15-Electromagnetism

Electromagnetism

Force on a current-carrying conductor

Cause – current in a conductor placed in an external magnetic field

Effect – a force (using FLHR) acts on the conductor

Magnitude of F = BIL sinθ where θ is the angle made between conductor and B.
(OR F = BIL where B and L are 90° to one another.)

Direction of F – perpendicular to both B and L (or I) OR perpendicular to plane containing B and L (or I). F = 0 when I=0.
Force on a moving charge

Cause – moving charge in an external magnetic field

Effect – a force (using FLHR) acts on the charge

Magnitude of F = Bqv sinθ where θ is the angle made between v and B.
(OR F = Bqv where B and v are 90° to one another.)

Direction of F – perpendicular to both B and v OR perpendicular to plane containing B and v.

F is a deflecting force, i.e. it changes the direction of motion of the moving charge but does not alter the magnitude of v.

F = 0 when v=0.

Magnetic fields due to (produced by) currents
-Learn to sketch the flux patterns due to – a long straight wire, a flat circular coil (inside coil) and a long solenoid (inside solenoid).
-Learn to sketch resultant flux patter due to of two or more magnetic fields.

-The sketch MUST clearly show
- Direction of field (using right hand grip rule)
- Magnitude of field (draw a number of field lines to show how closer spacing represent stronger field and further spacing represent weaker field)

Uniform magnetic field is produced inside a solenoid. The presence of a ferrous core inside a solenoid will enhance the magnetic field.

Field produced by a conductor does not result in a force acting on itself because this field is an internal field to the conductor but is an external field to another current-carrying conductor. The latter will experience a force due to this field.



Definitions

Magnetic flux density – The force acting per unit current in a wire of unit length at right angles to the field. +

Tesla – the magnetic flux density of a uniform magnetic field when the force on a conductor 1 metre long, placed perpendicular to the field and carrying a current of 1 ampere, is 1 newton.


Velocity selector
Only charge particles with velocity v=E/B are selected (i.e. can pass through the crossed E and B fields undeflected) because electric force on charged particle is equal and opposite to magnetic force on particle.
oppose the flux-change causing it, and does oppose it if induced current flows.