 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.