question:
What are electric fields?
Objectives:
Use a computer simulation to draw electric fields and infer that electric
fields point toward negative charges and away from positive charges.
Explain how the presence of multiple charges in a region affect electric
field diagrams.
Apply Coulomb’s law to calculate the force between charged particles.
The Electric field
Like gravity, the force between
electric charges is carried by a
field, called the electric field.
By convention, we draw the
electric field
to represent the
force on an imaginary positive
test charge.
Drawing the electric field
Electric field lines follow the
direction of the force on a
positive test charge.
The strength of the electric
field is shown by the spacing
of the field lines.
The field is strong where the
field lines are close together
and weak where the lines
are far apart.
Coulomb’s law and electric field
The object that creates the field
is called the
source charge
(
q
1
).
The charge you place to test the
force is the test charge (
q
2
).
As with gravity, we can rewrite
Coulomb’s law so that the
electric field is a separate
quantity in the formula.
Units of the electric field
1 newton per coulomb is
the same as
1 volt per
meter
.
A voltage difference of 1
volt over a space of 1
meter makes an electric
field of 1 V/m.
The force on a charge in an electric field
The force from the electric field
accelerates the charge on
which it is acting.
An electron only accelerates
for a short distance before it
collides with a copper atom.
This is why the constant force
from the electric field results in
a constant drift velocity for
electrons.
A raindrop has a static charge of 0.0001 C. In a thunderstorm, the
raindrop experiences an electric field of 1,000 V/m. What would be
the force on the drop?
Calculating electric field on a raindrop
1.
Looking for:
…force in newtons on the drop
2.
Given:
…charge (0.0001 C) and electric field
(1,000 V/m)
3.
Relationships:
Use
F
=
qE
4.
Solution:
F
= (0.0001 C)
×
(1,000 V/m) = 0.1 N
Electric shielding
When a circular conductor is placed in an electric
field,
no
electric field is detected inside the
conductor.
Electric shielding
If you unwrap a computer network wire, you will find smaller
wires wrapped by aluminum foil.
The aluminum foil is a conductor and
shields
the wires inside
from electrical interference.
Weather is Magnetic
Solar wind and magnetic storms are associated with sunspots.
Sunspots occur when magnetic fields
—
caused by the movement of
gas within the Sun
—
break the Sun’s surface.
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