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 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.
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
 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.
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 (q1).
 The
charge you place to test the
force is the test charge (q2).
 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.
Calculating electric field on a raindrop
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?
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.