Physics 100
Fall 2012
 Magnets and Variables
Announcements
 Exam 2 One week from today.
Homework due last Monday and next.
That is the last of the stuff on the test.
Next: Magnets, Magnetic Forces and
Magnetic Fields
Permanent Magnets
• Have north “poles” and south “poles”
• Like poles repel, opposites attract
• Most, but not all, metals are attracted to
magnets (and magnets are attracted to
metal)….and these metals are attracted to
either pole
• The needle on a compass is magnet and the
end that points north is named the north pole.
The Magnetic Force
• The force is strong.
• It is neither gravitational (too strong for that)
nor electrostatic (no excess charge need be
present).
• A new kind of force-the magnetic force.
The Earth Is a Big Bar Magnet
The Concept of a “Field”
 In science a field is a
collection of measurements
of the same quantity over a
region of space.
 Common examples
include temperature fields,
elevation fields, magnetic
fields and gravitational
Fields.
Magnetic Field Lines
• The direction of magnetic
field lines show the
direction of the magnetic
force (on a north pole).
• The density of field lines
shows the relative
strength of the magnetic
field.
The Earth Is a Big Bar Magnet
Current Carrying Wires
Produce Magnetic Forces
Current carrying wires produce magnetic forces.
What evidence do we have? Current carrying wires deflect
compass needles, can pick up iron objects and interact with
permanent magnets .
This picture shows the simplest of “electromagnets” (magnets
produced by running current through a wire).
Direction of Magnetic Field
Due to Current Carrying Wire
Direction is determined using a “right
hand rule”
Point the thumb of your right hand in the
direction of the CONVENTIONAL (i.e.
positive charge) current, then curl your
fingers. The direction of your curled
fingers is the direction of the magnetic
field (force).
X shows into the page. Dot shows out of
the page.
Direction of Magnetic Field
Due to Current Carrying Wire
Direction is determined using a “right
hand rule”
Point the thumb of your right hand in the
direction of the CONVENTIONAL (i.e.
positive charge) current, then curl your
fingers. The direction of your curled
fingers is the direction of the magnetic
field (force).
X shows into the page. Dot shows out of
the page.
The reason for coils of wire
The magnetic fields from each turn add
together to make a much stronger field
More Electromagnets
These coils of wire are called solenoids (or toroids if they are
in the shape of a doughnut rather than a cylinder) (or
inductors if they are circuit elements).
No matter where you see them, the purpose of these coils of
wire is to produce a magnetic field .
Experiments and Variables
• Scientists use experiments to look for
evidence to develop, support or contradict a
model or theory.
• Experiments are useful in finding cause and
effect relationships in nature.
• Scientists often design an experiment to find
out how changes to one quantity cause
changes in something else they are interested
in.
Experiments and Variables
• These changing quantities are called variables.
• A variable is any factor, trait, or condition that
can exist in differing amounts or types.
• An experiment has two kinds of variables:
controlled and uncontrolled.
• In an ideal experiment you have only two
uncontrolled variables: the one you choose to
change and the one you are studying.