16. Electromagnetism

Electromagnetism Magnetic Field – a region of space in which a magnetic pole, current -carrying conductor, or a moving charge experience a magnetic force. Magnetic Field Lines  Tangent of the lines points in the direction of the magnetic flux density, B  Closer lines indicate larger B  No two lines intersect one another Magnetic Flux Density – at a point is the force per unit current per unit length experienced by a straight current-carrying conductor placed at right angles in a uniform magnetic field. Tesla – is the magnetic flux density at a point in a magnetic field if a straight conductor carrying a current of one ampere placed at right angle to a uniform magnetic field experiences a force per unit length of one newton per metre. Magnetic Flux Density of a Long Straight Wire The magnetic flux density at point x, at a perpendicular distance r along a plane from the wire carrying current I is, I0B 2r   Tip: To determine direction of B, 1. Draw a straight line joining the wire to the point in question (Green Line) 2. Use RHGR a. Current into page, hence clockwise 3. Hence, as if “turning the green line clockwise”, B is perpendicular to the green line pointing as shown. Right-Hand Grip Rule (RHGR). Thumb as current, fingers as field directions Field direction at each point is tangential to the circular field lines. Perpendicular to radius Top View, current into page, clockwise field Top View, current out of page, anticlockwise field x r I B

Magnetic Flux Density of a Flat Circular Coil The magnetic flux density at point x, which is at the centre of the circular coil of radius, when a current I running through it is, NI0B 2r  Determine direction of field of a coil, use Right Hand Grip Rule, where now, thumb is the field direction and the fingers as the current direction. Magnetic Flux Density of a Solenoid The magnetic flux density within a long solenoid is uniform, I0Bn  where, Nn L Clockwise current, field into page within coil; out of page not within coil Anticlockwise current, field out of page within coil; into page not within coil B r I L N number of turns B L I

Ferromagnetic Material in Solenoid  Examples of ferromagnetic materials are iron, steel, nickel and cobalt  There are many tiny little magnets called magnetic dipoles within the material o Regions of a number of magnetic dipoles are called magnetic domains  When unmagnetised, the magnetic dipoles are oriented randomly  Under the influence of an external magnetic field, such as when the ferromagnetic material is placed inside of a solenoid, the magnetic dipoles will align with the field o The ferromagnetic material is now magnetised  The overall effect is that a ferrous core in a solenoid increases the magnetic flux density in and around the solenoid Other Magnetic Flux Density Patterns Two Parallel Straight Conductors Bar Magnets Two conductors wi