Magnetism

Magnetism Since ancient times, certain materials, called magnets, have been known to have the property of attracting tiny pieces of metal. This attractive property is called magnetism. N S Bar Magnet N S

Magnetic Poles S N Iron filings The strength of a magnet is concentrated at the ends, called north and south “poles” of the magnet. N S E W Compass Bar magnet A suspended magnet: N-seeking end and S-seeking end are N and S poles.

Magnetic Attraction-Repulsion Magnetic Forces: Like Poles Repel Unlike Poles Attract

Magnetic Field Lines We can describe magnetic field lines by imagining a tiny compass placed at nearby points. N S The direction of the magnetic field B at any point is the same as the direction indicated by this compass. Field B is strong where lines are dense and weak where lines are sparse.

B = Fmagnetic qv Magnetic Field F = magnetic force of charged particle q = magnitude of charge v = speed of charge

Field Lines Between Magnets Unlike poles N S Attraction Leave N and enter S N Repulsion Like poles

Magnetic Field of a Current Carrying Wire A current-carrying conductor produces a magnetic field Using the right hand rule the thumb is the direction of the current and your fingers are the direction of the field……clockwise or counterclockwise

Direction of Magnetic Force The right hand rule: With a flat right hand, point thumb in direction of velocity v, fingers in direction of B field. The flat hand pushes in the direction of force F. B v F B v F N S The force is greatest when the velocity v is perpendicular to the B field. The deflection decreases to zero for parallel motion. Use left hand for negative charges.

Indicating Direction of B-fields One way of indicating the directions of fields perpen-dicular to a plane is to use crosses X and dots · : A field directed into the paper is denoted by a cross “X” like the tail feathers of an arrow. X X X X X X X X X X X X X X X X · · · · A field directed out of the paper is denoted by a dot “ ” like the front tip end of an arrow. ·

Practice With Directions: What is the direction of the force F on the charge in each of the examples described below? Up F + v X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X v + Left F · · · · Up F · · · · - v - v F Right negative q

Electromagnetic Induction Process of inducing a current in a circuit with a changing magnetic field. Suppose a bar magnet is pushed into a coil of wire. As the magnet moves into the coil, the strength of the magnetic field within the coil increases, and a current is induced in the circuit. This induced current in turn produces its own magnetic field, whose direction can be found using the right hand rule.

3 Ways to Induce Current 1. Circuit is moved into or out of magnetic field 2. Circuit is rotated in the magnetic field 3. Intensity of magnetic field is varied

Magnetic Force v vsin Note that since the force is always perpendicular to the velocity, the magnetic force does no work and cannot increase or decrease the speed of a charge. B θ X FB into page

29-1,2 Magnetic Force *Charge moving perpendicular to B field experiences maximum force *Force proportional to  component of velocity vector *FORCE IS ALWAYS PERPENDICULAR TO THE MAGNETIC FIELD EVEN IF VELOCITY IS NOT *Charge moving parallel to B field experiences zero magnetic force FB